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1.1 ! root 1: This is Info file gcc.info, produced by Makeinfo-1.54 from the input ! 2: file gcc.texi. ! 3: ! 4: This file documents the use and the internals of the GNU compiler. ! 5: ! 6: Published by the Free Software Foundation 675 Massachusetts Avenue ! 7: Cambridge, MA 02139 USA ! 8: ! 9: Copyright (C) 1988, 1989, 1992, 1993 Free Software Foundation, Inc. ! 10: ! 11: Permission is granted to make and distribute verbatim copies of this ! 12: manual provided the copyright notice and this permission notice are ! 13: preserved on all copies. ! 14: ! 15: Permission is granted to copy and distribute modified versions of ! 16: this manual under the conditions for verbatim copying, provided also ! 17: that the sections entitled "GNU General Public License" and "Protect ! 18: Your Freedom--Fight `Look And Feel'" are included exactly as in the ! 19: original, and provided that the entire resulting derived work is ! 20: distributed under the terms of a permission notice identical to this ! 21: one. ! 22: ! 23: Permission is granted to copy and distribute translations of this ! 24: manual into another language, under the above conditions for modified ! 25: versions, except that the sections entitled "GNU General Public ! 26: License" and "Protect Your Freedom--Fight `Look And Feel'", and this ! 27: permission notice, may be included in translations approved by the Free ! 28: Software Foundation instead of in the original English. ! 29: ! 30: ! 31: File: gcc.info, Node: Passes, Next: RTL, Prev: Interface, Up: Top ! 32: ! 33: Passes and Files of the Compiler ! 34: ******************************** ! 35: ! 36: The overall control structure of the compiler is in `toplev.c'. This ! 37: file is responsible for initialization, decoding arguments, opening and ! 38: closing files, and sequencing the passes. ! 39: ! 40: The parsing pass is invoked only once, to parse the entire input. ! 41: The RTL intermediate code for a function is generated as the function ! 42: is parsed, a statement at a time. Each statement is read in as a ! 43: syntax tree and then converted to RTL; then the storage for the tree ! 44: for the statement is reclaimed. Storage for types (and the expressions ! 45: for their sizes), declarations, and a representation of the binding ! 46: contours and how they nest, remain until the function is finished being ! 47: compiled; these are all needed to output the debugging information. ! 48: ! 49: Each time the parsing pass reads a complete function definition or ! 50: top-level declaration, it calls either the function ! 51: `rest_of_compilation', or the function `rest_of_decl_compilation' in ! 52: `toplev.c', which are responsible for all further processing necessary, ! 53: ending with output of the assembler language. All other compiler ! 54: passes run, in sequence, within `rest_of_compilation'. When that ! 55: function returns from compiling a function definition, the storage used ! 56: for that function definition's compilation is entirely freed, unless it ! 57: is an inline function (*note An Inline Function is As Fast As a Macro: ! 58: Inline.). ! 59: ! 60: Here is a list of all the passes of the compiler and their source ! 61: files. Also included is a description of where debugging dumps can be ! 62: requested with `-d' options. ! 63: ! 64: * Parsing. This pass reads the entire text of a function definition, ! 65: constructing partial syntax trees. This and RTL generation are no ! 66: longer truly separate passes (formerly they were), but it is ! 67: easier to think of them as separate. ! 68: ! 69: The tree representation does not entirely follow C syntax, because ! 70: it is intended to support other languages as well. ! 71: ! 72: Language-specific data type analysis is also done in this pass, ! 73: and every tree node that represents an expression has a data type ! 74: attached. Variables are represented as declaration nodes. ! 75: ! 76: Constant folding and some arithmetic simplifications are also done ! 77: during this pass. ! 78: ! 79: The language-independent source files for parsing are ! 80: `stor-layout.c', `fold-const.c', and `tree.c'. There are also ! 81: header files `tree.h' and `tree.def' which define the format of ! 82: the tree representation. ! 83: ! 84: The source files to parse C are `c-parse.in', `c-decl.c', ! 85: `c-typeck.c', `c-aux-info.c', `c-convert.c', and `c-lang.c' along ! 86: with header files `c-lex.h', and `c-tree.h'. ! 87: ! 88: The source files for parsing C++ are `cp-parse.y', `cp-class.c', ! 89: `cp-cvt.c', `cp-decl.c', `cp-decl2.c', `cp-dem.c', `cp-except.c', ! 90: `cp-expr.c', `cp-init.c', `cp-lex.c', `cp-method.c', `cp-ptree.c', ! 91: `cp-search.c', `cp-tree.c', `cp-type2.c', and `cp-typeck.c', along ! 92: with header files `cp-tree.def', `cp-tree.h', and `cp-decl.h'. ! 93: ! 94: The special source files for parsing Objective C are ! 95: `objc-parse.y', `objc-actions.c', `objc-tree.def', and ! 96: `objc-actions.h'. Certain C-specific files are used for this as ! 97: well. ! 98: ! 99: The file `c-common.c' is also used for all of the above languages. ! 100: ! 101: * RTL generation. This is the conversion of syntax tree into RTL ! 102: code. It is actually done statement-by-statement during parsing, ! 103: but for most purposes it can be thought of as a separate pass. ! 104: ! 105: This is where the bulk of target-parameter-dependent code is found, ! 106: since often it is necessary for strategies to apply only when ! 107: certain standard kinds of instructions are available. The purpose ! 108: of named instruction patterns is to provide this information to ! 109: the RTL generation pass. ! 110: ! 111: Optimization is done in this pass for `if'-conditions that are ! 112: comparisons, boolean operations or conditional expressions. Tail ! 113: recursion is detected at this time also. Decisions are made about ! 114: how best to arrange loops and how to output `switch' statements. ! 115: ! 116: The source files for RTL generation include `stmt.c', `calls.c', ! 117: `expr.c', `explow.c', `expmed.c', `function.c', `optabs.c' and ! 118: `emit-rtl.c'. Also, the file `insn-emit.c', generated from the ! 119: machine description by the program `genemit', is used in this ! 120: pass. The header file `expr.h' is used for communication within ! 121: this pass. ! 122: ! 123: The header files `insn-flags.h' and `insn-codes.h', generated from ! 124: the machine description by the programs `genflags' and `gencodes', ! 125: tell this pass which standard names are available for use and ! 126: which patterns correspond to them. ! 127: ! 128: Aside from debugging information output, none of the following ! 129: passes refers to the tree structure representation of the function ! 130: (only part of which is saved). ! 131: ! 132: The decision of whether the function can and should be expanded ! 