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1.1 root 1: 1. Better optimization.
2:
1.1.1.4 root 3: * Constants in unused inline functions
4:
5: It would be nice to delay output of string constants so that string
6: constants mentioned in unused inline functions are never generated.
7: Perhaps this would also take care of string constants in dead code.
8:
9: The difficulty is in finding a clean way for the RTL which refers
10: to the constant (currently, only by an assembler symbol name)
11: to point to the constant and cause it to be output.
12:
1.1 root 13: * More cse
14:
15: The techniques for doing full global cse are described in the
16: red dragon book. It is likely to be slow and use a lot of memory,
17: but it might be worth offering as an additional option.
18:
19: It is probably possible to extend cse to a few very frequent cases
20: without so much expense.
21:
22: For example, it is not very hard to handle cse through if-then
23: statements with no else clauses. Here's how to do it. On reaching a
24: label, notice that the label's use-count is 1 and that the last
25: preceding jump jumps conditionally to this label. Now you know it
26: is a simple if-then statement. Remove from the hash table
27: all the expressions that were entered since that jump insn
28: and you can continue with cse.
29:
30: It is probably not hard to handle cse from the end of a loop
31: around to the beginning, and a few loops would be greatly sped
32: up by this.
33:
1.1.1.3 root 34: * Support more general tail-recursion among different functions.
1.1 root 35:
1.1.1.3 root 36: This might be possible under certain circumstances, such as when
37: the argument lists of the functions have the same lengths.
38: Perhaps it could be done with a special declaration.
39:
40: You would need to verify in the calling function that it does not
41: use the addresses of any local variables and does not use setjmp.
42:
43: * Put short statics vars at low addresses and use short addressing mode?
44: Useful on the 68000/68020 and perhaps on the 32000 series,
45: provided one has a linker that works with the feature.
46: This is said to make a 15% speedup on the 68000.
47: This brings to mind Hayes' changes for Stanford MIPS.
48:
49: * Detect dead stores into memory?
50:
51: A store into memory is dead if it is followed by another store into
1.1.1.5 ! root 52: the same location; and, in between, there is no reference to anything
! 53: that might be that location (including no reference to a variable
! 54: address).
1.1.1.3 root 55:
56: * Loop optimization.
57:
58: Strength reduction and iteration variable elimination could be
59: smarter. They should know how to decide which iteration variables are
60: not worth making explicit because they can be computed as part of an
61: address calculation. Based on this information, they should decide
1.1.1.5 ! root 62: when it is desirable to eliminate one iteration variable and create
1.1.1.3 root 63: another in its place.
64:
65: It should be possible to compute what the value of an iteration
66: variable will be at the end of the loop, and eliminate the variable
67: within the loop by computing that value at the loop end.
68:
69: When a loop has a simple increment that adds 1,
70: instead of jumping in after the increment,
71: decrement the loop count and jump to the increment.
72: This allows aob insns to be used.
1.1 root 73:
1.1.1.2 root 74: * Using constraints on values.
1.1 root 75:
1.1.1.2 root 76: Many operations could be simplified based on knowledge of the
77: minimum and maximum possible values of a register at any particular time.
78: These limits could come from the data types in the tree, via rtl generation,
79: or they can be deduced from operations that are performed. For example,
80: the result of an `and' operation one of whose operands is 7 must be in
81: the range 0 to 7. Compare instructions also tell something about the
82: possible values of the operand, in the code beyond the test.
83:
84: Value constraints can be used to determine the results of a further
85: comparison. They can also indicate that certain `and' operations are
86: redundant. Constraints might permit a decrement and branch
87: instruction that checks zeroness to be used when the user has
88: specified to exit if negative.
1.1 root 89:
90: * Smarter reload pass.
91:
92: The reload pass as currently written can reload values only into registers
93: that are reserved for reloading. This means that in order to use a
94: register for reloading it must spill everything out of that register.
95:
96: It would be straightforward, though complicated, for reload1.c to keep
97: track, during its scan, of which hard registers were available at each
98: point in the function, and use for reloading even registers that were
99: free only at the point they were needed. This would avoid much spilling
100: and make better code.
101:
102: * Change the type of a variable.
103:
104: Sometimes a variable is declared as `int', it is assigned only once
105: from a value of type `char', and then it is used only by comparison
106: against constants. On many machines, better code would result if
107: the variable had type `char'. If the compiler could detect this
108: case, it could change the declaration of the variable and change
109: all the places that use it.
110:
111: * Order of subexpressions.
112:
113: It might be possible to make better code by paying attention
114: to the order in which to generate code for subexpressions of an expression.
115:
1.1.1.3 root 116: * More code motion.
1.1 root 117:
1.1.1.3 root 118: Consider hoisting common code up past conditional branches or
119: tablejumps.
120:
121: * Trace scheduling.
122:
123: This technique is said to be able to figure out which way a jump
124: will usually go, and rearrange the code to make that path the
125: faster one.
