--- gcc/gcc.info-14 2018/04/24 17:55:42 1.1.1.3 +++ gcc/gcc.info-14 2018/04/24 18:41:37 1.1.1.9 @@ -1,9 +1,13 @@ -This is Info file gcc.info, produced by Makeinfo-1.47 from the input -file gcc.texi. +This is Info file gcc.info, produced by Makeinfo version 1.67 from the +input file gcc.texi. This file documents the use and the internals of the GNU compiler. - Copyright (C) 1988, 1989, 1992 Free Software Foundation, Inc. + Published by the Free Software Foundation 59 Temple Place - Suite 330 +Boston, MA 02111-1307 USA + + Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995 Free Software +Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are @@ -11,1042 +15,956 @@ preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided also -that the sections entitled "GNU General Public License" and "Boycott" -are included exactly as in the original, and provided that the entire +that the sections entitled "GNU General Public License," "Funding for +Free Software," and "Protect Your Freedom--Fight `Look And Feel'" are +included exactly as in the original, and provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Permission is granted to copy and distribute translations of this manual into another language, under the above conditions for modified versions, except that the sections entitled "GNU General Public -License" and "Boycott", and this permission notice, may be included in +License," "Funding for Free Software," and "Protect Your Freedom--Fight +`Look And Feel'", and this permission notice, may be included in translations approved by the Free Software Foundation instead of in the original English.  -File: gcc.info, Node: Register Classes, Next: Stack and Calling, Prev: Registers, Up: Target Macros - -Register Classes -================ - - On many machines, the numbered registers are not all equivalent. For -example, certain registers may not be allowed for indexed addressing; -certain registers may not be allowed in some instructions. These -machine restrictions are described to the compiler using "register -classes". - - You define a number of register classes, giving each one a name and -saying which of the registers belong to it. Then you can specify -register classes that are allowed as operands to particular instruction -patterns. - - In general, each register will belong to several classes. In fact, -one class must be named `ALL_REGS' and contain all the registers. -Another class must be named `NO_REGS' and contain no registers. Often -the union of two classes will be another class; however, this is not -required. - - One of the classes must be named `GENERAL_REGS'. There is nothing -terribly special about the name, but the operand constraint letters `r' -and `g' specify this class. If `GENERAL_REGS' is the same as -`ALL_REGS', just define it as a macro which expands to `ALL_REGS'. - - Order the classes so that if class X is contained in class Y then X -has a lower class number than Y. - - The way classes other than `GENERAL_REGS' are specified in operand -constraints is through machine-dependent operand constraint letters. -You can define such letters to correspond to various classes, then use -them in operand constraints. - - You should define a class for the union of two classes whenever some -instruction allows both classes. For example, if an instruction allows -either a floating point (coprocessor) register or a general register -for a certain operand, you should define a class `FLOAT_OR_GENERAL_REGS' -which includes both of them. Otherwise you will get suboptimal code. - - You must also specify certain redundant information about the -register classes: for each class, which classes contain it and which -ones are contained in it; for each pair of classes, the largest class -contained in their union. - - When a value occupying several consecutive registers is expected in a -certain class, all the registers used must belong to that class. -Therefore, register classes cannot be used to enforce a requirement for -a register pair to start with an even-numbered register. The way to -specify this requirement is with `HARD_REGNO_MODE_OK'. - - Register classes used for input-operands of bitwise-and or shift -instructions have a special requirement: each such class must have, for -each fixed-point machine mode, a subclass whose registers can transfer -that mode to or from memory. For example, on some machines, the -operations for single-byte values (`QImode') are limited to certain -registers. When this is so, each register class that is used in a -bitwise-and or shift instruction must have a subclass consisting of -registers from which single-byte values can be loaded or stored. This -is so that `PREFERRED_RELOAD_CLASS' can always have a possible value to -return. - -`enum reg_class' - An enumeral type that must be defined with all the register class - names as enumeral values. `NO_REGS' must be first. `ALL_REGS' - must be the last register class, followed by one more enumeral - value, `LIM_REG_CLASSES', which is not a register class but rather - tells how many classes there are. - - Each register class has a number, which is the value of casting - the class name to type `int'. The number serves as an index in - many of the tables described below. - -`N_REG_CLASSES' - The number of distinct register classes, defined as follows: - - #define N_REG_CLASSES (int) LIM_REG_CLASSES - -`REG_CLASS_NAMES' - An initializer containing the names of the register classes as C - string constants. These names are used in writing some of the - debugging dumps. - -`REG_CLASS_CONTENTS' - An initializer containing the contents of the register classes, as - integers which are bit masks. The Nth integer specifies the - contents of class N. The way the integer MASK is interpreted is - that register R is in the class if `MASK & (1 << R)' is 1. - - When the machine has more than 32 registers, an integer does not - suffice. Then the integers are replaced by sub-initializers, - braced groupings containing several integers. Each - sub-initializer must be suitable as an initializer for the type - `HARD_REG_SET' which is defined in `hard-reg-set.h'. - -`REGNO_REG_CLASS (REGNO)' - A C expression whose value is a register class containing hard - register REGNO. In general there is more that one such class; - choose a class which is "minimal", meaning that no smaller class - also contains the register. - -`BASE_REG_CLASS' - A macro whose definition is the name of the class to which a valid - base register must belong. A base register is one used in an - address which is the register value plus a displacement. - -`INDEX_REG_CLASS' - A macro whose definition is the name of the class to which a valid - index register must belong. An index register is one used in an - address where its value is either multiplied by a scale factor or - added to another register (as well as added to a displacement). - -`REG_CLASS_FROM_LETTER (CHAR)' - A C expression which defines the machine-dependent operand - constraint letters for register classes. If CHAR is such a - letter, the value should be the register class corresponding to - it. Otherwise, the value should be `NO_REGS'. The register - letter `r', corresponding to class `GENERAL_REGS', will not be - passed to this macro; you do not need to handle it. - -`REGNO_OK_FOR_BASE_P (NUM)' - A C expression which is nonzero if register number NUM is suitable - for use as a base register in operand addresses. It may be either - a suitable hard register or a pseudo register that has been - allocated such a hard register. - -`REGNO_OK_FOR_INDEX_P (NUM)' - A C expression which is nonzero if register number NUM is suitable - for use as an index register in operand addresses. It may be - either a suitable hard register or a pseudo register that has been - allocated such a hard register. - - The difference between an index register and a base register is - that the index register may be scaled. If an address involves the - sum of two registers, neither one of them scaled, then either one - may be labeled the "base" and the other the "index"; but whichever - labeling is used must fit the machine's constraints of which - registers may serve in each capacity. The compiler will try both - labelings, looking for one that is valid, and will reload one or - both registers only if neither labeling works. - -`PREFERRED_RELOAD_CLASS (X, CLASS)' - A C expression that places additional restrictions on the register - class to use when it is necessary to copy value X into a register - in class CLASS. The value is a register class; perhaps CLASS, or - perhaps another, smaller class. On many machines, the definition - - #define PREFERRED_RELOAD_CLASS(X,CLASS) CLASS - - is safe. - - Sometimes returning a more restrictive class makes better code. - For example, on the 68000, when X is an integer constant that is - in range for a `moveq' instruction, the value of this macro is - always `DATA_REGS' as long as CLASS includes the data registers. - Requiring a data register guarantees that a `moveq' will be used. - - If X is a `const_double', by returning `NO_REGS' you can force X - into a memory constant. This is useful on certain machines where - immediate floating values cannot be loaded into certain kinds of - registers. - -`LIMIT_RELOAD_CLASS (MODE, CLASS)' - A C expression that places additional restrictions on the register - class to use when it is necessary to be able to hold a value of - mode MODE in a reload register for which class CLASS would - ordinarily be used. - - Unlike `PREFERRED_RELOAD_CLASS', this macro should be used when - there are certain modes that simply can't go in certain reload - classes. - - The value is a register class; perhaps CLASS, or perhaps another, - smaller class. - - Don't define this macro unless the target machine has limitations - which require the macro to do something nontrivial. - -`SECONDARY_RELOAD_CLASS (CLASS, MODE, X)' -`SECONDARY_INPUT_RELOAD_CLASS (CLASS, MODE, X)' -`SECONDARY_OUTPUT_RELOAD_CLASS (CLASS, MODE, X)' - Many machines have some registers that cannot be copied directly - to or from memory or even from other types of registers. An - example is the `MQ' register, which on most machines, can only be - copied to or from general registers, but not memory. Some - machines allow copying all registers to and from memory, but - require a scratch register for stores to some memory locations - (e.g., those with symbolic address on the RT, and those with - certain symbolic address on the Sparc when compiling PIC). In - some cases, both an intermediate and a scratch register are - required. - - You should define these macros to indicate to the reload