133: inline in its subsequent callers is made at the end of rtl ! 134: generation. The function must meet certain criteria, currently ! 135: related to the size of the function and the types and number of ! 136: parameters it has. Note that this function may contain loops, ! 137: recursive calls to itself (tail-recursive functions can be ! 138: inlined!), gotos, in short, all constructs supported by GNU CC. ! 139: The file `integrate.c' contains the code to save a function's rtl ! 140: for later inlining and to inline that rtl when the function is ! 141: called. The header file `integrate.h' is also used for this ! 142: purpose. ! 143: ! 144: The option `-dr' causes a debugging dump of the RTL code after ! 145: this pass. This dump file's name is made by appending `.rtl' to ! 146: the input file name. ! 147: ! 148: * Jump optimization. This pass simplifies jumps to the following ! 149: instruction, jumps across jumps, and jumps to jumps. It deletes ! 150: unreferenced labels and unreachable code, except that unreachable ! 151: code that contains a loop is not recognized as unreachable in this ! 152: pass. (Such loops are deleted later in the basic block analysis.) ! 153: It also converts some code originally written with jumps into ! 154: sequences of instructions that directly set values from the ! 155: results of comparisons, if the machine has such instructions. ! 156: ! 157: Jump optimization is performed two or three times. The first time ! 158: is immediately following RTL generation. The second time is after ! 159: CSE, but only if CSE says repeated jump optimization is needed. ! 160: The last time is right before the final pass. That time, ! 161: cross-jumping and deletion of no-op move instructions are done ! 162: together with the optimizations described above. ! 163: ! 164: The source file of this pass is `jump.c'. ! 165: ! 166: The option `-dj' causes a debugging dump of the RTL code after ! 167: this pass is run for the first time. This dump file's name is ! 168: made by appending `.jump' to the input file name. ! 169: ! 170: * Register scan. This pass finds the first and last use of each ! 171: register, as a guide for common subexpression elimination. Its ! 172: source is in `regclass.c'. ! 173: ! 174: * Jump threading. This pass detects a condition jump that branches ! 175: to an identical or inverse test. Such jumps can be `threaded' ! 176: through the second conditional test. The source code for this ! 177: pass is in `jump.c'. This optimization is only performed if ! 178: `-fthread-jumps' is enabled. ! 179: ! 180: * Common subexpression elimination. This pass also does constant ! 181: propagation. Its source file is `cse.c'. If constant propagation ! 182: causes conditional jumps to become unconditional or to become ! 183: no-ops, jump optimization is run again when CSE is finished. ! 184: ! 185: The option `-ds' causes a debugging dump of the RTL code after ! 186: this pass. This dump file's name is made by appending `.cse' to ! 187: the input file name. ! 188: ! 189: * Loop optimization. This pass moves constant expressions out of ! 190: loops, and optionally does strength-reduction and loop unrolling ! 191: as well. Its source files are `loop.c' and `unroll.c', plus the ! 192: header `loop.h' used for communication between them. Loop ! 193: unrolling uses some functions in `integrate.c' and the header ! 194: `integrate.h'. ! 195: ! 196: The option `-dL' causes a debugging dump of the RTL code after ! 197: this pass. This dump file's name is made by appending `.loop' to ! 198: the input file name. ! 199: ! 200: * If `-frerun-cse-after-loop' was enabled, a second common ! 201: subexpression elimination pass is performed after the loop ! 202: optimization pass. Jump threading is also done again at this time ! 203: if it was specified. ! 204: ! 205: The option `-dt' causes a debugging dump of the RTL code after ! 206: this pass. This dump file's name is made by appending `.cse2' to ! 207: the input file name. ! 208: ! 209: * Stupid register allocation is performed at this point in a ! 210: nonoptimizing compilation. It does a little data flow analysis as ! 211: well. When stupid register allocation is in use, the next pass ! 212: executed is the reloading pass; the others in between are skipped. ! 213: The source file is `stupid.c'. ! 214: ! 215: * Data flow analysis (`flow.c'). This pass divides the program into ! 216: basic blocks (and in the process deletes unreachable loops); then ! 217: it computes which pseudo-registers are live at each point in the ! 218: program, and makes the first instruction that uses a value point at ! 219: the instruction that computed the value. ! 220: ! 221: This pass also deletes computations whose results are never used, ! 222: and combines memory references with add or subtract instructions ! 223: to make autoincrement or autodecrement addressing. ! 224: ! 225: The option `-df' causes a debugging dump of the RTL code after ! 226: this pass. This dump file's name is made by appending `.flow' to ! 227: the input file name. If stupid register allocation is in use, this ! 228: dump file reflects the full results of such allocation. ! 229: ! 230: * Instruction combination (`combine.c'). This pass attempts to ! 231: combine groups of two or three instructions that are related by ! 232: data flow into single instructions. It combines the RTL ! 233: expressions for the instructions by substitution, simplifies the ! 234: result using algebra, and then attempts to match the result ! 235: against the machine description. ! 236: ! 237: The option `-dc' causes a debugging dump of the RTL code after ! 238: this pass. This dump file's name is made by appending `.combine' ! 239: to the input file name. ! 240: ! 241: * Instruction scheduling (`sched.c'). This pass looks for ! 242: instructions whose output will not be available by the time that ! 243: it is used in subsequent instructions. (Memory loads and floating ! 244: point instructions often have this behavior on RISC machines). It ! 245: re-orders instructions within a basic block to try to separate the ! 246: definition and use of items that otherwise would cause pipeline ! 247: stalls. ! 248: ! 249: Instruction scheduling is performed twice. The first time is ! 250: immediately after instruction combination and the second is ! 251: immediately after reload. ! 252: ! 253: The option `-dS' causes a debugging dump of the RTL code after this ! 254: pass is run for the first time. The dump file's name is made by ! 255: appending `.sched' to the input file name. ! 256: ! 257: * Register class preferencing. The RTL code is scanned to find out ! 258: which register class is best for each pseudo register. The source ! 259: file is `regclass.c'. ! 260: ! 261: * Local register allocation (`local-alloc.c'). This pass allocates ! 262: hard registers to pseudo registers that are used only within one ! 