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diff --git a/gcc/PROJECTS b/gcc/PROJECTS deleted file mode 100644 index ee9be0241ed..00000000000 --- a/gcc/PROJECTS +++ /dev/null @@ -1,448 +0,0 @@ -0. Improved efficiency. - -* Parse and output array initializers an element at a time, freeing -storage after each, instead of parsing the whole initializer first and -then outputting. This would reduce memory usage for large -initializers. - -* See if the techniques describe in Oct 1991 SIGPLAN Notices -(Frazer and Hanson) are applicable to GCC. - -1. Better optimization. - -* Constants in unused inline functions - -It would be nice to delay output of string constants so that string -constants mentioned in unused inline functions are never generated. -Perhaps this would also take care of string constants in dead code. - -The difficulty is in finding a clean way for the RTL which refers -to the constant (currently, only by an assembler symbol name) -to point to the constant and cause it to be output. - -* More cse - -The techniques for doing full global cse are described in the red -dragon book, or (a different version) in Frederick Chow's thesis from -Stanford. It is likely to be slow and use a lot of memory, but it -might be worth offering as an additional option. - -It is probably possible to extend cse to a few very frequent cases -without so much expense. - -For example, it is not very hard to handle cse through if-then -statements with no else clauses. Here's how to do it. On reaching a -label, notice that the label's use-count is 1 and that the last -preceding jump jumps conditionally to this label. Now you know it -is a simple if-then statement. Remove from the hash table -all the expressions that were entered since that jump insn -and you can continue with cse. - -It is probably not hard to handle cse from the end of a loop -around to the beginning, and a few loops would be greatly sped -up by this. - -* Optimize a sequence of if statements whose conditions are exclusive. - -It is possible to optimize - - if (x == 1) ...; - if (x == 2) ...; - if (x == 3) ...; - -into - - if (x == 1) ...; - else if (x == 2) ...; - else if (x == 3) ...; - -provided that x is not altered by the contents of the if statements. - -It's not certain whether this is worth doing. Perhaps programmers -nearly always write the else's themselves, leaving few opportunities -to improve anything. - -* Un-cse. - -Perhaps we should have an un-cse step right after cse, which tries to -replace a reg with its value if the value can be substituted for the -reg everywhere, if that looks like an improvement. Which is if the -reg is used only a few times. Use rtx_cost to determine if the -change is really an improvement. - -* Clean up how cse works. - -The scheme is that each value has just one hash entry. The -first_same_value and next_same_value chains are no longer needed. - -For arithmetic, each hash table elt has the following slots: - -* Operation. This is an rtx code. -* Mode. -* Operands 0, 1 and 2. These point to other hash table elements. - -So, if we want to enter (PLUS:SI (REG:SI 30) (CONST_INT 104)), we -first enter (CONST_INT 104) and find the entry that (REG:SI 30) now -points to. Then we put these elts into operands 0 and 1 of a new elt. -We put PLUS and SI into the new elt. - -Registers and mem refs would never be entered into the table as such. -However, the values they contain would be entered. There would be a -table indexed by regno which points at the hash entry for the value in -that reg. - -The hash entry index now plays the role of a qty number. -We still need qty_first_reg, reg_next_eqv, etc. to record which regs -share a particular qty. - -When a reg is used whose contents are unknown, we need to create a -hash table entry whose contents say "unknown", as a place holder for -whatever the reg contains. If that reg is added to something, then -the hash entry for the sum will refer to the "unknown" entry. Use -UNKNOWN for the rtx code in this entry. This replaces make_new_qty. - -For a constant, a unique hash entry would be made based on the -value of the constant. - -What about MEM? Each time a memory address is referenced, we need a -qty (a hash table elt) to represent what is in it. (Just as for a -register.) If this isn't known, create one, just as for a reg whose -contents are unknown. - -We need a way to find all mem refs that still contain a certain value. -Do this with a chain of hash elts (for memory addresses) that point to -locations that hold the value. The hash elt for the value itself should -point to the start of the chain. It would be good for the hash elt -for an address to point to the hash elt for the contents of that address -(but this ptr can be null if the contents have never been entered). - -With this data structure, nothing need ever be invalidated except -the lists of which regs or mems hold a particular value. It is easy -to see if there is a reg or mem that is equiv to a particular value. -If the value is constant, it is always explicitly constant. - -* Support more general tail-recursion among different functions. - -This might be possible under certain circumstances, such as when -the argument lists of the functions have the same lengths. -Perhaps it could be done with a special declaration. - -You would need to verify in the calling function that it does not -use the addresses of any local variables and does not use setjmp. - -* Put short statics vars at low addresses and use short addressing mode? - -Useful on the 68000/68020 and perhaps on the 32000 series, -provided one has a linker that works with the feature. -This is said to make a 15% speedup on the 68000. - -* Keep global variables in registers. - -Here is a scheme for doing this. A global variable, or a local variable -whose address is taken, can be kept in a register for an entire function -if it does not use non-constant memory addresses and (for globals only) -does not call other functions. If the entire function does not meet -this criterion, a loop may. - -The VAR_DECL for such a variable would have to have two RTL expressions: -the true home in memory, and the pseudo-register used temporarily. -It is necessary to emit insns to copy the memory location into the -pseudo-register at the beginning of the