phase - that it may need to allocate at least one register for a reload in - addition to the register to contain the data. Specifically, if - copying X to a register CLASS in MODE requires an intermediate - register, you should define `SECONDARY_INPUT_RELOAD_CLASS' to - return the largest register class all of whose registers can be - used as intermediate registers or scratch registers. - - If copying a register CLASS in MODE to X requires an intermediate - or scratch register, you should define - `SECONDARY_OUTPUT_RELOAD_CLASS' to return the largest register - class required. If the requirements for input and output reloads - are the same, the macro `SECONDARY_RELOAD_CLASS' should be used - instead of defining both macros identically. - - The values returned by these macros are often `GENERAL_REGS'. - Return `NO_REGS' if no spare register is needed; i.e., if X can be - directly copied to or from a register of CLASS in MODE without - requiring a scratch register. Do not define this macro if it - would always return `NO_REGS'. - - If a scratch register is required (either with or without an - intermediate register), you should define patterns for - `reload_inM' or `reload_outM', as required (*note Standard - Names::.. These patterns, which will normally be implemented with - a `define_expand', should be similar to the `movM' patterns, - except that operand 2 is the scratch register. - - Define constraints for the reload register and scratch register - that contain a single register class. If the original reload - register (whose class is CLASS) can meet the constraint given in - the pattern, the value returned by these macros is used for the - class of the scratch register. Otherwise, two additional reload - registers are required. Their classes are obtained from the - constraints in the insn pattern. - - X might be a pseudo-register or a `subreg' of a pseudo-register, - which could either be in a hard register or in memory. Use - `true_regnum' to find out; it will return -1 if the pseudo is in - memory and the hard register number if it is in a register. - - These macros should not be used in the case where a particular - class of registers can only be copied to memory and not to another - class of registers. In that case, secondary reload registers are - not needed and would not be helpful. Instead, a stack location - must be used to perform the copy and the `movM' pattern should use - memory as a intermediate storage. This case often occurs between - floating-point and general registers. - -`SMALL_REGISTER_CLASSES' - Normally the compiler will avoid choosing spill registers from - registers that have been explicitly mentioned in the rtl (these - registers are normally those used to pass parameters and return - values). However, some machines have so few registers of certain - classes that there would not be enough registers to use as spill - registers if this were done. - - On those machines, you should define `SMALL_REGISTER_CLASSES'. - When it is defined, the compiler allows registers explicitly used - in the rtl to be used as spill registers but prevents the compiler - from extending the lifetime of these registers. - - Defining this macro is always safe, but unnecessarily defining - this macro will reduce the amount of optimizations that can be - performed in some cases. If this macro is not defined but needs - to be, the compiler will run out of reload registers and print a - fatal error message. - - For most machines, this macro should not be defined. - -`CLASS_MAX_NREGS (CLASS, MODE)' - A C expression for the maximum number of consecutive registers of - class CLASS needed to hold a value of mode MODE. - - This is closely related to the macro `HARD_REGNO_NREGS'. In fact, - the value of the macro `CLASS_MAX_NREGS (CLASS, MODE)' should be - the maximum value of `HARD_REGNO_NREGS (REGNO, MODE)' for all - REGNO values in the class CLASS. - - This macro helps control the handling of multiple-word values in - the reload pass. +File: gcc.info, Node: Machine Modes, Next: Constants, Prev: Flags, Up: RTL - Three other special macros describe which operands fit which -constraint letters. +Machine Modes +============= -`CONST_OK_FOR_LETTER_P (VALUE, C)' - A C expression that defines the machine-dependent operand - constraint letters that specify particular ranges of integer - values. If C is one of those letters, the expression should check - that VALUE, an integer, is in the appropriate range and return 1 - if so, 0 otherwise. If C is not one of those letters, the value - should be 0 regardless of VALUE. - -`CONST_DOUBLE_OK_FOR_LETTER_P (VALUE, C)' - A C expression that defines the machine-dependent operand - constraint letters that specify particular ranges of - `const_double' values. - - If C is one of those letters, the expression should check that - VALUE, an RTX of code `const_double', is in the appropriate range - and return 1 if so, 0 otherwise. If C is not one of those - letters, the value should be 0 regardless of VALUE. - - `const_double' is used for all floating-point constants and for - `DImode' fixed-point constants. A given letter can accept either - or both kinds of values. It can use `GET_MODE' to distinguish - between these kinds. - -`EXTRA_CONSTRAINT (VALUE, C)' - A C expression that defines the optional machine-dependent - constraint letters that can be used to segregate specific types of - operands, usually memory references, for the target machine. - Normally this macro will not be defined. If it is required for a - particular target machine, it should return 1 if VALUE corresponds - to the operand type represented by the constraint letter C. If C - is not defined as an extra constraint, the value returned should - be 0 regardless of VALUE. - - For example, on the ROMP, load instructions cannot have their - output in r0 if the memory reference contains a symbolic address. - Constraint letter `Q' is defined as representing a memory address - that does *not* contain a symbolic address. An alternative is - specified with a `Q' constraint on the input and `r' on the - output. The next alternative specifies `m' on the input and a - register class that does not include r0 on the output. + A machine mode describes a size of data object and the +representation used for it. In the C code, machine modes are +represented by an enumeration type, `enum machine_mode', defined in +`machmode.def'. Each RTL expression has room for a machine mode and so +do certain kinds of tree expressions (declarations and types, to be +precise). + + In debugging dumps and machine descriptions, the machine mode of an +RTL expression is written after the expression code with a colon to +separate them. The letters `mode' which appear at the end of each +machine mode name are omitted. For example, `(reg:SI 38)' is a `reg' +expression with machine mode `SImode'. If the mode is `VOIDmode', it +is not written at all. + + Here is a table of machine modes. The term "byte" below refers to an +object of `BITS_PER_UNIT' bits (*note Storage Layout::.). + +`QImode' + "Quarter-Integer" mode represents a single byte treated as an + integer. + +`HImode' + "Half-Integer" mode represents a two-byte integer. + +`PSImode' + "Partial Single Integer" mode represents an integer which occupies + four bytes but which doesn't really use all four. On some + machines, this is the right mode to use for pointers. + +`SImode' + "Single Integer" mode represents a four-byte integer. + +`PDImode' + "Partial Double Integer" mode represents an integer which occupies + eight bytes but which doesn't really use all eight. On some + machines, this is the right mode to use for certain pointers. + +`DImode' + "Double Integer" mode represents an eight-byte integer. + +`TImode' + "Tetra Integer" (?) mode represents a sixteen-byte integer. + +`SFmode' + "Single Floating" mode represents a single-precision (four byte) + floating point number. + +`DFmode' + "Double Floating" mode represents a double-precision (eight byte) + floating point number. + +`XFmode' + "Extended Floating" mode represents a triple-precision (twelve + byte) floating point number. This mode is used for IEEE extended + floating point. On some systems not all bits within these bytes + will actually be used. + +`TFmode' + "Tetra Floating" mode represents a quadruple-precision (sixteen + byte) floating point number. + +`CCmode' + "Condition Code" mode represents the value of a condition code, + which is a machine-specific set of bits used to represent the + result of a comparison operation. Other machine-specific modes + may also be used for the condition code. These modes are not used + on machines that use `cc0' (see *note Condition Code::.). + +`BLKmode' + "Block" mode represents values that are aggregates to which none of + the other modes apply. In RTL, only memory references can have + this mode, and only if they appear in string-move or vector + instructions. On machines which have no such instructions, + `BLKmode' will not appear in RTL. + +`VOIDmode' + Void mode means the absence of a mode or an unspecified mode. For + example, RTL expressions of code `const_int' have mode `VOIDmode' + because they can be taken to have whatever mode the context + requires. In debugging dumps of RTL, `VOIDmode' is expressed by + the absence of any mode. + +`SCmode, DCmode, XCmode, TCmode' + These modes stand for a complex number represented as a pair of + floating point values. The floating point values are in `SFmode', + `DFmode', `XFmode', and `TFmode', respectively. + +`CQImode, CHImode, CSImode, CDImode, CTImode, COImode' + These modes stand for a complex number represented as a pair of + integer values. The integer values are in `QImode', `HImode', + `SImode', `DImode', `TImode', and `OImode', respectively. + + The machine description defines `Pmode' as a C macro which expands +into the machine mode used for addresses. Normally this is the mode +whose size is `BITS_PER_WORD', `SImode' on 32-bit machines. + + The only modes which a machine description must support are +`QImode', and the modes corresponding to `BITS_PER_WORD', +`FLOAT_TYPE_SIZE' and `DOUBLE_TYPE_SIZE'. The compiler will attempt to +use `DImode' for 8-byte structures and unions, but this can be +prevented by overriding the definition of `MAX_FIXED_MODE_SIZE'. +Alternatively, you can have the compiler use `TImode' for 16-byte +structures and unions. Likewise, you can arrange for the C type `short +int' to avoid using `HImode'. + + Very few explicit references to machine modes remain in the compiler +and these few references will soon be removed. Instead, the machine +modes are divided into mode classes. These