263: basic block. Because the basic block is linear, it can use fast ! 264: and powerful techniques to do a very good job. ! 265: ! 266: The option `-dl' causes a debugging dump of the RTL code after ! 267: this pass. This dump file's name is made by appending `.lreg' to ! 268: the input file name. ! 269: ! 270: * Global register allocation (`global.c'). This pass allocates hard ! 271: registers for the remaining pseudo registers (those whose life ! 272: spans are not contained in one basic block). ! 273: ! 274: * Reloading. This pass renumbers pseudo registers with the hardware ! 275: registers numbers they were allocated. Pseudo registers that did ! 276: not get hard registers are replaced with stack slots. Then it ! 277: finds instructions that are invalid because a value has failed to ! 278: end up in a register, or has ended up in a register of the wrong ! 279: kind. It fixes up these instructions by reloading the ! 280: problematical values temporarily into registers. Additional ! 281: instructions are generated to do the copying. ! 282: ! 283: The reload pass also optionally eliminates the frame pointer and ! 284: inserts instructions to save and restore call-clobbered registers ! 285: around calls. ! 286: ! 287: Source files are `reload.c' and `reload1.c', plus the header ! 288: `reload.h' used for communication between them. ! 289: ! 290: The option `-dg' causes a debugging dump of the RTL code after ! 291: this pass. This dump file's name is made by appending `.greg' to ! 292: the input file name. ! 293: ! 294: * Instruction scheduling is repeated here to try to avoid pipeline ! 295: stalls due to memory loads generated for spilled pseudo registers. ! 296: ! 297: The option `-dR' causes a debugging dump of the RTL code after ! 298: this pass. This dump file's name is made by appending `.sched2' ! 299: to the input file name. ! 300: ! 301: * Jump optimization is repeated, this time including cross-jumping ! 302: and deletion of no-op move instructions. ! 303: ! 304: The option `-dJ' causes a debugging dump of the RTL code after ! 305: this pass. This dump file's name is made by appending `.jump2' to ! 306: the input file name. ! 307: ! 308: * Delayed branch scheduling. This optional pass attempts to find ! 309: instructions that can go into the delay slots of other ! 310: instructions, usually jumps and calls. The source file name is ! 311: `reorg.c'. ! 312: ! 313: The option `-dd' causes a debugging dump of the RTL code after ! 314: this pass. This dump file's name is made by appending `.dbr' to ! 315: the input file name. ! 316: ! 317: * Conversion from usage of some hard registers to usage of a register ! 318: stack may be done at this point. Currently, this is supported only ! 319: for the floating-point registers of the Intel 80387 coprocessor. ! 320: The source file name is `reg-stack.c'. ! 321: ! 322: The options `-dk' causes a debugging dump of the RTL code after ! 323: this pass. This dump file's name is made by appending `.stack' to ! 324: the input file name. ! 325: ! 326: * Final. This pass outputs the assembler code for the function. It ! 327: is also responsible for identifying spurious test and compare ! 328: instructions. Machine-specific peephole optimizations are ! 329: performed at the same time. The function entry and exit sequences ! 330: are generated directly as assembler code in this pass; they never ! 331: exist as RTL. ! 332: ! 333: The source files are `final.c' plus `insn-output.c'; the latter is ! 334: generated automatically from the machine description by the tool ! 335: `genoutput'. The header file `conditions.h' is used for ! 336: communication between these files. ! 337: ! 338: * Debugging information output. This is run after final because it ! 339: must output the stack slot offsets for pseudo registers that did ! 340: not get hard registers. Source files are `dbxout.c' for DBX ! 341: symbol table format, `sdbout.c' for SDB symbol table format, and ! 342: `dwarfout.c' for DWARF symbol table format. ! 343: ! 344: Some additional files are used by all or many passes: ! 345: ! 346: * Every pass uses `machmode.def' and `machmode.h' which define the ! 347: machine modes. ! 348: ! 349: * Several passes use `real.h', which defines the default ! 350: representation of floating point constants and how to operate on ! 351: them. ! 352: ! 353: * All the passes that work with RTL use the header files `rtl.h' and ! 354: `rtl.def', and subroutines in file `rtl.c'. The tools `gen*' also ! 355: use these files to read and work with the machine description RTL. ! 356: ! 357: * Several passes refer to the header file `insn-config.h' which ! 358: contains a few parameters (C macro definitions) generated ! 359: automatically from the machine description RTL by the tool ! 360: `genconfig'. ! 361: ! 362: * Several passes use the instruction recognizer, which consists of ! 363: `recog.c' and `recog.h', plus the files `insn-recog.c' and ! 364: `insn-extract.c' that are generated automatically from the machine ! 365: description by the tools `genrecog' and `genextract'. ! 366: ! 367: * Several passes use the header files `regs.h' which defines the ! 368: information recorded about pseudo register usage, and ! 369: `basic-block.h' which defines the information recorded about basic ! 370: blocks. ! 371: ! 372: * `hard-reg-set.h' defines the type `HARD_REG_SET', a bit-vector ! 373: with a bit for each hard register, and some macros to manipulate ! 374: it. This type is just `int' if the machine has few enough hard ! 375: registers; otherwise it is an array of `int' and some of the ! 376: macros expand into loops. ! 377: ! 378: * Several passes use instruction attributes. A definition of the ! 379: attributes defined for a particular machine is in file ! 380: `insn-attr.h', which is generated from the machine description by ! 381: the program `genattr'. The file `insn-attrtab.c' contains ! 382: subroutines to obtain the attribute values for insns. It is ! 383: generated from the machine description by the program `genattrtab'. ! 384: ! 385: ! 386: File: gcc.info, Node: RTL, Next: Machine Desc, Prev: Passes, Up: Top ! 387: ! 388: RTL Representation ! 389: ****************** ! 390: ! 391: Most of the work of the compiler is done on an intermediate ! 392: representation called register transfer language. In this language, ! 393: the instructions to be output are described, pretty much one by one, in ! 394: an algebraic form that describes what the instruction does. ! 395: ! 396: RTL is inspired by Lisp lists. It has both an internal form, made ! 397: up of structures that point at other structures, and a textual form ! 398: that is used in the machine description and in printed debugging dumps. ! 399: The textual form uses nested parentheses to indicate the pointers in ! 400: the internal form. ! 401: ! 402: * Menu: ! 403: ! 