function or loop, and perhaps -back out at the end. These insns should have REG_EQUIV notes so that, -if the pseudo-register does not get a hard register, it is spilled into -the memory location which exists in any case. - -The easiest way to set up these insns is to modify the routine -put_var_into_stack so that it does not apply to the entire function -(sparing any loops which contain nothing dangerous) and to call it at -the end of the function regardless of where in the function the -address of a local variable is taken. It would be called -unconditionally at the end of the function for all relevant global -variables. - -For debugger output, the thing to do is to invent a new binding level -around the appropriate loop and define the variable name as a register -variable with that scope. - -* Live-range splitting. - -Currently a variable is allocated a hard register either for the full -extent of its use or not at all. Sometimes it would be good to -allocate a variable a hard register for just part of a function; for -example, through a particular loop where the variable is mostly used, -or outside of a particular loop where the variable is not used. (The -latter is nice because it might let the variable be in a register most -of the time even though the loop needs all the registers.) - -It might not be very hard to do this in global.c when a variable -fails to get a hard register for its entire life span. - -The first step is to find a loop in which the variable is live, but -which is not the whole life span or nearly so. It's probably best to -use a loop in which the variable is heavily used. - -Then create a new pseudo-register to represent the variable in that loop. -Substitute this for the old pseudo-register there, and insert move insns -to copy between the two at the loop entry and all exits. (When several -such moves are inserted at the same place, some new feature should be -added to say that none of those registers conflict merely because of -overlap between the new moves. And the reload pass should reorder them -so that a store precedes a load, for any given hard register.) - -After doing this for all the reasonable candidates, run global-alloc -over again. With luck, one of the two pseudo-registers will be fit -somewhere. It may even have a much higher priority due to its reduced -life span. - -There will be no room in general for the new pseudo-registers in -basic_block_live_at_start, so there will need to be a second such -matrix exclusively for the new ones. Various other vectors indexed by -register number will have to be made bigger, or there will have to be -secondary extender vectors just for global-alloc. - -A simple new feature could arrange that both pseudo-registers get the -same stack slot if they both fail to get hard registers. - -Other compilers split live ranges when they are not connected, or -try to split off pieces `at the edge'. I think splitting around loops -will provide more speedup. - -Creating a fake binding block and a new like-named variable with -shorter life span and different address might succeed in describing -this technique for the debugger. - -* Detect dead stores into memory? - -A store into memory is dead if it is followed by another store into -the same location; and, in between, there is no reference to anything -that might be that location (including no reference to a variable -address). - -* Loop optimization. - -Strength reduction and iteration variable elimination could be -smarter. They should know how to decide which iteration variables are -not worth making explicit because they can be computed as part of an -address calculation. Based on this information, they should decide -when it is desirable to eliminate one iteration variable and create -another in its place. - -It should be possible to compute what the value of an iteration -variable will be at the end of the loop, and eliminate the variable -within the loop by computing that value at the loop end. - -When a loop has a simple increment that adds 1, -instead of jumping in after the increment, -decrement the loop count and jump to the increment. -This allows aob insns to be used. - -* Using constraints on values. - -Many operations could be simplified based on knowledge of the -minimum and maximum possible values of a register at any particular time. -These limits could come from the data types in the tree, via rtl generation, -or they can be deduced from operations that are performed. For example, -the result of an `and' operation one of whose operands is 7 must be in -the range 0 to 7. Compare instructions also tell something about the -possible values of the operand, in the code beyond the test. - -Value constraints can be used to determine the results of a further -comparison. They can also indicate that certain `and' operations are -redundant. Constraints might permit a decrement and branch -instruction that checks zeroness to be used when the user has -specified to exit if negative. - -* Smarter reload pass. - -The reload pass as currently written can reload values only into registers -that are reserved for reloading. This means that in order to use a -register for reloading it must spill everything out of that register. - -It would be straightforward, though complicated, for reload1.c to keep -track, during its scan, of which hard registers were available at each -point in the function, and use for reloading even registers that were -free only at the point they were needed. This would avoid much spilling -and make better code. - -* Change the type of a variable. - -Sometimes a variable is declared as `int', it is assigned only once -from a value of type `char', and then it is used only by comparison -against constants. On many machines, better code would result if -the variable had type `char'. If the compiler could detect this -case, it could change the declaration of the variable and change -all the places that use it. - -* Better handling for very sparse switches. - -There may be cases where it would be better to compile a switch -statement to use a fixed hash table rather than the current -combination of jump tables and binary search. - -* Order of subexpressions. - -It might be possible to make better code by paying attention -to the order in which to generate