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gcse.c: Update various comments.
* gcse.c: Update various comments.
(current_function_calls_longjmp): Delete declaration.
From-SVN: r25674
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Jeffrey A Law
Jeff Law
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commit
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@ -1,5 +1,8 @@
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Wed Mar 10 20:28:29 1999 Jeffrey A Law (law@cygnus.com)
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* gcse.c: Update various comments.
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(current_function_calls_longjmp): Delete declaration.
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* gcse.c (run_jump_opt_after_gcse): New variable.
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(gcse_main): Returns an integer.
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(hash_scan_set): Record initializations from CONST_DOUBLEs too.
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127
gcc/gcse.c
127
gcc/gcse.c
@ -1,4 +1,4 @@
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/* Global common subexpression elimination
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/* Global common subexpression elimination/Partial redundancy elimination
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and global constant/copy propagation for GNU compiler.
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Copyright (C) 1997, 1998, 1999 Free Software Foundation, Inc.
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@ -24,18 +24,17 @@ Boston, MA 02111-1307, USA. */
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- do rough calc of how many regs are needed in each block, and a rough
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calc of how many regs are available in each class and use that to
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throttle back the code in cases where RTX_COST is minimal.
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- memory aliasing support
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- dead store elimination
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- a store to the same address as a load does not kill the load if the
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source of the store is also the destination of the load. Handling this
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allows more load motion, particularly out of loops.
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- ability to realloc sbitmap vectors would allow one initial computation
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of reg_set_in_block with only subsequent additions, rather than
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recomputing it for each pass
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NOTES
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- the classic gcse implementation is kept in for now for comparison
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*/
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/* References searched while implementing this.
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This list will eventually be deleted but I wanted to have a record of it
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for now.
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Compilers Principles, Techniques and Tools
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Aho, Sethi, Ullman
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@ -49,12 +48,6 @@ Boston, MA 02111-1307, USA. */
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Frederick Chow
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Stanford Ph.D. thesis, Dec. 1983
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xxx
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Elimination Algorithms for Data Flow Analysis
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B.G. Ryder, M.C. Paull
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ACM Computing Surveys, Vol. 18, Num. 3, Sep. 1986
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reread
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A Fast Algorithm for Code Movement Optimization
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D.M. Dhamdhere
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SIGPLAN Notices, Vol. 23, Num. 10, Oct. 1988
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@ -74,11 +67,6 @@ reread
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R. Cytron, J. Ferrante, B.K. Rosen, M.N. Wegman, and F.K. Zadeck
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ACM TOPLAS, Vol. 13, Num. 4, Oct. 1991
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yyy
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How to Analyze Large Programs Efficiently and Informatively
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D.M. Dhamdhere, B.K. Rosen, F.K. Zadeck
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ACM SIGPLAN Notices Vol. 27, Num. 7, Jul. 1992, '92 Conference on PLDI
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Lazy Code Motion
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J. Knoop, O. Ruthing, B. Steffen
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ACM SIGPLAN Notices Vol. 27, Num. 7, Jul. 1992, '92 Conference on PLDI
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@ -134,13 +122,24 @@ yyy
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Michael Wolfe
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Addison-Wesley, 1996
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People wishing to speed up the code here should read xxx, yyy.
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Advanced Compiler Design and Implementation
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Steven Muchnick
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Morgan Kaufmann, 1997
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People wishing to speed up the code here should read:
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Elimination Algorithms for Data Flow Analysis
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B.G. Ryder, M.C. Paull
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ACM Computing Surveys, Vol. 18, Num. 3, Sep. 1986
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How to Analyze Large Programs Efficiently and Informatively
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D.M. Dhamdhere, B.K. Rosen, F.K. Zadeck
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ACM SIGPLAN Notices Vol. 27, Num. 7, Jul. 1992, '92 Conference on PLDI
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People wishing to do something different can find various possibilities
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in the above papers and elsewhere.
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*/
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#include "config.h"
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/* Must precede rtl.h for FFS. */
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#include "system.h"
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#include "rtl.h"
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@ -174,11 +173,10 @@ yyy
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heuristics into gcse.c. */
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#define FOLLOW_BACK_EDGES 1
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/* We support two GCSE implementations: Classic GCSE (i.e. Dragon Book)
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and PRE (Partial Redundancy Elimination) GCSE (based on Fred Chow's thesis).
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The default is PRE.
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/* We support GCSE via Partial Redundancy Elimination. PRE optimizations
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are a superset of those done by GCSE.
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In either case we perform the following steps:
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We perform the following steps:
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1) Compute basic block information.
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@ -202,10 +200,7 @@ yyy
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Function want_to_gcse_p says what these are.
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PRE handles moving invariant expressions out of loops (by treating them as
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partially redundant). This feature of PRE is disabled here (by not
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propagating dataflow information along back edges) because loop.c has more
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involved (and thus typically better) heuristics for what to move out of
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loops.
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partially redundant).