404: * RTL Objects:: Expressions vs vectors vs strings vs integers. ! 405: * Accessors:: Macros to access expression operands or vector elts. ! 406: * Flags:: Other flags in an RTL expression. ! 407: * Machine Modes:: Describing the size and format of a datum. ! 408: * Constants:: Expressions with constant values. ! 409: * Regs and Memory:: Expressions representing register contents or memory. ! 410: * Arithmetic:: Expressions representing arithmetic on other expressions. ! 411: * Comparisons:: Expressions representing comparison of expressions. ! 412: * Bit Fields:: Expressions representing bitfields in memory or reg. ! 413: * Conversions:: Extending, truncating, floating or fixing. ! 414: * RTL Declarations:: Declaring volatility, constancy, etc. ! 415: * Side Effects:: Expressions for storing in registers, etc. ! 416: * Incdec:: Embedded side-effects for autoincrement addressing. ! 417: * Assembler:: Representing `asm' with operands. ! 418: * Insns:: Expression types for entire insns. ! 419: * Calls:: RTL representation of function call insns. ! 420: * Sharing:: Some expressions are unique; others *must* be copied. ! 421: * Reading RTL:: Reading textual RTL from a file. ! 422: ! 423: ! 424: File: gcc.info, Node: RTL Objects, Next: Accessors, Prev: RTL, Up: RTL ! 425: ! 426: RTL Object Types ! 427: ================ ! 428: ! 429: RTL uses five kinds of objects: expressions, integers, wide integers, ! 430: strings and vectors. Expressions are the most important ones. An RTL ! 431: expression ("RTX", for short) is a C structure, but it is usually ! 432: referred to with a pointer; a type that is given the typedef name `rtx'. ! 433: ! 434: An integer is simply an `int'; their written form uses decimal ! 435: digits. A wide integer is an integral object whose type is ! 436: `HOST_WIDE_INT' (*note Config::.); their written form uses decimal ! 437: digits. ! 438: ! 439: A string is a sequence of characters. In core it is represented as a ! 440: `char *' in usual C fashion, and it is written in C syntax as well. ! 441: However, strings in RTL may never be null. If you write an empty ! 442: string in a machine description, it is represented in core as a null ! 443: pointer rather than as a pointer to a null character. In certain ! 444: contexts, these null pointers instead of strings are valid. Within RTL ! 445: code, strings are most commonly found inside `symbol_ref' expressions, ! 446: but they appear in other contexts in the RTL expressions that make up ! 447: machine descriptions. ! 448: ! 449: A vector contains an arbitrary number of pointers to expressions. ! 450: The number of elements in the vector is explicitly present in the ! 451: vector. The written form of a vector consists of square brackets ! 452: (`[...]') surrounding the elements, in sequence and with whitespace ! 453: separating them. Vectors of length zero are not created; null pointers ! 454: are used instead. ! 455: ! 456: Expressions are classified by "expression codes" (also called RTX ! 457: codes). The expression code is a name defined in `rtl.def', which is ! 458: also (in upper case) a C enumeration constant. The possible expression ! 459: codes and their meanings are machine-independent. The code of an RTX ! 460: can be extracted with the macro `GET_CODE (X)' and altered with ! 461: `PUT_CODE (X, NEWCODE)'. ! 462: ! 463: The expression code determines how many operands the expression ! 464: contains, and what kinds of objects they are. In RTL, unlike Lisp, you ! 465: cannot tell by looking at an operand what kind of object it is. ! 466: Instead, you must know from its context--from the expression code of ! 467: the containing expression. For example, in an expression of code ! 468: `subreg', the first operand is to be regarded as an expression and the ! 469: second operand as an integer. In an expression of code `plus', there ! 470: are two operands, both of which are to be regarded as expressions. In ! 471: a `symbol_ref' expression, there is one operand, which is to be ! 472: regarded as a string. ! 473: ! 474: Expressions are written as parentheses containing the name of the ! 475: expression type, its flags and machine mode if any, and then the ! 476: operands of the expression (separated by spaces). ! 477: ! 478: Expression code names in the `md' file are written in lower case, ! 479: but when they appear in C code they are written in upper case. In this ! 480: manual, they are shown as follows: `const_int'. ! 481: ! 482: In a few contexts a null pointer is valid where an expression is ! 483: normally wanted. The written form of this is `(nil)'. ! 484: ! 485: ! 486: File: gcc.info, Node: Accessors, Next: Flags, Prev: RTL Objects, Up: RTL ! 487: ! 488: Access to Operands ! 489: ================== ! 490: ! 491: For each expression type `rtl.def' specifies the number of contained ! 492: objects and their kinds, with four possibilities: `e' for expression ! 493: (actually a pointer to an expression), `i' for integer, `w' for wide ! 494: integer, `s' for string, and `E' for vector of expressions. The ! 495: sequence of letters for an expression code is called its "format". ! 496: Thus, the format of `subreg' is `ei'. ! 497: ! 498: A few other format characters are used occasionally: ! 499: ! 500: `u' ! 501: `u' is equivalent to `e' except that it is printed differently in ! 502: debugging dumps. It is used for pointers to insns. ! 503: ! 504: `n' ! 505: `n' is equivalent to `i' except that it is printed differently in ! 506: debugging dumps. It is used for the line number or code number of ! 507: a `note' insn. ! 508: ! 509: `S' ! 510: `S' indicates a string which is optional. In the RTL objects in ! 511: core, `S' is equivalent to `s', but when the object is read, from ! 512: an `md' file, the string value of this operand may be omitted. An ! 513: omitted string is taken to be the null string. ! 514: ! 515: `V' ! 516: `V' indicates a vector which is optional. In the RTL objects in ! 517: core, `V' is equivalent to `E', but when the object is read from ! 518: an `md' file, the vector value of this operand may be omitted. An ! 519: omitted vector is effectively the same as a vector of no elements. ! 520: ! 521: `0' ! 522: `0' means a slot whose contents do not fit any normal category. ! 523: `0' slots are not printed at all in dumps, and are often used in ! 524: special ways by small parts of the compiler. ! 525: ! 526: There are macros to get the number of operands, the format, and the ! 527: class of an expression code: ! 528: ! 529: `GET_RTX_LENGTH (CODE)' ! 530: Number of operands of an RTX of code CODE. ! 531: ! 532: `GET_RTX_FORMAT (CODE)' ! 533: The format of an RTX of code CODE, as a C string. ! 534: ! 535: `GET_RTX_CLASS (CODE)' ! 536: A single character representing the type of RTX operation that code ! 537: CODE performs. ! 538: ! 539: The following classes are defined: ! 540: ! 