+++ /dev/null
-Haifa scheduler (haifa-sched.c, loop.[ch], unroll.[ch], genattrtab.c):
-(contact law@cygnus.com before starting any serious haifa work)
-
- * Fix all the formatting problems. Simple, mindless work.
-
- * Fix/add comments throughout the code. Many of the comments are from
- the old scheduler and are out of date and misleading. Many new hunks
- of code don't have sufficient comments and documentation. Those which
- do have comments need to be rewritten to use complete sentences and
- proper formatting.
-
- * Someone needs make one (or more) passes over the scheduler as a whole to
- just clean it up. Try to move the machine dependent bits into the target
- files where they belong, avoid re-creating functions where or near
- equivalents already exist (ie is_conditional_branch and friends), etc., etc.
-
- * Document the new scheduling options. Remove those options which are
- not really useful (like reverse scheduling for example). In general
- the haifa scheduler adds _way_ too many options. I'm definitely of the
- opinion that gcc already has too many -foptions, and haifa doesn't help
- that situation.
-
- * Testing and benchmarking. We've converted a few ports to using the
- Haifa scheduler (hppa, sparc, ppc, alpha). We need to continue testing
- and benchmarking the new scheduler on additional targets.
-
- We need to have some kind of docs for how to best describe a machine to
- the haifa scheduler to get good performance. Some existing ports have
- been tuned to deal with the old scheduler -- they may need to be tuned
- to generate good schedules with haifa.
-
-
-
-Improvements to global cse and partial redundancy elimination:
-
-The current implementation of global cse uses a lazy code motion algorithm
-from Muchnick's "Advanced Compiler Design and Implementation".
-
-Longer term we want to convert to an edge based LCM algorithm using the
-new structures defined by flow.c. This allows for better expression
-placement and provides edge splitting "for free".
-
-lcm also provides the underlying framework for several additional optimizations
-such as shrink wrapping, spill code motion, dead store elimination, and generic
-load/store motion (all the other examples are subcases of load/store motion).
-
-It can probably also be used to improve the reg-stack pass of the compiler.
-
-Contact law@cygnus.com if you're interested in working on lazy code motion.
-
--------------
-
-The old PROJECTS file. Stuff I know has been done has been deleted.
-Stuff in progress has a contact name associated with it.
-has been
-
-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.
-
-* 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.)
-
-Contact meissner@cygnus.com before starting any work on live range
-splitting.
-
-* 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).
-
-This can be modeled as a partial redundancy elimination/lazy code motion
-problem. Contact law@cygnus.com before working on dead store elimination
-optimizations.
-
-* 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.
-
-* 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.
-
-Contact law@cygnus.com before working on code hoisting.
-
-* 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
-
projects / gcc.git / commitdiff