1.1 root 126:
127: * Distributive law.
128:
1.1.1.2 root 129: The C expression *(X + 4 * (Y + C)) compiles better on certain
130: machines if rewritten as *(X + 4*C + 4*Y) because of known addressing
131: modes. It may be tricky to determine when, and for which machines, to
132: use each alternative.
133:
1.1.1.3 root 134: Some work has been done on this, in combine.c.
135:
1.1.1.2 root 136: * Jump-execute-next.
137:
138: Many recent machines have jumps which optionally execute the following
139: instruction before the instruction jumped to, either conditionally or
140: unconditionally. To take advantage of this capability requires a new
141: compiler pass that would reorder instructions when possible. After
142: reload may be a good place for it.
143:
144: On some machines, the result of a load from memory is not available
145: until after the following instruction. The easiest way to support
146: these machines is to output each RTL load instruction as two assembler
147: instructions, the second being a no-op. Putting useful instructions
148: after the load instructions may be a similar task to putting them
149: after jump instructions.
150:
151: * Pipeline scheduling.
152:
153: On many machines, code gets faster if instructions are reordered
154: so that pipelines are kept full. Doing the best possible job of this
155: requires knowing which functional units each kind of instruction executes
156: in and how long the functional unit stays busy with it. Then the
157: goal is to reorder the instructions to keep many functional units
158: busy but never feed them so fast they must wait.
1.1 root 159:
1.1.1.3 root 160: * Can optimize by changing if (x) y; else z; into z; if (x) y;
161: if z and x do not interfere and z has no effects not undone by y.
162: This is desirable if z is faster than jumping.
163:
164: * For a two-insn loop on the 68020, such as
165: foo: movb a2@+,a3@+
166: jne foo
167: it is better to insert dbeq d0,foo before the jne.
168: d0 can be a junk register. The challenge is to fit this into
169: a portable framework: when can you detect this situation and
170: still be able to allocate a junk register?
171:
1.1 root 172: 2. Simpler porting.
173:
174: Right now, describing the target machine's instructions is done
175: cleanly, but describing its addressing mode is done with several
176: ad-hoc macro definitions. Porting would be much easier if there were
177: an RTL description for addressing modes like that for instructions.
178: Tools analogous to genflags and genrecog would generate macros from
179: this description.
180:
181: There would be one pattern in the address-description file for each
182: kind of addressing, and this pattern would have:
183:
184: * the RTL expression for the address
185: * C code to verify its validity (since that may depend on
186: the exact data).
187: * C code to print the address in assembler language.
188: * C code to convert the address into a valid one, if it is not valid.
189: (This would replace LEGITIMIZE_ADDRESS).
190: * Register constraints for all indeterminates that appear
191: in the RTL expression.
192:
193: 3. Other languages.
194:
1.1.1.2 root 195: Front ends for Pascal, Fortran, Algol, Cobol, Modula-2 and Ada are
196: desirable.
1.1 root 197:
1.1.1.2 root 198: Pascal, Modula-2 and Ada require the implementation of functions
199: within functions. Some of the mechanisms for this already exist.
1.1 root 200:
201: 4. Generalize the machine model.
202:
1.1.1.4 root 203: * Some new compiler features may be needed to do a good job on machines
1.1.1.2 root 204: where static data needs to be addressed using base registers.
1.1 root 205:
1.1.1.4 root 206: * Some machines have two stacks in different areas of memory, one used
1.1.1.2 root 207: for scalars and another for large objects. The compiler does not
208: now have a way to understand this.
1.1.1.3 root 209:
210: 5. Precompilation of header files.
211:
212: In the future, many programs will use thousands of lines of header files.
213: Compiling the headers might be slower than compiling the guts of any one
214: source file. Here is a scheme for precompiling header files to make
215: compilation faster for a sequence of headers which is often used.
216:
217: A precompiled header corresponds to a sequence of header files. The
218: preprocessor recognizes when the input starts with a sequence of
219: `#include' commands and searches a data base for a precompiled header
220: corresponding to that sequence. The modtimes of all these files are
221: stored in the data base so that one can tell whether the precompiled
222: header is still valid.
223:
224: For robustness, each directory should have its own collection of
225: precompiled headers and its own data base of them. Probably each
226: precompiled header would be a file and the data base would be one
227: more file.
228:
229: The data base records the entire collection of predefined macros and
230: their definitions, except for __FILE__, __LINE__ and __DATE__, for
231: each precompiled header. If this collection does not match the setup
232: at the start of the current compilation (including the results of -D
233: and -U switches), the precompiled header is inapplicable. It might
234: be possible to have distinct precompiled headers for the same sequence
235: of header files but different collections of predefined macros.
236:
237: The state of any option that affects macro processing, such as -ansi
238: or -traditional, must also be recorded, and the precompiled header is
239: valid only if these options match.