are represented by the +enumeration type `enum mode_class' defined in `machmode.h'. The +possible mode classes are: + +`MODE_INT' + Integer modes. By default these are `QImode', `HImode', `SImode', + `DImode', and `TImode'. + +`MODE_PARTIAL_INT' + The "partial integer" modes, `PSImode' and `PDImode'. + +`MODE_FLOAT' + floating point modes. By default these are `SFmode', `DFmode', + `XFmode' and `TFmode'. + +`MODE_COMPLEX_INT' + Complex integer modes. (These are not currently implemented). + +`MODE_COMPLEX_FLOAT' + Complex floating point modes. By default these are `SCmode', + `DCmode', `XCmode', and `TCmode'. + +`MODE_FUNCTION' + Algol or Pascal function variables including a static chain. + (These are not currently implemented). + +`MODE_CC' + Modes representing condition code values. These are `CCmode' plus + any modes listed in the `EXTRA_CC_MODES' macro. *Note Jump + Patterns::, also see *Note Condition Code::. + +`MODE_RANDOM' + This is a catchall mode class for modes which don't fit into the + above classes. Currently `VOIDmode' and `BLKmode' are in + `MODE_RANDOM'. + + Here are some C macros that relate to machine modes: + +`GET_MODE (X)' + Returns the machine mode of the RTX X. + +`PUT_MODE (X, NEWMODE)' + Alters the machine mode of the RTX X to be NEWMODE. + +`NUM_MACHINE_MODES' + Stands for the number of machine modes available on the target + machine. This is one greater than the largest numeric value of any + machine mode. + +`GET_MODE_NAME (M)' + Returns the name of mode M as a string. + +`GET_MODE_CLASS (M)' + Returns the mode class of mode M. + +`GET_MODE_WIDER_MODE (M)' + Returns the next wider natural mode. For example, the expression + `GET_MODE_WIDER_MODE (QImode)' returns `HImode'. + +`GET_MODE_SIZE (M)' + Returns the size in bytes of a datum of mode M. + +`GET_MODE_BITSIZE (M)' + Returns the size in bits of a datum of mode M. + +`GET_MODE_MASK (M)' + Returns a bitmask containing 1 for all bits in a word that fit + within mode M. This macro can only be used for modes whose + bitsize is less than or equal to `HOST_BITS_PER_INT'. + +`GET_MODE_ALIGNMENT (M))' + Return the required alignment, in bits, for an object of mode M. + +`GET_MODE_UNIT_SIZE (M)' + Returns the size in bytes of the subunits of a datum of mode M. + This is the same as `GET_MODE_SIZE' except in the case of complex + modes. For them, the unit size is the size of the real or + imaginary part. + +`GET_MODE_NUNITS (M)' + Returns the number of units contained in a mode, i.e., + `GET_MODE_SIZE' divided by `GET_MODE_UNIT_SIZE'. + +`GET_CLASS_NARROWEST_MODE (C)' + Returns the narrowest mode in mode class C. + + The global variables `byte_mode' and `word_mode' contain modes whose +classes are `MODE_INT' and whose bitsizes are either `BITS_PER_UNIT' or +`BITS_PER_WORD', respectively. On 32-bit machines, these are `QImode' +and `SImode', respectively.  -File: gcc.info, Node: Stack and Calling, Next: Varargs, Prev: Register Classes, Up: Target Macros +File: gcc.info, Node: Constants, Next: Regs and Memory, Prev: Machine Modes, Up: RTL -Describing Stack Layout and Calling Conventions -=============================================== +Constant Expression Types +========================= -* Menu: + The simplest RTL expressions are those that represent constant +values. -* Frame Layout:: -* Frame Registers:: -* Elimination:: -* Stack Arguments:: -* Register Arguments:: -* Scalar Return:: -* Aggregate Return:: -* Caller Saves:: -* Function Entry:: -* Profiling:: +`(const_int I)' + This type of expression represents the integer value I. I is + customarily accessed with the macro `INTVAL' as in `INTVAL (EXP)', + which is equivalent to `XWINT (EXP, 0)'. + + There is only one expression object for the integer value zero; it + is the value of the variable `const0_rtx'. Likewise, the only + expression for integer value one is found in `const1_rtx', the only + expression for integer value two is found in `const2_rtx', and the + only expression for integer value negative one is found in + `constm1_rtx'. Any attempt to create an expression of code + `const_int' and value zero, one, two or negative one will return + `const0_rtx', `const1_rtx', `const2_rtx' or `constm1_rtx' as + appropriate. + + Similarly, there is only one object for the integer whose value is + `STORE_FLAG_VALUE'. It is found in `const_true_rtx'. If + `STORE_FLAG_VALUE' is one, `const_true_rtx' and `const1_rtx' will + point to the same object. If `STORE_FLAG_VALUE' is -1, + `const_true_rtx' and `constm1_rtx' will point to the same object. + +`(const_double:M ADDR I0 I1 ...)' + Represents either a floating-point constant of mode M or an + integer constant too large to fit into `HOST_BITS_PER_WIDE_INT' + bits but small enough to fit within twice that number of bits (GNU + CC does not provide a mechanism to represent even larger + constants). In the latter case, M will be `VOIDmode'. + + ADDR is used to contain the `mem' expression that corresponds to + the location in memory that at which the constant can be found. If + it has not been allocated a memory location, but is on the chain + of all `const_double' expressions in this compilation (maintained + using an undisplayed field), ADDR contains `const0_rtx'. If it is + not on the chain, ADDR contains `cc0_rtx'. ADDR is customarily + accessed with the macro `CONST_DOUBLE_MEM' and the chain field via + `CONST_DOUBLE_CHAIN'. + + If M is `VOIDmode', the bits of the value are stored in I0 and I1. + I0 is customarily accessed with the macro `CONST_DOUBLE_LOW' and + I1 with `CONST_DOUBLE_HIGH'. + + If the constant is floating point (regardless of its precision), + then the number of integers used to store the value depends on the + size of `REAL_VALUE_TYPE' (*note Cross-compilation::.). The + integers represent a floating point number, but not precisely in + the target machine's or host machine's floating point format. To + convert them to the precise bit pattern used by the target + machine, use the macro `REAL_VALUE_TO_TARGET_DOUBLE' and friends + (*note Data Output::.). + + The macro `CONST0_RTX (MODE)' refers to an expression with value 0 + in mode MODE. If mode MODE is of mode class `MODE_INT', it + returns `const0_rtx'. Otherwise, it returns a `CONST_DOUBLE' + expression in mode MODE. Similarly, the macro `CONST1_RTX (MODE)' + refers to an expression with value 1 in mode MODE and similarly + for `CONST2_RTX'. + +`(const_string STR)' + Represents a constant string with value STR. Currently this is + used only for insn attributes (*note Insn Attributes::.) since + constant strings in C are placed in memory. + +`(symbol_ref:MODE SYMBOL)' + Represents the value of an assembler label for data. SYMBOL is a + string that describes the name of the assembler label. If it + starts with a `*', the label is the rest of SYMBOL not including + the `*'. Otherwise, the label is SYMBOL, usually prefixed with + `_'. + + The `symbol_ref' contains a mode, which is usually `Pmode'. + Usually that is the only mode for which a symbol is directly valid. + +`(label_ref LABEL)' + Represents the value of an assembler label for code. It contains + one operand, an expression, which must be a `code_label' that + appears in the instruction sequence to identify the place where + the label should go. + + The reason for using a distinct expression type for code label + references is so that jump optimization can distinguish them. + +`(const:M EXP)' + Represents a constant that is the result of an assembly-time + arithmetic computation. The operand, EXP, is an expression that + contains only constants (`const_int', `symbol_ref' and `label_ref' + expressions) combined with `plus' and `minus'. However, not all + combinations are valid, since the assembler cannot do arbitrary + arithmetic on relocatable symbols. + + M should be `Pmode'. + +`(high:M EXP)' + Represents the high-order bits of EXP, usually a `symbol_ref'. + The number of bits is machine-dependent and is normally the number + of bits specified in an instruction that initializes the high + order bits of a register. It is used with `lo_sum' to represent + the typical two-instruction sequence used in RISC machines to + reference a global memory location. - -File: gcc.info, Node: Frame Layout, Next: Frame Registers, Up: Stack and Calling - -Basic Stack Layout ------------------- - -`STACK_GROWS_DOWNWARD' - Define this macro if pushing a word onto the stack moves the stack - pointer to a smaller address. - - When we say, "define this macro if ...," it means that the - compiler checks this macro only with `#ifdef' so the precise - definition used does not matter. - -`FRAME_GROWS_DOWNWARD' - Define this macro if the addresses of local variable slots are at - negative offsets from the frame pointer. - -`ARGS_GROW_DOWNWARD' - Define this macro if successive arguments to a function occupy - decreasing addresses on the stack. - -`STARTING_FRAME_OFFSET' - Offset from the frame pointer to the first local variable slot to - be allocated. - - If `FRAME_GROWS_DOWNWARD', the next slot's offset is found by - subtracting the length of the first slot from - `STARTING_FRAME_OFFSET'. Otherwise, it is found by adding the - length of the first slot to the value `STARTING_FRAME_OFFSET'. - -`STACK_POINTER_OFFSET' - Offset from the stack pointer register to the first location at - which outgoing arguments are placed. If not specified, the - default value of zero is used. This is the proper value for most - machines. - - If `ARGS_GROW_DOWNWARD', this is the offset to the location above - the first location at which outgoing arguments are placed. - -`FIRST_PARM_OFFSET (FUNDECL)' - Offset from the argument pointer register to the first argument's - address. On some machines it may depend on the data type of the - function. - - If `ARGS_GROW_DOWNWARD', this is the offset to the location above - the first argument's address. - -`STACK_DYNAMIC_OFFSET (FUNDECL)' - Offset from the stack pointer register to an item dynamically - allocated on the stack, e.g., by `alloca'. - - The default value for this macro is `STACK_POINTER_OFFSET' plus the - length of the outgoing arguments. The default is correct for most - machines. See `function.c' for details. - -`DYNAMIC_CHAIN_ADDRESS (FRAMEADDR)' - A C expression whose value is RTL representing the address in a - stack frame where the pointer to the caller's frame is stored. - Assume that FRAMEADDR is an RTL expression for the address of the - stack frame itself. - - If you don't define this macro, the default is to return the value - of FRAMEADDR--that is, the stack frame address is also the address - of the stack word that points to the previous frame. + M should be `Pmode'.  -File: gcc.info, Node: Frame Registers, Next: Elimination, Prev: Frame Layout, Up: Stack and Calling +File: gcc.info, Node: Regs and Memory, Next: Arithmetic, Prev: Constants, Up: RTL -Registers That Address the Stack Frame --------------------------------------- +Registers and Memory +==================== -`STACK_POINTER_REGNUM' - The register number of the stack pointer register, which must also - be a fixed register according to `FIXED_REGISTERS'. On most - machines, the hardware determines which register this is. - -`FRAME_POINTER_REGNUM' - The register number of the frame pointer register, which is used to - access automatic variables in the stack frame. On some machines, - the hardware determines which register this is. On other - machines, you can choose any register you wish for this purpose. - -`ARG_POINTER_REGNUM' - The register number of the arg pointer register, which is used to - access the function's argument list. On some machines, this is - the same as the frame pointer register. On some machines, the - hardware determines which register this is. On other machines, - you can choose any register you wish for this purpose. If this is - not the same register as the frame pointer register, then you must - mark it as a fixed register according to `FIXED_REGISTERS', or - arrange to be able to eliminate it (*note Elimination::.). - -`STATIC_CHAIN_REGNUM' -`STATIC_CHAIN_INCOMING_REGNUM' - Register numbers used for passing a function's static chain - pointer. If register windows are used, - `STATIC_CHAIN_INCOMING_REGNUM' is the register number as seen by - the called function, while `STATIC_CHAIN_REGNUM' is the register - number as seen by the calling function. If these registers are - the same, `STATIC_CHAIN_INCOMING_REGNUM' need not be defined. - - The static chain register need not be a fixed register. - - If the static chain is passed in memory, these macros should not be - defined; instead, the next two macros should be defined. - -`STATIC_CHAIN' -`STATIC_CHAIN_INCOMING' - If the static chain is passed in memory, these macros provide rtx - giving `mem' expressions that denote where they are stored. - `STATIC_CHAIN' and `STATIC_CHAIN_INCOMING' give the locations as - seen by the calling and called functions, respectively. Often the - former will be at an offset from the stack pointer and the latter - at an offset from the frame pointer. - - The variables `stack_pointer_rtx', `frame_pointer_rtx', and - `arg_pointer_rtx' will have been initialized prior to the use of - these macros and should be used to refer to those items. + Here are the RTL expression types for describing access to machine +registers and to main memory. - If the static chain is passed in a register, the two previous - macros should be defined instead. +`(reg:M N)' + For small values of the integer N (those that are less than + `FIRST_PSEUDO_REGISTER'), this stands for a reference to machine + register number N: a "hard register". For larger values of N, it + stands for a temporary value or "pseudo register". The compiler's + strategy is to generate code assuming an unlimited number of such + pseudo registers, and later convert them into hard registers or + into memory references. + + M is the machine mode of the reference. It is necessary because + machines can generally refer to each register in more than one + mode. For example, a register may contain a full word but there + may be instructions to refer to it as a half word or as a single + byte, as well as instructions to refer to it as a floating point + number of various precisions. + + Even for a register that the machine can access in only one mode, + the mode must always be specified. + + The symbol `FIRST_PSEUDO_REGISTER' is defined by the machine + description, since the number of hard registers on the machine is + an invariant characteristic of the machine. Note, however, that + not all of the machine registers must be general registers. All + the machine registers that can be used for storage of data are + given hard register numbers, even those that can be used only in + certain instructions or can hold only certain types of data. + + A hard register may be accessed in various modes throughout one + function, but each pseudo register is given a natural mode and is + accessed only in that mode. When it is necessary to describe an + access to a pseudo register using a nonnatural mode, a `subreg' + expression is used. + + A `reg' expression with a machine mode that specifies more than + one word of data may actually stand for several consecutive + registers. If in addition the register number specifies a + hardware register, then it actually represents several consecutive + hardware registers starting with the specified one. + + Each pseudo register number used in a function's RTL code is + represented by a unique `reg' expression. + + Some pseudo register numbers, those within the range of + `FIRST_VIRTUAL_REGISTER' to `LAST_VIRTUAL_REGISTER' only appear + during the RTL generation phase and are eliminated before the + optimization phases. These represent locations in the stack frame + that cannot be determined until RTL generation for the function + has been completed. The following virtual register numbers are + defined: + + `VIRTUAL_INCOMING_ARGS_REGNUM' + This points to the first word of the incoming arguments + passed on the stack. Normally these arguments are placed + there by the caller, but the callee may have pushed some + arguments that were previously passed in registers. + + When RTL generation is complete, this virtual register is + replaced by the sum of the register given by + `ARG_POINTER_REGNUM' and the value of `FIRST_PARM_OFFSET'. + + `VIRTUAL_STACK_VARS_REGNUM' + If `FRAME_GROWS_DOWNWARD' is defined, this points to + immediately above the first variable on the stack. + Otherwise, it points to the first variable on the stack. + + `VIRTUAL_STACK_VARS_REGNUM' is replaced with the sum of the + register given by `FRAME_POINTER_REGNUM' and the value + `STARTING_FRAME_OFFSET'. + + `VIRTUAL_STACK_DYNAMIC_REGNUM' + This points to the location of dynamically allocated memory + on the stack immediately after the stack pointer has been + adjusted by the amount of memory desired. + + This virtual register is replaced by the sum of the register + given by `STACK_POINTER_REGNUM' and the value + `STACK_DYNAMIC_OFFSET'. + + `VIRTUAL_OUTGOING_ARGS_REGNUM' + This points to the location in the stack at which outgoing + arguments should be written when the stack is pre-pushed + (arguments pushed using push insns should always use + `STACK_POINTER_REGNUM'). + + This virtual register is replaced by the sum of the register + given by `STACK_POINTER_REGNUM' and the value + `STACK_POINTER_OFFSET'. + +`(subreg:M REG WORDNUM)' + `subreg' expressions are used to refer to a register in a machine + mode other than its natural one, or to refer to one register of a + multi-word `reg' that actually refers to several registers. + + Each pseudo-register has a natural mode. If it is necessary to + operate on it in a different mode--for example, to perform a + fullword move instruction on a pseudo-register that contains a + single byte--the pseudo-register must be enclosed in a `subreg'. + In such a case, WORDNUM is zero. + + Usually M is at least as narrow as the mode of REG, in which case + it is restricting consideration to only the bits of REG that are + in M. + + Sometimes M is wider than the mode of REG. These `subreg' + expressions are often called "paradoxical". They are used in + cases where we want to refer to an object in a wider mode but do + not care what value the additional bits have. The reload pass + ensures that paradoxical references are only made to hard + registers. - -File: gcc.info, Node: Elimination, Next: Stack Arguments, Prev: Frame Registers, Up: Stack and Calling + The other use of `subreg' is to extract the individual registers of + a multi-register value. Machine modes such as `DImode' and + `TImode' can indicate values longer than a word, values which + usually require two or more consecutive registers. To access one + of the registers, use a `subreg' with mode `SImode' and a WORDNUM + that says which register. + + Storing in a non-paradoxical `subreg' has undefined results for + bits belonging to the same word as the `subreg'. This laxity makes + it easier to generate efficient code for such instructions. To + represent an instruction that preserves all the bits outside of + those in the `subreg', use `strict_low_part' around the `subreg'. + + The compilation parameter `WORDS_BIG_ENDIAN', if set to 1, says + that word number zero is the most significant part; otherwise, it + is the least significant part. + + Between the combiner pass and the reload pass, it is possible to + have a paradoxical `subreg' which contains a `mem' instead of a + `reg' as its first operand. After the reload pass, it is also + possible to have a non-paradoxical `subreg' which contains a + `mem'; this usually occurs when the `mem' is a stack slot which + replaced a pseudo register. + + Note that it is not valid to access a `DFmode' value in `SFmode' + using a `subreg'. On some machines the most significant part of a + `DFmode' value does not have the same format as a single-precision + floating value. + + It is also not valid to access a single word of a multi-word value + in a hard register when less registers can hold the value than + would be expected from its size. For example, some 32-bit + machines have floating-point registers that can hold an entire + `DFmode' value. If register 10 were such a register `(subreg:SI + (reg:DF 10) 1)' would be invalid because there is no way to + convert that reference to a single machine register. The reload + pass prevents `subreg' expressions such as these from being formed. + + The first operand of a `subreg' expression is customarily accessed + with the `SUBREG_REG' macro and the second operand is customarily + accessed with the `SUBREG_WORD' macro. + +`(scratch:M)' + This represents a scratch register that will be required for the + execution of a single instruction and not used subsequently. It is + converted into a `reg' by either the local register allocator or + the reload pass. -Eliminating Frame Pointer and Arg Pointer ------------------------------------------ + `scratch' is usually present inside a `clobber' operation (*note + Side Effects::.). -`FRAME_POINTER_REQUIRED' - A C expression which is nonzero if a function must have and use a - frame pointer. This expression is evaluated in the reload pass. - If its value is nonzero the function will have a frame pointer. - - The expression