541: `o' ! 542: An RTX code that represents an actual object, such as `reg' or ! 543: `mem'. `subreg' is not in this class. ! 544: ! 545: `<' ! 546: An RTX code for a comparison. The codes in this class are ! 547: `NE', `EQ', `LE', `LT', `GE', `GT', `LEU', `LTU', `GEU', ! 548: `GTU'. ! 549: ! 550: `1' ! 551: An RTX code for a unary arithmetic operation, such as `neg'. ! 552: ! 553: `c' ! 554: An RTX code for a commutative binary operation, other than ! 555: `NE' and `EQ' (which have class `<'). ! 556: ! 557: `2' ! 558: An RTX code for a noncommutative binary operation, such as ! 559: `MINUS'. ! 560: ! 561: `b' ! 562: An RTX code for a bitfield operation, either `ZERO_EXTRACT' or ! 563: `SIGN_EXTRACT'. ! 564: ! 565: `3' ! 566: An RTX code for other three input operations, such as ! 567: `IF_THEN_ELSE'. ! 568: ! 569: `i' ! 570: An RTX code for a machine insn (`INSN', `JUMP_INSN', and ! 571: `CALL_INSN'). ! 572: ! 573: `m' ! 574: An RTX code for something that matches in insns, such as ! 575: `MATCH_DUP'. ! 576: ! 577: `x' ! 578: All other RTX codes. ! 579: ! 580: Operands of expressions are accessed using the macros `XEXP', ! 581: `XINT', `XWINT' and `XSTR'. Each of these macros takes two arguments: ! 582: an expression-pointer (RTX) and an operand number (counting from zero). ! 583: Thus, ! 584: ! 585: XEXP (X, 2) ! 586: ! 587: accesses operand 2 of expression X, as an expression. ! 588: ! 589: XINT (X, 2) ! 590: ! 591: accesses the same operand as an integer. `XSTR', used in the same ! 592: fashion, would access it as a string. ! 593: ! 594: Any operand can be accessed as an integer, as an expression or as a ! 595: string. You must choose the correct method of access for the kind of ! 596: value actually stored in the operand. You would do this based on the ! 597: expression code of the containing expression. That is also how you ! 598: would know how many operands there are. ! 599: ! 600: For example, if X is a `subreg' expression, you know that it has two ! 601: operands which can be correctly accessed as `XEXP (X, 0)' and `XINT (X, ! 602: 1)'. If you did `XINT (X, 0)', you would get the address of the ! 603: expression operand but cast as an integer; that might occasionally be ! 604: useful, but it would be cleaner to write `(int) XEXP (X, 0)'. `XEXP ! 605: (X, 1)' would also compile without error, and would return the second, ! 606: integer operand cast as an expression pointer, which would probably ! 607: result in a crash when accessed. Nothing stops you from writing `XEXP ! 608: (X, 28)' either, but this will access memory past the end of the ! 609: expression with unpredictable results. ! 610: ! 611: Access to operands which are vectors is more complicated. You can ! 612: use the macro `XVEC' to get the vector-pointer itself, or the macros ! 613: `XVECEXP' and `XVECLEN' to access the elements and length of a vector. ! 614: ! 615: `XVEC (EXP, IDX)' ! 616: Access the vector-pointer which is operand number IDX in EXP. ! 617: ! 618: `XVECLEN (EXP, IDX)' ! 619: Access the length (number of elements) in the vector which is in ! 620: operand number IDX in EXP. This value is an `int'. ! 621: ! 622: `XVECEXP (EXP, IDX, ELTNUM)' ! 623: Access element number ELTNUM in the vector which is in operand ! 624: number IDX in EXP. This value is an RTX. ! 625: ! 626: It is up to you to make sure that ELTNUM is not negative and is ! 627: less than `XVECLEN (EXP, IDX)'. ! 628: ! 629: All the macros defined in this section expand into lvalues and ! 630: therefore can be used to assign the operands, lengths and vector ! 631: elements as well as to access them. ! 632: ! 633: ! 634: File: gcc.info, Node: Flags, Next: Machine Modes, Prev: Accessors, Up: RTL ! 635: ! 636: Flags in an RTL Expression ! 637: ========================== ! 638: ! 639: RTL expressions contain several flags (one-bit bitfields) that are ! 640: used in certain types of expression. Most often they are accessed with ! 641: the following macros: ! 642: ! 643: `MEM_VOLATILE_P (X)' ! 644: In `mem' expressions, nonzero for volatile memory references. ! 645: Stored in the `volatil' field and printed as `/v'. ! 646: ! 647: `MEM_IN_STRUCT_P (X)' ! 648: In `mem' expressions, nonzero for reference to an entire ! 649: structure, union or array, or to a component of one. Zero for ! 650: references to a scalar variable or through a pointer to a scalar. ! 651: Stored in the `in_struct' field and printed as `/s'. ! 652: ! 653: `REG_LOOP_TEST_P' ! 654: In `reg' expressions, nonzero if this register's entire life is ! 655: contained in the exit test code for some loop. Stored in the ! 656: `in_struct' field and printed as `/s'. ! 657: ! 658: `REG_USERVAR_P (X)' ! 659: In a `reg', nonzero if it corresponds to a variable present in the ! 660: user's source code. Zero for temporaries generated internally by ! 661: the compiler. Stored in the `volatil' field and printed as `/v'. ! 662: ! 663: `REG_FUNCTION_VALUE_P (X)' ! 664: Nonzero in a `reg' if it is the place in which this function's ! 665: value is going to be returned. (This happens only in a hard ! 666: register.) Stored in the `integrated' field and printed as `/i'. ! 667: ! 668: The same hard register may be used also for collecting the values ! 669: of functions called by this one, but `REG_FUNCTION_VALUE_P' is zero ! 670: in this kind of use. ! 671: ! 672: `SUBREG_PROMOTED_VAR_P' ! 673: Nonzero in a `subreg' if it was made when accessing an object that ! 674: was promoted to a wider mode in accord with the `PROMOTED_MODE' ! 675: machine description macro (*note Storage Layout::.). In this ! 676: case, the mode of the `subreg' is the declared mode of the object ! 677: and the mode of `SUBREG_REG' is the mode of the register that ! 678: holds the object. Promoted variables are always either sign- or ! 679: zero-extended to the wider mode on every assignment. Stored in ! 680: the `in_struct' field and printed as `/s'. ! 681: ! 682: `SUBREG_PROMOTED_UNSIGNED_P' ! 683: Nonzero in a `subreg' that has `SUBREG_PROMOTED_VAR_P' nonzero if ! 684: the object being referenced is kept zero-extended and zero if it ! 685: is kept sign-extended. Stored in the `unchanging' field and ! 686: printed as `/u'. ! 687: ! 688: `RTX_UNCHANGING_P (X)' ! 689: Nonzero in a `reg' or `mem' if the value is not changed. (This ! 690: flag is not set for memory references via pointers to constants. ! 691: Such pointers only guarantee that the object will not be changed ! 692: explicitly by the current function. The object might be changed by ! 693: other functions or by aliasing.) Stored in the `unchanging' field ! 694: and printed as `/u'. ! 695: ! 696: `RTX_INTEGRATED_P (INSN)' ! 697: Nonzero in an insn if it resulted from an in-line function call. ! 698: Stored in the `integrated' field and printed as `/i'. This may be ! 699: deleted; nothing currently depends on it. ! 700: ! 701: `SYMBOL_REF_USED (X)' ! 