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Eventually it would be nice to replace cse.c/gcse.c with SSA (static single
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assignment) based GVN (global value numbering). L. T. Simpson's paper
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@ -254,7 +249,7 @@ yyy
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argue it is not. The number of iterations for the algorithm to converge
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is typically 2-4 so I don't view it as that expensive (relatively speaking).
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PRE GCSE depends heavily on the seconds CSE pass to clean up the copies
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PRE GCSE depends heavily on the second CSE pass to clean up the copies
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we create. To make an expression reach the place where it's redundant,
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the result of the expression is copied to a new register, and the redundant
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expression is deleted by replacing it with this new register. Classic GCSE
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@ -263,35 +258,6 @@ yyy
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**********************
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When -fclassic-gcse, we perform a classic global CSE pass.
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It is based on the algorithms in the Dragon book, and is based on code
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written by Devor Tevi at Intel.
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The steps for Classic GCSE are:
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1) Build the hash table of expressions we wish to GCSE (expr_hash_table).
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Also recorded are reaching definition "gen" statements (rd_gen)
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2) Compute the reaching definitions (reaching_defs).
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This is a bitmap for each basic block indexed by INSN_CUID that is 1
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for each statement containing a definition that reaches the block.
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3) Compute the available expressions (ae_in).
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This is a bitmap for each basic block indexed by expression number
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that is 1 for each expression that is available at the beginning of
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the block.
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4) Perform GCSE.
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This is done by scanning each instruction looking for sets of the form
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(set (pseudo-reg) (expression)) and checking if `expression' is in the
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hash table. If it is, and if the expression is available, and if only
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one computation of the expression reaches the instruction, we substitute
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the expression for a register containing its value. If there is no
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such register, but the expression is expensive enough we create an
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instruction to copy the result of the expression into and use that.
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**********************
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A fair bit of simplicity is created by creating small functions for simple
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tasks, even when the function is only called in one place. This may
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measurably slow things down [or may not] by creating more function call
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@ -307,6 +273,23 @@ yyy
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/* -dG dump file. */
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static FILE *gcse_file;
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/* Note whether or not we should run jump optimization after gcse. We
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want to do this for two cases.
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* If we changed any jumps via cprop.
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* If we added any labels via edge splitting. */
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static int run_jump_opt_after_gcse;
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/* Element I is a list of I's predecessors/successors. */
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static int_list_ptr *s_preds;
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static int_list_ptr *s_succs;
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/* Element I is the number of predecessors/successors of basic block I. */
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static int *num_preds;
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static int *num_succs;
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/* Bitmaps are normally not included in debugging dumps.
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However it's useful to be able to print them from GDB.
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We could create special functions for this, but it's simpler to
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@ -325,23 +308,6 @@ static char can_copy_p[(int) NUM_MACHINE_MODES];
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/* Non-zero if can_copy_p has been initialized. */
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static int can_copy_init_p;
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/* Element I is a list of I's predecessors/successors. */
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static int_list_ptr *s_preds;
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static int_list_ptr *s_succs;
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/* Element I is the number of predecessors/successors of basic block I. */
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static int *num_preds;
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static int *num_succs;
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/* Note whether or not we should run jump optimization after gcse. We
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want to do this for two cases.
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* If we changed any jumps via cprop.
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* If we added any labels via edge splitting. */
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static int run_jump_opt_after_gcse;
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/* Hash table of expressions. */
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struct expr
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@ -354,8 +320,9 @@ struct expr
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struct expr *next_same_hash;
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/* List of anticipatable occurrences in basic blocks in the function.
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An "anticipatable occurrence" is one that is the first occurrence in the
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basic block and the operands are not modified in the basic block prior
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to the occurrence. */
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basic block, the operands are not modified in the basic block prior
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to the occurrence and the output is not used between the start of
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the block and the occurrence. */
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struct occr *antic_occr;
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/* List of available occurrence in basic blocks in the function.
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An "available occurrence" is one that is the last occurrence in the
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@ -444,11 +411,10 @@ static int n_sets;
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For simplicity, GCSE is done on sets of pseudo-regs only. PRE GCSE only
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requires knowledge of which blocks kill which regs [and thus could use
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a bitmap instead of the lists `reg_set_table' uses]. The classic GCSE
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uses the information in lists.
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a bitmap instead of the lists `reg_set_table' uses].
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If the classic GCSE pass is deleted `reg_set_table' and could be turned
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into an array of bitmaps (num-bbs x num-regs)
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`reg_set_table' and could be turned into an array of bitmaps
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(num-bbs x num-regs)
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[however perhaps it may be useful to keep the data as is].
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One advantage of recording things this way is that `reg_set_table' is
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fairly sparse with respect to pseudo regs but for hard regs could be
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@ -515,7 +481,6 @@ static int copy_prop_count;
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extern char *current_function_name;
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extern int current_function_calls_setjmp;
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extern int current_function_calls_longjmp;
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/* These variables are used by classic GCSE.
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Normally they'd be defined a bit later, but `rd_gen' needs to
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