can in principle examine the current function and - decide according to the facts, but on most machines the constant 0 - or the constant 1 suffices. Use 0 when the machine allows code to - be generated with no frame pointer, and doing so saves some time - or space. Use 1 when there is no possible advantage to avoiding a - frame pointer. - - In certain cases, the compiler does not know how to produce valid - code without a frame pointer. The compiler recognizes those cases - and automatically gives the function a frame pointer regardless of - what `FRAME_POINTER_REQUIRED' says. You don't need to worry about - them. - - In a function that does not require a frame pointer, the frame - pointer register can be allocated for ordinary usage, unless you - mark it as a fixed register. See `FIXED_REGISTERS' for more - information. - - This macro is ignored and need not be defined if `ELIMINABLE_REGS' - is defined. - -`INITIAL_FRAME_POINTER_OFFSET (DEPTH-VAR)' - A C statement to store in the variable DEPTH-VAR the difference - between the frame pointer and the stack pointer values immediately - after the function prologue. The value would be computed from - information such as the result of `get_frame_size ()' and the - tables of registers `regs_ever_live' and `call_used_regs'. - - If `ELIMINABLE_REGS' is defined, this macro will be not be used and - need not be defined. Otherwise, it must be defined even if - `FRAME_POINTER_REQUIRED' is defined to always be true; in that - case, you may set DEPTH-VAR to anything. - -`ELIMINABLE_REGS' - If defined, this macro specifies a table of register pairs used to - eliminate unneeded registers that point into the stack frame. If - it is not defined, the only elimination attempted by the compiler - is to replace references to the frame pointer with references to - the stack pointer. - - The definition of this macro is a list of structure - initializations, each of which specifies an original and - replacement register. - - On some machines, the position of the argument pointer is not - known until the compilation is completed. In such a case, a - separate hard register must be used for the argument pointer. - This register can be eliminated by replacing it with either the - frame pointer or the argument pointer, depending on whether or not - the frame pointer has been eliminated. - - In this case, you might specify: - #define ELIMINABLE_REGS \ - {{ARG_POINTER_REGNUM, STACK_POINTER_REGNUM}, \ - {ARG_POINTER_REGNUM, FRAME_POINTER_REGNUM}, \ - {FRAME_POINTER_REGNUM, STACK_POINTER_REGNUM}} - - Note that the elimination of the argument pointer with the stack - pointer is specified first since that is the preferred elimination. - -`CAN_ELIMINATE (FROM-REG, TO-REG)' - A C expression that returns non-zero if the compiler is allowed to - try to replace register number FROM-REG with register number - TO-REG. This macro need only be defined if `ELIMINABLE_REGS' is - defined, and will usually be the constant 1, since most of the - cases preventing register elimination are things that the compiler - already knows about. - -`INITIAL_ELIMINATION_OFFSET (FROM-REG, TO-REG, OFFSET-VAR)' - This macro is similar to `INITIAL_FRAME_POINTER_OFFSET'. It - specifies the initial difference between the specified pair of - registers. This macro must be defined if `ELIMINABLE_REGS' is - defined. - -`LONGJMP_RESTORE_FROM_STACK' - Define this macro if the `longjmp' function restores registers from - the stack frames, rather than from those saved specifically by - `setjmp'. Certain quantities must not be kept in registers across - a call to `setjmp' on such machines. +`(cc0)' + This refers to the machine's condition code register. It has no + operands and may not have a machine mode. There are two ways to + use it: + + * To stand for a complete set of condition code flags. This is + best on most machines, where each comparison sets the entire + series of flags. + + With this technique, `(cc0)' may be validly used in only two + contexts: as the destination of an assignment (in test and + compare instructions) and in comparison operators comparing + against zero (`const_int' with value zero; that is to say, + `const0_rtx'). + + * To stand for a single flag that is the result of a single + condition. This is useful on machines that have only a + single flag bit, and in which comparison instructions must + specify the condition to test. + + With this technique, `(cc0)' may be validly used in only two + contexts: as the destination of an assignment (in test and + compare instructions) where the source is a comparison + operator, and as the first operand of `if_then_else' (in a + conditional branch). + + There is only one expression object of code `cc0'; it is the value + of the variable `cc0_rtx'. Any attempt to create an expression of + code `cc0' will return `cc0_rtx'. + + Instructions can set the condition code implicitly. On many + machines, nearly all instructions set the condition code based on + the value that they compute or store. It is not necessary to + record these actions explicitly in the RTL because the machine + description includes a prescription for recognizing the + instructions that do so (by means of the macro + `NOTICE_UPDATE_CC'). *Note Condition Code::. Only instructions + whose sole purpose is to set the condition code, and instructions + that use the condition code, need mention `(cc0)'. + + On some machines, the condition code register is given a register + number and a `reg' is used instead of `(cc0)'. This is usually the + preferable approach if only a small subset of instructions modify + the condition code. Other machines store condition codes in + general registers; in such cases a pseudo register should be used. + + Some machines, such as the Sparc and RS/6000, have two sets of + arithmetic instructions, one that sets and one that does not set + the condition code. This is best handled by normally generating + the instruction that does not set the condition code, and making a + pattern that both performs the arithmetic and sets the condition + code register (which would not be `(cc0)' in this case). For + examples, search for `addcc' and `andcc' in `sparc.md'. + +`(pc)' + This represents the machine's program counter. It has no operands + and may not have a machine mode. `(pc)' may be validly used only + in certain specific contexts in jump instructions. + + There is only one expression object of code `pc'; it is the value + of the variable `pc_rtx'. Any attempt to create an expression of + code `pc' will return `pc_rtx'. + + All instructions that do not jump alter the program counter + implicitly by incrementing it, but there is no need to mention + this in the RTL. + +`(mem:M ADDR)' + This RTX represents a reference to main memory at an address + represented by the expression ADDR. M specifies how large a unit + of memory is accessed.  -File: gcc.info, Node: Stack Arguments, Next: Register Arguments, Prev: Elimination, Up: Stack and Calling - -Passing Function Arguments on the Stack ---------------------------------------- +File: gcc.info, Node: Arithmetic, Next: Comparisons, Prev: Regs and Memory, Up: RTL - The macros in this section control how arguments are passed on the -stack. See the following section for other macros that control passing -certain arguments in registers. +RTL Expressions for Arithmetic +============================== -`PROMOTE_PROTOTYPES' - Define this macro if an argument declared as `char' or `short' in - a prototype should actually be passed as an `int'. In addition to - avoiding errors in certain cases of mismatch, it also makes for - better code on certain machines. - -`PUSH_ROUNDING (NPUSHED)' - A C expression that is the number of bytes actually pushed onto the - stack when an instruction attempts to push NPUSHED bytes. - - If the target machine does not have a push instruction, do not - define this macro. That directs GNU CC to use an alternate - strategy: to allocate the entire argument block and then store the - arguments into it. - - On some machines, the definition - - #define PUSH_ROUNDING(BYTES) (BYTES) - - will suffice. But on other machines, instructions that appear to - push one byte actually push two bytes in an attempt to maintain - alignment. Then the definition should be - - #define PUSH_ROUNDING(BYTES) (((BYTES) + 1) & ~1) - -`ACCUMULATE_OUTGOING_ARGS' - If defined, the maximum amount of space required for outgoing - arguments will be computed and placed into the variable - `current_function_outgoing_args_size'. No space will be pushed - onto the stack for each call; instead, the function prologue should - increase the stack frame size by this amount. - - It is not proper to define both `PUSH_ROUNDING' and - `ACCUMULATE_OUTGOING_ARGS'. - -`REG_PARM_STACK_SPACE (FNDECL)' - Define this macro if functions should assume that stack space has - been allocated for arguments even when their values are passed in - registers. - - The value of this macro is the size, in bytes, of the area - reserved for ! arguments passed in registers for the function - represented by FNDECL. - - This space can either be allocated by the caller or be a part of - the machine-dependent stack frame: `OUTGOING_REG_PARM_STACK_SPACE' - says which. - -`MAYBE_REG_PARM_STACK_SPACE' -`FINAL_REG_PARM_STACK_SPACE (CONST_SIZE, VAR_SIZE)' - Define these macros in addition to the one above if functions might - allocate stack space for arguments even when their values are - passed in registers. These should be used when the stack space - allocated for arguments in registers is not a simple constant - independent of the function declaration. - - The value of the first macro is the size, in bytes, of the area - that we should initially assume would be reserved for arguments - passed in registers. - - The value of the second macro is the actual size, in bytes, of the - area that will be reserved for arguments passed in registers. - This takes two arguments: an integer representing the number of - bytes of fixed sized arguments on the stack, and a tree - representing the number of bytes of variable sized arguments on - the stack. - - When these macros are defined, `REG_PARM_STACK_SPACE' will only be - called for libcall functions, the current function, or for a - function being called when it is known that such stack space must - be allocated. In each case this value can be easily computed. - - When deciding whether a called function needs such stack space, - and how much space to reserve, GNU CC uses these two macros - instead of `REG_PARM_STACK_SPACE'. - -`OUTGOING_REG_PARM_STACK_SPACE' - Define this if it is the responsibility of the caller to