702: In a `symbol_ref', indicates that X has been used. This is ! 703: normally only used to ensure that X is only declared external ! 704: once. Stored in the `used' field. ! 705: ! 706: `SYMBOL_REF_FLAG (X)' ! 707: In a `symbol_ref', this is used as a flag for machine-specific ! 708: purposes. Stored in the `volatil' field and printed as `/v'. ! 709: ! 710: `LABEL_OUTSIDE_LOOP_P' ! 711: In `label_ref' expressions, nonzero if this is a reference to a ! 712: label that is outside the innermost loop containing the reference ! 713: to the label. Stored in the `in_struct' field and printed as `/s'. ! 714: ! 715: `INSN_DELETED_P (INSN)' ! 716: In an insn, nonzero if the insn has been deleted. Stored in the ! 717: `volatil' field and printed as `/v'. ! 718: ! 719: `INSN_ANNULLED_BRANCH_P (INSN)' ! 720: In an `insn' in the delay slot of a branch insn, indicates that an ! 721: annulling branch should be used. See the discussion under ! 722: `sequence' below. Stored in the `unchanging' field and printed as ! 723: `/u'. ! 724: ! 725: `INSN_FROM_TARGET_P (INSN)' ! 726: In an `insn' in a delay slot of a branch, indicates that the insn ! 727: is from the target of the branch. If the branch insn has ! 728: `INSN_ANNULLED_BRANCH_P' set, this insn should only be executed if ! 729: the branch is taken. For annulled branches with this bit clear, ! 730: the insn should be executed only if the branch is not taken. ! 731: Stored in the `in_struct' field and printed as `/s'. ! 732: ! 733: `CONSTANT_POOL_ADDRESS_P (X)' ! 734: Nonzero in a `symbol_ref' if it refers to part of the current ! 735: function's "constants pool". These are addresses close to the ! 736: beginning of the function, and GNU CC assumes they can be addressed ! 737: directly (perhaps with the help of base registers). Stored in the ! 738: `unchanging' field and printed as `/u'. ! 739: ! 740: `CONST_CALL_P (X)' ! 741: In a `call_insn', indicates that the insn represents a call to a ! 742: const function. Stored in the `unchanging' field and printed as ! 743: `/u'. ! 744: ! 745: `LABEL_PRESERVE_P (X)' ! 746: In a `code_label', indicates that the label can never be deleted. ! 747: Labels referenced by a non-local goto will have this bit set. ! 748: Stored in the `in_struct' field and printed as `/s'. ! 749: ! 750: `SCHED_GROUP_P (INSN)' ! 751: During instruction scheduling, in an insn, indicates that the ! 752: previous insn must be scheduled together with this insn. This is ! 753: used to ensure that certain groups of instructions will not be ! 754: split up by the instruction scheduling pass, for example, `use' ! 755: insns before a `call_insn' may not be separated from the ! 756: `call_insn'. Stored in the `in_struct' field and printed as `/s'. ! 757: ! 758: These are the fields which the above macros refer to: ! 759: ! 760: `used' ! 761: Normally, this flag is used only momentarily, at the end of RTL ! 762: generation for a function, to count the number of times an ! 763: expression appears in insns. Expressions that appear more than ! 764: once are copied, according to the rules for shared structure ! 765: (*note Sharing::.). ! 766: ! 767: In a `symbol_ref', it indicates that an external declaration for ! 768: the symbol has already been written. ! 769: ! 770: In a `reg', it is used by the leaf register renumbering code to ! 771: ensure that each register is only renumbered once. ! 772: ! 773: `volatil' ! 774: This flag is used in `mem', `symbol_ref' and `reg' expressions and ! 775: in insns. In RTL dump files, it is printed as `/v'. ! 776: ! 777: In a `mem' expression, it is 1 if the memory reference is volatile. ! 778: Volatile memory references may not be deleted, reordered or ! 779: combined. ! 780: ! 781: In a `symbol_ref' expression, it is used for machine-specific ! 782: purposes. ! 783: ! 784: In a `reg' expression, it is 1 if the value is a user-level ! 785: variable. 0 indicates an internal compiler temporary. ! 786: ! 787: In an insn, 1 means the insn has been deleted. ! 788: ! 789: `in_struct' ! 790: In `mem' expressions, it is 1 if the memory datum referred to is ! 791: all or part of a structure or array; 0 if it is (or might be) a ! 792: scalar variable. A reference through a C pointer has 0 because ! 793: the pointer might point to a scalar variable. This information ! 794: allows the compiler to determine something about possible cases of ! 795: aliasing. ! 796: ! 797: In an insn in the delay slot of a branch, 1 means that this insn ! 798: is from the target of the branch. ! 799: ! 800: During instruction scheduling, in an insn, 1 means that this insn ! 801: must be scheduled as part of a group together with the previous ! 802: insn. ! 803: ! 804: In `reg' expressions, it is 1 if the register has its entire life ! 805: contained within the test expression of some loop. ! 806: ! 807: In `subreg' expressions, 1 means that the `subreg' is accessing an ! 808: object that has had its mode promoted from a wider mode. ! 809: ! 810: In `label_ref' expressions, 1 means that the referenced label is ! 811: outside the innermost loop containing the insn in which the ! 812: `label_ref' was found. ! 813: ! 814: In `code_label' expressions, it is 1 if the label may never be ! 815: deleted. This is used for labels which are the target of ! 816: non-local gotos. ! 817: ! 818: In an RTL dump, this flag is represented as `/s'. ! 819: ! 820: `unchanging' ! 821: In `reg' and `mem' expressions, 1 means that the value of the ! 822: expression never changes. ! 823: ! 824: In `subreg' expressions, it is 1 if the `subreg' references an ! 825: unsigned object whose mode has been promoted to a wider mode. ! 826: ! 827: In an insn, 1 means that this is an annulling branch. ! 828: ! 829: In a `symbol_ref' expression, 1 means that this symbol addresses ! 830: something in the per-function constants pool. ! 831: ! 832: In a `call_insn', 1 means that this instruction is a call to a ! 833: const function. ! 834: ! 835: In an RTL dump, this flag is represented as `/u'. ! 836: ! 837: `integrated' ! 838: In some kinds of expressions, including insns, this flag means the ! 839: rtl was produced by procedure integration. ! 840: ! 841: In a `reg' expression, this flag indicates the register containing ! 842: the value to be returned by the current function. On machines ! 843: that pass parameters in registers, the same register number may be ! 844: used for parameters as well, but this flag is not set on such uses. ! 845: ! 846: ! 847: File: gcc.info, Node: Machine Modes, Next: Constants, Prev: Flags, Up: RTL ! 848: ! 849: Machine Modes ! 850: ============= ! 851: ! 852: A machine mode describes a size of data object and the ! 853: representation used for it. In the C code, machine modes are ! 854: represented by an enumeration type, `enum machine_mode', defined in ! 855: `machmode.def'. Each RTL expression has room for a machine mode and so ! 