code for subexpressions of an expression. - -* More code motion. - -Consider hoisting common code up past conditional branches or -tablejumps. - -* Trace scheduling. - -This technique is said to be able to figure out which way a jump -will usually go, and rearrange the code to make that path the -faster one. - -* Distributive law. - -The C expression *(X + 4 * (Y + C)) compiles better on certain -machines if rewritten as *(X + 4*C + 4*Y) because of known addressing -modes. It may be tricky to determine when, and for which machines, to -use each alternative. - -Some work has been done on this, in combine.c. - -* Can optimize by changing if (x) y; else z; into z; if (x) y; -if z and x do not interfere and z has no effects not undone by y. -This is desirable if z is faster than jumping. - -* For a two-insn loop on the 68020, such as - foo: movb a2@+,a3@+ - jne foo -it is better to insert dbeq d0,foo before the jne. -d0 can be a junk register. The challenge is to fit this into -a portable framework: when can you detect this situation and -still be able to allocate a junk register? - -2. Simpler porting. - -Right now, describing the target machine's instructions is done -cleanly, but describing its addressing mode is done with several -ad-hoc macro definitions. Porting would be much easier if there were -an RTL description for addressing modes like that for instructions. -Tools analogous to genflags and genrecog would generate macros from -this description. - -There would be one pattern in the address-description file for each -kind of addressing, and this pattern would have: - - * the RTL expression for the address - * C code to verify its validity (since that may depend on - the exact data). - * C code to print the address in assembler language. - * C code to convert the address into a valid one, if it is not valid. - (This would replace LEGITIMIZE_ADDRESS). - * Register constraints for all indeterminates that appear - in the RTL expression. - -3. Other languages. - -Front ends for Pascal, Fortran, Algol, Cobol, Modula-2 and Ada are -desirable. - -Pascal, Modula-2 and Ada require the implementation of functions -within functions. Some of the mechanisms for this already exist. - -4. More extensions. - -* Generated unique labels. Have some way of generating distinct labels -for use in extended asm statements. I don't know what a good syntax would -be. - -* A way of defining a structure containing a union, in which the choice of -union alternative is controlled by a previous structure component. - -Here is a possible syntax for this. - -struct foo { - enum { INT, DOUBLE } code; - auto union { case INT: int i; case DOUBLE: double d;} value : code; -}; - -* Allow constructor expressions as lvalues, like this: - - (struct foo) {a, b, c} = foo(); - -This would call foo, which returns a structure, and then store the -several components of the structure into the variables a, b, and c. - -5. Generalize the machine model. - -* Some new compiler features may be needed to do a good job on machines -where static data needs to be addressed using base registers. - -* Some machines have two stacks in different areas of memory, one used -for scalars and another for large objects. The compiler does not -now have a way to understand this. - -6. Useful warnings. - -* Warn about statements that are undefined because the order of -evaluation of increment operators makes a big difference. Here is an -example: - - *foo++ = hack (*foo); - -7. Better documentation of how GCC works and how to port it. - -Here is an outline proposed by Allan Adler. - -I. Overview of this document -II. The machines on which GCC is implemented - A. Prose description of those characteristics of target machines and - their operating systems which are pertinent to the implementation - of GCC. - i. target machine characteristics - ii. comparison of this system of machine characteristics with - other systems of machine specification currently in use - B. Tables of the characteristics of the target machines on which - GCC is implemented. - C. A priori restrictions on the values of characteristics of target - machines, with special reference to those parts of the source code - which entail those restrictions - i. restrictions on individual characteristics - ii. restrictions involving relations between various characteristics - D. The use of GCC as a cross-compiler - i. cross-compilation to existing machines - ii. cross-compilation to non-existent machines - E. Assumptions which are made regarding the target machine - i. assumptions regarding the architecture of the target machine - ii. assumptions regarding the operating system of the target machine - iii. assumptions regarding software resident on the target machine - iv. where in the source code these assumptions are in effect made -III. A systematic approach to writing the files tm.h and xm.h - A. Macros which require special care or skill - B. Examples, with special reference to the underlying reasoning -IV. A systematic approach to writing the machine description file md - A. Minimal viable sets of insn descriptions - B. Examples, with special reference to the underlying reasoning -V. Uses of the file aux-output.c -VI. Specification of what constitutes correct performance of an - implementation of GCC - A. The components of GCC - B. The itinerary of a C program through GCC - C. A system of benchmark programs - D. What your RTL and assembler should look like with these benchmarks - E. Fine tuning for speed and size of compiled code -VII. A systematic procedure for debugging an implementation of GCC - A. Use of GDB - i. the macros in the file .gdbinit for GCC - ii. obstacles to the use of GDB - a. functions implemented as macros can't be called in GDB - B. Debugging without GDB - i. How to turn off the normal operation of GCC and access specific - parts of GCC - C. Debugging tools - D. Debugging the parser - i. how machine macros and insn definitions affect the parser - E. Debugging the recognizer - i. how machine macros and insn definitions affect the recognizer - -ditto for other components - -VIII. Data types used by GCC, with special reference to restrictions not - specified in the formal definition of the data type -IX. References to the literature for the algorithms used in GCC - |