allocate - the area reserved for arguments passed in registers. - - If `ACCUMULATE_OUTGOING_ARGS' is defined, this macro controls - whether the space for these arguments counts in the value of - `current_function_outgoing_args_size'. - -`STACK_PARMS_IN_REG_PARM_AREA' - Define this macro if `REG_PARM_STACK_SPACE' is defined but stack - parameters don't skip the area specified by `REG_PARM_STACK_SPACE'. - - Normally, when a parameter is not passed in registers, it is - placed on the stack beyond the `REG_PARM_STACK_SPACE' area. - Defining this macro suppresses this behavior and causes the - parameter to be passed on the stack in its natural location. - -`RETURN_POPS_ARGS (FUNTYPE, STACK-SIZE)' - A C expression that should indicate the number of bytes of its own - arguments that a function pops on returning, or 0 if the function - pops no arguments and the caller must therefore pop them all after - the function returns. - - FUNTYPE is a C variable whose value is a tree node that describes - the function in question. Normally it is a node of type - `FUNCTION_TYPE' that describes the data type of the function. From - this it is possible to obtain the data types of the value and - arguments (if known). - - When a call to a library function is being considered, FUNTYPE - will contain an identifier node for the library function. Thus, if - you need to distinguish among various library functions, you can - do so by their names. Note that "library function" in this - context means a function used to perform arithmetic, whose name is - known specially in the compiler and was not mentioned in the C - code being compiled. - - STACK-SIZE is the number of bytes of arguments passed on the - stack. If a variable number of bytes is passed, it is zero, and - argument popping will always be the responsibility of the calling - function. - - On the Vax, all functions always pop their arguments, so the - definition of this macro is STACK-SIZE. On the 68000, using the - standard calling convention, no functions pop their arguments, so - the value of the macro is always 0 in this case. But an - alternative calling convention is available in which functions - that take a fixed number of arguments pop them but other functions - (such as `printf') pop nothing (the caller pops all). When this - convention is in use, FUNTYPE is examined to determine whether a - function takes a fixed number of arguments. + Unless otherwise specified, all the operands of arithmetic +expressions must be valid for mode M. An operand is valid for mode M +if it has mode M, or if it is a `const_int' or `const_double' and M is +a mode of class `MODE_INT'. + + For commutative binary operations, constants should be placed in the +second operand. + +`(plus:M X Y)' + Represents the sum of the values represented by X and Y carried + out in machine mode M. + +`(lo_sum:M X Y)' + Like `plus', except that it represents that sum of X and the + low-order bits of Y. The number of low order bits is + machine-dependent but is normally the number of bits in a `Pmode' + item minus the number of bits set by the `high' code (*note + Constants::.). + + M should be `Pmode'. + +`(minus:M X Y)' + Like `plus' but represents subtraction. + +`(compare:M X Y)' + Represents the result of subtracting Y from X for purposes of + comparison. The result is computed without overflow, as if with + infinite precision. + + Of course, machines can't really subtract with infinite precision. + However, they can pretend to do so when only the sign of the + result will be used, which is the case when the result is stored + in the condition code. And that is the only way this kind of + expression may validly be used: as a value to be stored in the + condition codes. + + The mode M is not related to the modes of X and Y, but instead is + the mode of the condition code value. If `(cc0)' is used, it is + `VOIDmode'. Otherwise it is some mode in class `MODE_CC', often + `CCmode'. *Note Condition Code::. + + Normally, X and Y must have the same mode. Otherwise, `compare' + is valid only if the mode of X is in class `MODE_INT' and Y is a + `const_int' or `const_double' with mode `VOIDmode'. The mode of X + determines what mode the comparison is to be done in; thus it must + not be `VOIDmode'. + + If one of the operands is a constant, it should be placed in the + second operand and the comparison code adjusted as appropriate. + + A `compare' specifying two `VOIDmode' constants is not valid since + there is no way to know in what mode the comparison is to be + performed; the comparison must either be folded during the + compilation or the first operand must be loaded into a register + while its mode is still known. + +`(neg:M X)' + Represents the negation (subtraction from zero) of the value + represented by X, carried out in mode M. + +`(mult:M X Y)' + Represents the signed product of the values represented by X and Y + carried out in machine mode M. + + Some machines support a multiplication that generates a product + wider than the operands. Write the pattern for this as + + (mult:M (sign_extend:M X) (sign_extend:M Y)) + + where M is wider than the modes of X and Y, which need not be the + same. + + Write patterns for unsigned widening multiplication similarly using + `zero_extend'. + +`(div:M X Y)' + Represents the quotient in signed division of X by Y, carried out + in machine mode M. If M is a floating point mode, it represents + the exact quotient; otherwise, the integerized quotient. + + Some machines have division instructions in which the operands and + quotient widths are not all the same; you should represent such + instructions using `truncate' and `sign_extend' as in, + + (truncate:M1 (div:M2 X (sign_extend:M2 Y))) + +`(udiv:M X Y)' + Like `div' but represents unsigned division. + +`(mod:M X Y)' +`(umod:M X Y)' + Like `div' and `udiv' but represent the remainder instead of the + quotient. + +`(smin:M X Y)' +`(smax:M X Y)' + Represents the smaller (for `smin') or larger (for `smax') of X + and Y, interpreted as signed integers in mode M. + +`(umin:M X Y)' +`(umax:M X Y)' + Like `smin' and `smax', but the values are interpreted as unsigned + integers. + +`(not:M X)' + Represents the bitwise complement of the value represented by X, + carried out in mode M, which must be a fixed-point machine mode. + +`(and:M X Y)' + Represents the bitwise logical-and of the values represented by X + and Y, carried out in machine mode M, which must be a fixed-point + machine mode. + +`(ior:M X Y)' + Represents the bitwise inclusive-or of the values represented by X + and Y, carried out in machine mode M, which must be a fixed-point + mode. + +`(xor:M X Y)' + Represents the bitwise exclusive-or of the values represented by X + and Y, carried out in machine mode M, which must be a fixed-point + mode. + +`(ashift:M X C)' + Represents the result of arithmetically shifting X left by C + places. X have mode M, a fixed-point machine mode. C be a + fixed-point mode or be a constant with mode `VOIDmode'; which mode + is determined by the mode called for in the machine description + entry for the left-shift instruction. For example, on the Vax, + the mode of C is `QImode' regardless of M. + +`(lshiftrt:M X C)' +`(ashiftrt:M X C)' + Like `ashift' but for right shift. Unlike the case for left shift, + these two operations are distinct. + +`(rotate:M X C)' +`(rotatert:M X C)' + Similar but represent left and right rotate. If C is a constant, + use `rotate'. + +`(abs:M X)' + Represents the absolute value of X, computed in mode M. + +`(sqrt:M X)' + Represents the square root of X, computed in mode M. Most often M + will be a floating point mode. + +`(ffs:M X)' + Represents one plus the index of the least significant 1-bit in X, + represented as an integer of mode M. (The value is zero if X is + zero.) The mode of X need not be M; depending on the target + machine, various mode combinations may be valid.  -File: gcc.info, Node: Register Arguments, Next: Scalar Return, Prev: Stack Arguments, Up: Stack and Calling +File: gcc.info, Node: Comparisons, Next: Bit Fields, Prev: Arithmetic, Up: RTL -Passing Arguments in Registers ------------------------------- +Comparison Operations +===================== - This section describes the macros which let you control how various -types of arguments are passed in registers or how they are arranged in -the stack. - -`FUNCTION_ARG (CUM, MODE, TYPE, NAMED)' - A C expression that controls whether a function argument is passed - in a register, and which register. - - The arguments are CUM, which summarizes all the previous - arguments; MODE, the machine mode of the argument; TYPE, the data - type of the argument as a tree node or 0 if that is not known - (which happens for C support library functions); and NAMED, which - is 1 for an ordinary argument and 0 for nameless arguments that - correspond to `...' in the called function's prototype. - - The value of the expression should either be a `reg' RTX for the - hard register in which to pass the argument, or zero to pass the - argument on the stack. - - For machines like the Vax and 68000, where normally all arguments - are pushed, zero suffices as a definition. - - The usual way to make the ANSI library `stdarg.h' work on a machine - where some arguments are usually passed in registers, is to cause - nameless arguments to be passed on the stack instead. This is done - by making `FUNCTION_ARG' return 0 whenever NAMED is 0. - - You may use the macro `MUST_PASS_IN_STACK (MODE, TYPE)' in the - definition of this macro to determine if this argument is of a - type that must be passed in the stack. If `REG_PARM_STACK_SPACE' - is not defined and `FUNCTION_ARG' returns non-zero for such an - argument, the compiler will abort. If `REG_PARM_STACK_SPACE' is - defined, the argument will be computed in the stack and then - loaded into a register. - -`FUNCTION_INCOMING_ARG (CUM, MODE, TYPE, NAMED)' - Define this macro if the target machine has "register windows", so - that the register in which a function sees an arguments is not - necessarily the same as the one in which the caller passed the - argument. - - For such machines, `FUNCTION_ARG' computes the register in which - the caller passes the value, and `FUNCTION_INCOMING_ARG' should be - defined in a similar fashion to tell the function being called - where the arguments will arrive. - - If `FUNCTION_INCOMING_ARG' is not defined, `FUNCTION_ARG' serves - both purposes. - -`FUNCTION_ARG_PARTIAL_NREGS (CUM, MODE, TYPE, NAMED)' - A C expression for the number of words, at the beginning of an - argument, must be put in registers. The value must be zero for - arguments that are