856: do certain kinds of tree expressions (declarations and types, to be ! 857: precise). ! 858: ! 859: In debugging dumps and machine descriptions, the machine mode of an ! 860: RTL expression is written after the expression code with a colon to ! 861: separate them. The letters `mode' which appear at the end of each ! 862: machine mode name are omitted. For example, `(reg:SI 38)' is a `reg' ! 863: expression with machine mode `SImode'. If the mode is `VOIDmode', it ! 864: is not written at all. ! 865: ! 866: Here is a table of machine modes. The term "byte" below refers to an ! 867: object of `BITS_PER_UNIT' bits (*note Storage Layout::.). ! 868: ! 869: `QImode' ! 870: "Quarter-Integer" mode represents a single byte treated as an ! 871: integer. ! 872: ! 873: `HImode' ! 874: "Half-Integer" mode represents a two-byte integer. ! 875: ! 876: `PSImode' ! 877: "Partial Single Integer" mode represents an integer which occupies ! 878: four bytes but which doesn't really use all four. On some ! 879: machines, this is the right mode to use for pointers. ! 880: ! 881: `SImode' ! 882: "Single Integer" mode represents a four-byte integer. ! 883: ! 884: `PDImode' ! 885: "Partial Double Integer" mode represents an integer which occupies ! 886: eight bytes but which doesn't really use all eight. On some ! 887: machines, this is the right mode to use for certain pointers. ! 888: ! 889: `DImode' ! 890: "Double Integer" mode represents an eight-byte integer. ! 891: ! 892: `TImode' ! 893: "Tetra Integer" (?) mode represents a sixteen-byte integer. ! 894: ! 895: `SFmode' ! 896: "Single Floating" mode represents a single-precision (four byte) ! 897: floating point number. ! 898: ! 899: `DFmode' ! 900: "Double Floating" mode represents a double-precision (eight byte) ! 901: floating point number. ! 902: ! 903: `XFmode' ! 904: "Extended Floating" mode represents a triple-precision (twelve ! 905: byte) floating point number. This mode is used for IEEE extended ! 906: floating point. ! 907: ! 908: `TFmode' ! 909: "Tetra Floating" mode represents a quadruple-precision (sixteen ! 910: byte) floating point number. ! 911: ! 912: `CCmode' ! 913: "Condition Code" mode represents the value of a condition code, ! 914: which is a machine-specific set of bits used to represent the ! 915: result of a comparison operation. Other machine-specific modes ! 916: may also be used for the condition code. These modes are not used ! 917: on machines that use `cc0' (see *note Condition Code::.). ! 918: ! 919: `BLKmode' ! 920: "Block" mode represents values that are aggregates to which none of ! 921: the other modes apply. In RTL, only memory references can have ! 922: this mode, and only if they appear in string-move or vector ! 923: instructions. On machines which have no such instructions, ! 924: `BLKmode' will not appear in RTL. ! 925: ! 926: `VOIDmode' ! 927: Void mode means the absence of a mode or an unspecified mode. For ! 928: example, RTL expressions of code `const_int' have mode `VOIDmode' ! 929: because they can be taken to have whatever mode the context ! 930: requires. In debugging dumps of RTL, `VOIDmode' is expressed by ! 931: the absence of any mode. ! 932: ! 933: `SCmode, DCmode, XCmode, TCmode' ! 934: These modes stand for a complex number represented as a pair of ! 935: floating point values. The floating point values are in `SFmode', ! 936: `DFmode', `XFmode', and `TFmode', respectively. ! 937: ! 938: `CQImode, CHImode, CSImode, CDImode, CTImode, COImode' ! 939: These modes stand for a complex number represented as a pair of ! 940: integer values. The integer values are in `QImode', `HImode', ! 941: `SImode', `DImode', `TImode', and `OImode', respectively. ! 942: ! 943: The machine description defines `Pmode' as a C macro which expands ! 944: into the machine mode used for addresses. Normally this is the mode ! 945: whose size is `BITS_PER_WORD', `SImode' on 32-bit machines. ! 946: ! 947: The only modes which a machine description must support are ! 948: `QImode', and the modes corresponding to `BITS_PER_WORD', ! 949: `FLOAT_TYPE_SIZE' and `DOUBLE_TYPE_SIZE'. The compiler will attempt to ! 950: use `DImode' for 8-byte structures and unions, but this can be ! 951: prevented by overriding the definition of `MAX_FIXED_MODE_SIZE'. ! 952: Alternatively, you can have the compiler use `TImode' for 16-byte ! 953: structures and unions. Likewise, you can arrange for the C type `short ! 954: int' to avoid using `HImode'. ! 955: ! 956: Very few explicit references to machine modes remain in the compiler ! 957: and these few references will soon be removed. Instead, the machine ! 958: modes are divided into mode classes. These are represented by the ! 959: enumeration type `enum mode_class' defined in `machmode.h'. The ! 960: possible mode classes are: ! 961: ! 962: `MODE_INT' ! 963: Integer modes. By default these are `QImode', `HImode', `SImode', ! 964: `DImode', and `TImode'. ! 965: ! 966: `MODE_PARTIAL_INT' ! 967: The "partial integer" modes, `PSImode' and `PDImode'. ! 968: ! 969: `MODE_FLOAT' ! 970: floating point modes. By default these are `SFmode', `DFmode', ! 971: `XFmode' and `TFmode'. ! 972: ! 973: `MODE_COMPLEX_INT' ! 974: Complex integer modes. (These are not currently implemented). ! 975: ! 976: `MODE_COMPLEX_FLOAT' ! 977: Complex floating point modes. By default these are `SCmode', ! 978: `DCmode', `XCmode', and `TCmode'. ! 979: ! 980: `MODE_FUNCTION' ! 981: Algol or Pascal function variables including a static chain. ! 982: (These are not currently implemented). ! 983: ! 984: `MODE_CC' ! 985: Modes representing condition code values. These are `CCmode' plus ! 986: any modes listed in the `EXTRA_CC_MODES' macro. *Note Jump ! 987: Patterns::, also see *Note Condition Code::. ! 988: ! 989: `MODE_RANDOM' ! 990: This is a catchall mode class for modes which don't fit into the ! 991: above classes. Currently `VOIDmode' and `BLKmode' are in ! 992: `MODE_RANDOM'. ! 993: ! 994: Here are some C macros that relate to machine modes: ! 995: ! 996: `GET_MODE (X)' ! 997: Returns the machine mode of the RTX X. ! 998: ! 999: `PUT_MODE (X, NEWMODE)' ! 1000: Alters the machine mode of the RTX X to be NEWMODE. ! 1001: ! 1002: `NUM_MACHINE_MODES' ! 1003: Stands for the number of machine modes available on the target ! 1004: machine. This is one greater than the largest numeric value of any ! 1005: machine mode. ! 1006: ! 1007: `GET_MODE_NAME (M)' ! 1008: Returns the name of mode M as a string. ! 1009: ! 1010: `GET_MODE_CLASS (M)' ! 1011: Returns the mode class of mode M. ! 1012: ! 1013: `GET_MODE_WIDER_MODE (M)' ! 1014: Returns the next wider natural mode. For example, the expression ! 1015: `GET_MODE_WIDER_MODE (QImode)' returns `HImode'. ! 1016: ! 1017: `GET_MODE_SIZE (M)' ! 1018: Returns the size in bytes of a datum of mode M. ! 1019: ! 1020: `GET_MODE_BITSIZE (M)' ! 