240:
241: The precompiled header contains an ordered series of strings. Some
242: strings are marked "unconditional"; these must be compiled each time
243: the precompiled header is used. Other strings are have keys, which
244: are identifiers. A string with keys must be compiled if at least one
245: of its keys is mentioned in the input. The order these strings appear
246: in the precompiled header is called their intrinsic order.
247:
248: The C preprocessor reads in the precompiled header file and scan all
249: the strings, making for each key an entry in the same symbol table
250: used for macros, pointing at a list of all the strings for which it is
251: a key. Each string must have a flag (one flag per string, not one per
252: key per string). The same code in `rescan' that detects references to
253: macros would detect a reference to a key and flag all of the strings
254: that it belongs to as needing to be output.
255:
256: Each of these strings is immediately recursively macroexpanded (i.e.
257: `rescan' is called), but the output from this is discarded. The
258: expansion is to detect any other keys mentioned in the string, and to
259: define any macros for which the string contains a #define. The key's
260: symbol table entry is be deleted to save time if the key is
261: encountered again, and to avoid an infinite recursion.
262:
263: The unconditional strings are macroexpanded with `rescan' (but the
264: output is discarded) at some time before anything is actually output.
265:
266: At the end of compilation, before any of the actual input text is
267: output, the list of strings is scanned in the intrinsic order, and
268: each string that is unconditional or flagged is output verbatim,
269: except that any #define lines are discarded.
270:
271: Precompiled headers would be constructed by explicit request with a
272: special tool. The first step is to run cpp on the sequence of header
273: files' contents. This would use a new option that would cause all
274: #define lines to be output unchanged as well as defining the macro.
275: The second step is to divide the output into strings, some keyed and
276: some unconditional. This division is done without changing the order
277: of the text being divided up.
1.1.1.4 root 278:
1.1.1.5 ! root 279: [email protected] has some ideas on this subject also.
! 280:
1.1.1.4 root 281: 6. Other possibly nice features.
282:
283: * cpp could have a #provide directive.
284: #provide would have the same syntax as #include,
285: and it would nullify any future #include directive
286: with the same argument. Thus, the file foo.h
287: could contain #provide <foo> to prevent itself from
288: being included twice.
289:
290: This is much cleaner than the alternative sometimes implemented,
291: which is to require the user to use something other than #include
292: in order to ensure inclusion only once.
293:
294: 7. Better documentation of how GCC works and how to port it.
295:
296: Here is an outline proposed by Allan Adler.
297:
298: I. Overview of this document
299: II. The machines on which GCC is implemented
300: A. Prose description of those characteristics of target machines and
301: their operating systems which are pertinent to the implementation
302: of GCC.
303: i. target machine characteristics
304: ii. comparison of this system of machine characteristics with
305: other systems of machine specification currently in use
306: B. Tables of the characteristics of the target machines on which
307: GCC is implemented.
308: C. A priori restrictions on the values of characteristics of target
309: machines, with special reference to those parts of the source code
310: which entail those restrictions
311: i. restrictions on individual characteristics
312: ii. restrictions involving relations between various characteristics
313: D. The use of GCC as a cross-compiler
314: i. cross-compilation to existing machines
315: ii. cross-compilation to non-existent machines
316: E. Assumptions which are made regarding the target machine
317: i. assumptions regarding the architecture of the target machine
318: ii. assumptions regarding the operating system of the target machine
319: iii. assumptions regarding software resident on the target machine
320: iv. where in the source code these assumptions are in effect made
321: III. A systematic approach to writing the files tm.h and xm.h
322: A. Macros which require special care or skill
323: B. Examples, with special reference to the underlying reasoning
324: IV. A systematic approach to writing the machine description file md
325: A. Minimal viable sets of insn descriptions
326: B. Examples, with special reference to the underlying reasoning
327: V. Uses of the file aux-output.c
328: VI. Specification of what constitutes correct performance of an
329: implementation of GCC
330: A. The components of GCC
331: B. The itinerary of a C program through GCC
332: C. A system of benchmark programs
1.1.1.5 ! root 333: D. What your RTL and assembler should look like with these benchmarks
1.1.1.4 root 334: E. Fine tuning for speed and size of compiled code
335: VII. A systematic procedure for debugging an implementation of GCC
336: A. Use of GDB
337: i. the macros in the file .gdbinit for GCC
338: ii. obstacles to the use of GDB
339: a. functions implemented as macros can't be called in GDB
340: B. Debugging without GDB
341: i. How to turn off the normal operation of GCC and access specific
342: parts of GCC
343: C. Debugging tools
344: D. Debugging the parser
345: i. how machine macros and insn definitions affect the parser
346: E. Debugging the recognizer
347: i. how machine macros and insn definitions affect the recognizer
348:
349: ditto for other components
350:
351: VIII. Data types used by GCC, with special reference to restrictions not
352: specified in the formal definition of the data type
353: IX. References to the literature for the algorithms used in GCC
354:
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