passed entirely in registers or that are - entirely pushed on the stack. - - On some machines, certain arguments must be passed partially in - registers and partially in memory. On these machines, typically - the first N words of arguments are passed in registers, and the - rest on the stack. If a multi-word argument (a `double' or a - structure) crosses that boundary, its first few words must be - passed in registers and the rest must be pushed. This macro tells - the compiler when this occurs, and how many of the words should go - in registers. - - `FUNCTION_ARG' for these arguments should return the first - register to be used by the caller for this argument; likewise - `FUNCTION_INCOMING_ARG', for the called function. - -`FUNCTION_ARG_PASS_BY_REFERENCE (CUM, MODE, TYPE, NAMED)' - A C expression that indicates when an argument must be passed by - reference. If nonzero for an argument, a copy of that argument is - made in memory and a pointer to the argument is passed instead of - the argument itself. The pointer is passed in whatever way is - appropriate for passing a pointer to that type. - - On machines where `REG_PARM_STACK_SPACE' is not defined, a suitable - definition of this macro might be - #define FUNCTION_ARG_PASS_BY_REFERENCE(CUM, MODE, TYPE, NAMED) \ - MUST_PASS_IN_STACK (MODE, TYPE) - -`CUMULATIVE_ARGS' - A C type for declaring a variable that is used as the first - argument of `FUNCTION_ARG' and other related values. For some - target machines, the type `int' suffices and can hold the number - of bytes of argument so far. - - There is no need to record in `CUMULATIVE_ARGS' anything about the - arguments that have been passed on the stack. The compiler has - other variables to keep track of that. For target machines on - which all arguments are passed on the stack, there is no need to - store anything in `CUMULATIVE_ARGS'; however, the data structure - must exist and should not be empty, so use `int'. - -`INIT_CUMULATIVE_ARGS (CUM, FNTYPE, LIBNAME)' - A C statement (sans semicolon) for initializing the variable CUM - for the state at the beginning of the argument list. The variable - has type `CUMULATIVE_ARGS'. The value of FNTYPE is the tree node - for the data type of the function which will receive the args, or 0 - if the args are to a compiler support library function. - - When processing a call to a compiler support library function, - LIBNAME identifies which one. It is a `symbol_ref' rtx which - contains the name of the function, as a string. LIBNAME is 0 when - an ordinary C function call is being processed. Thus, each time - this macro is called, either LIBNAME or FNTYPE is nonzero, but - never both of them at once. - -`INIT_CUMULATIVE_INCOMING_ARGS (CUM, FNTYPE, LIBNAME)' - Like `INIT_CUMULATIVE_ARGS' but overrides it for the purposes of - finding the arguments for the function being compiled. If this - macro is undefined, `INIT_CUMULATIVE_ARGS' is used instead. - - The argument LIBNAME exists for symmetry with - `INIT_CUMULATIVE_ARGS'. The value passed for LIBNAME is always 0, - since library routines with special calling conventions are never - compiled with GNU CC. - -`FUNCTION_ARG_ADVANCE (CUM, MODE, TYPE, NAMED)' - A C statement (sans semicolon) to update the summarizer variable - CUM to advance past an argument in the argument list. The values - MODE, TYPE and NAMED describe that argument. Once this is done, - the variable CUM is suitable for analyzing the *following* - argument with `FUNCTION_ARG', etc. - - This macro need not do anything if the argument in question was - passed on the stack. The compiler knows how to track the amount - of stack space used for arguments without any special help. - -`FUNCTION_ARG_PADDING (MODE, TYPE)' - If defined, a C expression which determines whether, and in which - direction, to pad out an argument with extra space. The value - should be of type `enum direction': either `upward' to pad above - the argument, `downward' to pad below, or `none' to inhibit - padding. - - This macro does not control the *amount* of padding; that is - always just enough to reach the next multiple of - `FUNCTION_ARG_BOUNDARY'. - - This macro has a default definition which is right for most - systems. For little-endian machines, the default is to pad upward. - For big-endian machines, the default is to pad downward for an - argument of constant size shorter than an `int', and upward - otherwise. - -`FUNCTION_ARG_BOUNDARY (MODE, TYPE)' - If defined, a C expression that gives the alignment boundary, in - bits, of an argument with the specified mode and type. If it is - not defined, `PARM_BOUNDARY' is used for all arguments. - -`FUNCTION_ARG_REGNO_P (REGNO)' - A C expression that is nonzero if REGNO is the number of a hard - register in which function arguments are sometimes passed. This - does *not* include implicit arguments such as the static chain and - the structure-value address. On many machines, no registers can be - used for this purpose since all function arguments are pushed on - the stack. + Comparison operators test a relation on two operands and are +considered to represent a machine-dependent nonzero value described by, +but not necessarily equal to, `STORE_FLAG_VALUE' (*note Misc::.) if the +relation holds, or zero if it does not. The mode of the comparison +operation is independent of the mode of the data being compared. If +the comparison operation is being tested (e.g., the first operand of an +`if_then_else'), the mode must be `VOIDmode'. If the comparison +operation is producing data to be stored in some variable, the mode +must be in class `MODE_INT'. All comparison operations producing data +must use the same mode, which is machine-specific. + + There are two ways that comparison operations may be used. The +comparison operators may be used to compare the condition codes `(cc0)' +against zero, as in `(eq (cc0) (const_int 0))'. Such a construct +actually refers to the result of the preceding instruction in which the +condition codes were set. The instructing setting the condition code +must be adjacent to the instruction using the condition code; only +`note' insns may separate them. + + Alternatively, a comparison operation may directly compare two data +objects. The mode of the comparison is determined by the operands; they +must both be valid for a common machine mode. A comparison with both +operands constant would be invalid as the machine mode could not be +deduced from it, but such a comparison should never exist in RTL due to +constant folding. + + In the example above, if `(cc0)' were last set to `(compare X Y)', +the comparison operation is identical to `(eq X Y)'. Usually only one +style of comparisons is supported on a particular machine, but the +combine pass will try to merge the operations to produce the `eq' shown +in case it exists in the context of the particular insn involved. + + Inequality comparisons come in two flavors, signed and unsigned. +Thus, there are distinct expression codes `gt' and `gtu' for signed and +unsigned greater-than. These can produce different results for the same +pair of integer values: for example, 1 is signed greater-than -1 but not +unsigned greater-than, because -1 when regarded as unsigned is actually +`0xffffffff' which is greater than 1. + + The signed comparisons are also used for floating point values. +Floating point comparisons are distinguished by the machine modes of +the operands. + +`(eq:M X Y)' + 1 if the values represented by X and Y are equal, otherwise 0. + +`(ne:M X Y)' + 1 if the values represented by X and Y are not equal, otherwise 0. + +`(gt:M X Y)' + 1 if the X is greater than Y. If they are fixed-point, the + comparison is done in a signed sense. + +`(gtu:M X Y)' + Like `gt' but does unsigned comparison, on fixed-point numbers + only. + +`(lt:M X Y)' +`(ltu:M X Y)' + Like `gt' and `gtu' but test for "less than". + +`(ge:M X Y)' +`(geu:M X Y)' + Like `gt' and `gtu' but test for "greater than or equal". + +`(le:M X Y)' +`(leu:M X Y)' + Like `gt' and `gtu' but test for "less than or equal". + +`(if_then_else COND THEN ELSE)' + This is not a comparison operation but is listed here because it is + always used in conjunction with a comparison operation. To be + precise, COND is a comparison expression. This expression + represents a choice, according to COND, between the value + represented by THEN and the one represented by ELSE. + + On most machines, `if_then_else' expressions are valid only to + express conditional jumps. + +`(cond [TEST1 VALUE1 TEST2 VALUE2 ...] DEFAULT)' + Similar to `if_then_else', but more general. Each of TEST1, + TEST2, ... is performed in turn. The result of this expression is + the VALUE corresponding to the first non-zero test, or DEFAULT if + none of the tests are non-zero expressions. - -File: gcc.info, Node: Scalar Return, Next: Aggregate Return, Prev: Register Arguments, Up: Stack and Calling + This is currently not valid for instruction patterns and is + supported only for insn attributes. *Note Insn Attributes::. -How Scalar Function Values Are Returned ---------------------------------------- + +File: gcc.info, Node: Bit Fields, Next: Conversions, Prev: Comparisons, Up: RTL - This section discusses the macros that control returning scalars as -values--values that can fit in registers. +Bit Fields +========== -`TRADITIONAL_RETURN_FLOAT' - Define this macro if `-traditional' should not cause functions - declared to return `float' to convert the value to `double'. - -`FUNCTION_VALUE (VALTYPE, FUNC)' - A C expression to create an RTX representing the place where a - function returns a value of data type VALTYPE. VALTYPE is a tree - node representing a data type. Write `TYPE_MODE (VALTYPE)' to get - the machine mode used to represent that type. On many machines, - only the mode is relevant. (Actually, on most machines, scalar - values are returned in the same place regardless of mode). - - If the precise function being called is known, FUNC is a tree node - (`FUNCTION_DECL') for it; otherwise, FUNC is a null pointer. This - makes it possible to use a different value-returning convention - for specific functions when all their calls are known. - - `FUNCTION_VALUE' is not used for return vales with aggregate data - types, because these are returned in another way. See - `STRUCT_VALUE_REGNUM' and related macros, below. - -`FUNCTION_OUTGOING_VALUE (VALTYPE, FUNC)' - Define this macro if the target machine has "register windows" so - that the register in which a function returns its value is not the - same as the one in which the caller sees the value. - - For such machines, `FUNCTION_VALUE' computes the register in which - the caller will see the value, and `FUNCTION_OUTGOING_VALUE' - should be defined in a similar fashion to tell the function where - to put the value. - - If `FUNCTION_OUTGOING_VALUE' is not defined, `FUNCTION_VALUE' - serves both purposes. - - `FUNCTION_OUTGOING_VALUE' is not used for return vales with - aggregate data types, because these are returned in another way. - See `STRUCT_VALUE_REGNUM' and related macros, below. - -`LIBCALL_VALUE (MODE)' - A C expression to create an RTX representing the place where a - library function returns a value of mode MODE. If the precise - function being called is known, FUNC is a tree node - (`FUNCTION_DECL') for it; otherwise, FUNC is a null pointer. This - makes it possible to use a different value-returning convention - for specific functions when all their calls are known. - - Note that "library function" in this context means a compiler - support routine, used to perform arithmetic, whose name is known - specially by the compiler and was not mentioned in the C code being - compiled. - - The definition of `LIBRARY_VALUE' need not be concerned aggregate - data types, because none of the library functions returns such - types. - -`FUNCTION_VALUE_REGNO_P (REGNO)' - A C expression that is nonzero if REGNO is the number of a hard - register in which the values of called function may come back. - - A register whose use for returning values is limited to serving as - the second of a pair (for a value of type `double', say) need not - be recognized by this macro. So for most machines, this definition - suffices: - - #define FUNCTION_VALUE_REGNO_P(N) ((N) == 0) - - If the machine has register windows, so that the caller and the - called function use different registers for the return value, this - macro should recognize only the caller's register numbers. + Special expression codes exist to represent bitfield instructions. +These types of expressions are lvalues in RTL; they may appear on the +left side of an assignment, indicating insertion of a value into the +specified bit field. + +`(sign_extract:M LOC SIZE POS)' + This represents a reference to a sign-extended bit field contained + or starting in LOC (a memory or register reference). The bit field + is SIZE bits wide and starts at bit POS. The compilation option + `BITS_BIG_ENDIAN' says which end of the memory unit POS counts + from. + + If LOC is in memory, its mode must be a single-byte integer mode. + If LOC is in a register, the mode to use is specified by the + operand of the `insv' or `extv' pattern (*note Standard Names::.) + and is usually a full-word integer mode. + + The mode of POS is machine-specific and is also specified in the + `insv' or `extv' pattern. + + The mode M is the same as the mode that would be used for LOC if + it were a register. + +`(zero_extract:M LOC SIZE POS)' + Like `sign_extract' but refers to an unsigned or zero-extended bit + field. The same sequence of bits are extracted, but they are + filled to an entire word with zeros instead of by sign-extension.  -File: gcc.info, Node: Aggregate Return, Next: Caller Saves, Prev: Scalar Return, Up: Stack and Calling - -How Large Values Are Returned ------------------------------ +File: gcc.info, Node: Conversions, Next: RTL Declarations, Prev: Bit Fields, Up: RTL - When a function value's mode is `BLKmode' (and in some other cases), -the value is not returned according to `FUNCTION_VALUE' (*note Scalar -Return::.). Instead, the caller passes the address of a block of -memory in which the value should be stored. This address is called the -"structure value address". - - This section describes how to control returning structure values in -memory. - -`RETURN_IN_MEMORY (TYPE)' - A C expression which can inhibit the returning of certain function - values in registers, based on the type of value. A nonzero value - says to return the function value in memory, just as large - structures are always returned. Here TYPE will be a C expression - of type `tree', representing the data type of the value. - - Note that values of mode `BLKmode' are returned in memory - regardless of this macro. Also, the option `-fpcc-struct-return' - takes effect regardless of this macro. On most systems, it is - possible to leave the macro undefined; this causes a default - definition to be used, whose value is the constant 0. - -`STRUCT_VALUE_REGNUM' - If the structure value address is passed in a register, then - `STRUCT_VALUE_REGNUM' should be the number of that register. - -`STRUCT_VALUE' - If the structure value address is not passed in a register, define - `STRUCT_VALUE' as an expression returning an RTX for the place - where the address is passed. If it returns 0, the address is - passed as an "invisible" first argument. - -`STRUCT_VALUE_INCOMING_REGNUM' - On some architectures the place where the structure value address - is found by the called function is not the same place that the - caller put it. This can be due to register windows, or it could - be because the function prologue moves it to a different place. - - If the incoming location of the structure value address is in a - register, define this macro as the register number. - -`STRUCT_VALUE_INCOMING' - If the incoming location is not a register, define - `STRUCT_VALUE_INCOMING' as an expression for an RTX for where the - called function should find the value. If it should find the - value on the stack, define this to create a `mem' which refers to - the frame pointer. A definition of 0 means that the address is - passed as an "invisible" first argument. - -`PCC_STATIC_STRUCT_RETURN' - Define this macro if the usual system convention on the target - machine for returning structures and unions is for the called - function to return the address of a static variable containing the - value. GNU CC does not normally use this convention, even if it - is the usual one, but does use it if `-fpcc-struct-value' is - specified. +Conversions +=========== - Do not define this if the usual system convention is for the - caller to pass an address to the subroutine. + All conversions between machine modes must be represented by +explicit conversion operations. For example, an expression which is +the sum of a byte and a full word cannot be written as `(plus:SI +(reg:QI 34) (reg:SI 80))' because the `plus' operation requires two +operands of the same machine mode. Therefore, the byte-sized operand +is enclosed in a conversion operation, as in + + (plus:SI (sign_extend:SI (reg:QI 34)) (reg:SI 80)) + + The conversion operation is not a mere placeholder, because there +may be more than one way of converting from a given starting mode to +the desired final mode. The conversion operation code says how to do +it. + + For all conversion operations, X must not be `VOIDmode' because the +mode in which to do the conversion would not be known. The conversion +must either be done at compile-time or X must be placed into a register. + +`(sign_extend:M X)' + Represents the result of sign-extending the value X to machine + mode M. M must be a fixed-point mode and X a fixed-point value of + a mode narrower than M. + +`(zero_extend:M X)' + Represents the result of zero-extending the value X to machine + mode M. M must be a fixed-point mode and X a fixed-point value of + a mode narrower than M. + +`(float_extend:M X)' + Represents the result of extending the value X to machine mode M. + M must be a floating point mode and X a floating point value of a + mode narrower than M. + +`(truncate:M X)' + Represents the result of truncating the value X to machine mode M. + M must be a fixed-point mode and X a fixed-point value of a mode + wider than M. + +`(float_truncate:M X)' + Represents the result of truncating the value X to machine mode M. + M must be a floating point mode and X a floating point value of a + mode wider than M. + +`(float:M X)' + Represents the result of converting fixed point value X, regarded + as signed, to floating point mode M. + +`(unsigned_float:M X)' + Represents the result of converting fixed point value X, regarded + as unsigned, to floating point mode M. + +`(fix:M X)' + When M is a fixed point mode, represents the result of converting + floating point value X to mode M, regarded as signed. How + rounding is done is not specified, so this operation may be used + validly in compiling C code only for integer-valued operands. + +`(unsigned_fix:M X)' + Represents the result of converting floating point value X to + fixed point mode M, regarded as unsigned. How rounding is done is + not specified. + +`(fix:M X)' + When M is a floating point mode, represents the result of + converting floating point value X (valid for mode M) to an + integer, still represented in floating point mode M, by rounding + towards zero.  -File: gcc.info, Node: Caller Saves, Next: Function Entry, Prev: Aggregate Return, Up: Stack and Calling +File: gcc.info, Node: RTL Declarations, Next: Side Effects, Prev: Conversions, Up: RTL -Caller-Saves Register Allocation --------------------------------- +Declarations +============ - If you enable it, GNU CC can save registers around function calls. -This makes it possible to use call-clobbered registers to hold -variables that must live across calls. - -`DEFAULT_CALLER_SAVES' - Define this macro if function calls on the target machine do not - preserve any registers; in other words, if `CALL_USED_REGISTERS' - has 1 for all registers. This macro enables `-fcaller-saves' by - default. Eventually that option will be enabled by default on all - machines and both the option and this macro will be eliminated. - -`CALLER_SAVE_PROFITABLE (REFS, CALLS)' - A C expression to determine whether it is worthwhile to consider - placing a pseudo-register in a call-clobbered hard register and - saving and restoring it around each function call. The expression - should be 1 when this is worth doing, and 0 otherwise. + Declaration expression codes do not represent arithmetic operations +but rather state assertions about their operands. - If you don't define this macro, a default is used which is good on - most machines: `4 * CALLS < REFS'. +`(strict_low_part (subreg:M (reg:N R) 0))' + This expression code is used in only one context: as the + destination operand of a `set' expression. In addition, the + operand of this expression must be a non-paradoxical `subreg' + expression. + + The presence of `strict_low_part' says that the part of the + register which is meaningful in mode N, but is not part of mode M, + is not to be altered. Normally, an assignment to such a subreg is + allowed to have undefined effects on the rest of the register when + M is less than a word. - \ No newline at end of file