1021: Returns the size in bits of a datum of mode M. ! 1022: ! 1023: `GET_MODE_MASK (M)' ! 1024: Returns a bitmask containing 1 for all bits in a word that fit ! 1025: within mode M. This macro can only be used for modes whose ! 1026: bitsize is less than or equal to `HOST_BITS_PER_INT'. ! 1027: ! 1028: `GET_MODE_ALIGNMENT (M))' ! 1029: Return the required alignment, in bits, for an object of mode M. ! 1030: ! 1031: `GET_MODE_UNIT_SIZE (M)' ! 1032: Returns the size in bytes of the subunits of a datum of mode M. ! 1033: This is the same as `GET_MODE_SIZE' except in the case of complex ! 1034: modes. For them, the unit size is the size of the real or ! 1035: imaginary part. ! 1036: ! 1037: `GET_MODE_NUNITS (M)' ! 1038: Returns the number of units contained in a mode, i.e., ! 1039: `GET_MODE_SIZE' divided by `GET_MODE_UNIT_SIZE'. ! 1040: ! 1041: `GET_CLASS_NARROWEST_MODE (C)' ! 1042: Returns the narrowest mode in mode class C. ! 1043: ! 1044: The global variables `byte_mode' and `word_mode' contain modes whose ! 1045: classes are `MODE_INT' and whose bitsizes are either `BITS_PER_UNIT' or ! 1046: `BITS_PER_WORD', respectively. On 32-bit machines, these are `QImode' ! 1047: and `SImode', respectively. ! 1048: ! 1049: ! 1050: File: gcc.info, Node: Constants, Next: Regs and Memory, Prev: Machine Modes, Up: RTL ! 1051: ! 1052: Constant Expression Types ! 1053: ========================= ! 1054: ! 1055: The simplest RTL expressions are those that represent constant ! 1056: values. ! 1057: ! 1058: `(const_int I)' ! 1059: This type of expression represents the integer value I. I is ! 1060: customarily accessed with the macro `INTVAL' as in `INTVAL (EXP)', ! 1061: which is equivalent to `XWINT (EXP, 0)'. ! 1062: ! 1063: There is only one expression object for the integer value zero; it ! 1064: is the value of the variable `const0_rtx'. Likewise, the only ! 1065: expression for integer value one is found in `const1_rtx', the only ! 1066: expression for integer value two is found in `const2_rtx', and the ! 1067: only expression for integer value negative one is found in ! 1068: `constm1_rtx'. Any attempt to create an expression of code ! 1069: `const_int' and value zero, one, two or negative one will return ! 1070: `const0_rtx', `const1_rtx', `const2_rtx' or `constm1_rtx' as ! 1071: appropriate. ! 1072: ! 1073: Similarly, there is only one object for the integer whose value is ! 1074: `STORE_FLAG_VALUE'. It is found in `const_true_rtx'. If ! 1075: `STORE_FLAG_VALUE' is one, `const_true_rtx' and `const1_rtx' will ! 1076: point to the same object. If `STORE_FLAG_VALUE' is -1, ! 1077: `const_true_rtx' and `constm1_rtx' will point to the same object. ! 1078: ! 1079: `(const_double:M ADDR I0 I1 ...)' ! 1080: Represents either a floating-point constant of mode M or an ! 1081: integer constant too large to fit into `HOST_BITS_PER_WIDE_INT' ! 1082: bits but small enough to fit within twice that number of bits (GNU ! 1083: CC does not provide a mechanism to represent even larger ! 1084: constants). In the latter case, M will be `VOIDmode'. ! 1085: ! 1086: ADDR is used to contain the `mem' expression that corresponds to ! 1087: the location in memory that at which the constant can be found. If ! 1088: it has not been allocated a memory location, but is on the chain ! 1089: of all `const_double' expressions in this compilation (maintained ! 1090: using an undisplayed field), ADDR contains `const0_rtx'. If it is ! 1091: not on the chain, ADDR contains `cc0_rtx'. ADDR is customarily ! 1092: accessed with the macro `CONST_DOUBLE_MEM' and the chain field via ! 1093: `CONST_DOUBLE_CHAIN'. ! 1094: ! 1095: If M is `VOIDmode', the bits of the value are stored in I0 and I1. ! 1096: I0 is customarily accessed with the macro `CONST_DOUBLE_LOW' and ! 1097: I1 with `CONST_DOUBLE_HIGH'. ! 1098: ! 1099: If the constant is floating point (regardless of its precision), ! 1100: then the number of integers used to store the value depends on the ! 1101: size of `REAL_VALUE_TYPE' (*note Cross-compilation::.). The ! 1102: integers represent a floating point number, but not precisely in ! 1103: the target machine's or host machine's floating point format. To ! 1104: convert them to the precise bit pattern used by the target ! 1105: machine, use the macro `REAL_VALUE_TO_TARGET_DOUBLE' and friends ! 1106: (*note Data Output::.). ! 1107: ! 1108: The macro `CONST0_RTX (MODE)' refers to an expression with value 0 ! 1109: in mode MODE. If mode MODE is of mode class `MODE_INT', it ! 1110: returns `const0_rtx'. Otherwise, it returns a `CONST_DOUBLE' ! 1111: expression in mode MODE. Similarly, the macro `CONST1_RTX (MODE)' ! 1112: refers to an expression with value 1 in mode MODE and similarly ! 1113: for `CONST2_RTX'. ! 1114: ! 1115: `(const_string STR)' ! 1116: Represents a constant string with value STR. Currently this is ! 1117: used only for insn attributes (*note Insn Attributes::.) since ! 1118: constant strings in C are placed in memory. ! 1119: ! 1120: `(symbol_ref:MODE SYMBOL)' ! 1121: Represents the value of an assembler label for data. SYMBOL is a ! 1122: string that describes the name of the assembler label. If it ! 1123: starts with a `*', the label is the rest of SYMBOL not including ! 1124: the `*'. Otherwise, the label is SYMBOL, usually prefixed with ! 1125: `_'. ! 1126: ! 1127: The `symbol_ref' contains a mode, which is usually `Pmode'. ! 1128: Usually that is the only mode for which a symbol is directly valid. ! 1129: ! 1130: `(label_ref LABEL)' ! 1131: Represents the value of an assembler label for code. It contains ! 1132: one operand, an expression, which must be a `code_label' that ! 1133: appears in the instruction sequence to identify the place where ! 1134: the label should go. ! 1135: ! 1136: The reason for using a distinct expression type for code label ! 1137: references is so that jump optimization can distinguish them. ! 1138: ! 1139: `(const:M EXP)' ! 1140: Represents a constant that is the result of an assembly-time ! 1141: arithmetic computation. The operand, EXP, is an expression that ! 1142: contains only constants (`const_int', `symbol_ref' and `label_ref' ! 1143: expressions) combined with `plus' and `minus'. However, not all ! 1144: combinations are valid, since the assembler cannot do arbitrary ! 1145: arithmetic on relocatable symbols. ! 1146: ! 1147: M should be `Pmode'. ! 1148: ! 1149: `(high:M EXP)' ! 1150: Represents the high-order bits of EXP, usually a `symbol_ref'. ! 1151: The number of bits is machine-dependent and is normally the number ! 1152: of bits specified in an instruction that initializes the high ! 1153: order bits of a register. It is used with `lo_sum' to represent ! 1154: the typical two-instruction sequence used in RISC machines to ! 1155: reference a global memory location. ! 1156: ! 1157: M should be `Pmode'. ! 1158:
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