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Commit f4e584dc by Jeffrey A Law Committed by Jeff Law
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gcse.c: Update various comments. 

        * gcse.c: Update various comments.
        (current_function_calls_longjmp): Delete declaration.

From-SVN: r25674
parent e78d9500
Showing with 49 additions and 81 deletions
gcc/ChangeLog
Wed Mar 10 20:28:29 1999 Jeffrey A Law (law@cygnus.com)
* gcse.c: Update various comments.
(current_function_calls_longjmp): Delete declaration.
* gcse.c (run_jump_opt_after_gcse): New variable.
(gcse_main): Returns an integer.
(hash_scan_set): Record initializations from CONST_DOUBLEs too.
......
gcc/gcse.c
/* Global common subexpression elimination
/* Global common subexpression elimination/Partial redundancy elimination
and global constant/copy propagation for GNU compiler.
Copyright (C) 1997, 1998, 1999 Free Software Foundation, Inc.
......@@ -24,18 +24,17 @@ Boston, MA 02111-1307, USA. */
- do rough calc of how many regs are needed in each block, and a rough
calc of how many regs are available in each class and use that to
throttle back the code in cases where RTX_COST is minimal.
- memory aliasing support
- dead store elimination
- a store to the same address as a load does not kill the load if the
source of the store is also the destination of the load. Handling this
allows more load motion, particularly out of loops.
- ability to realloc sbitmap vectors would allow one initial computation
of reg_set_in_block with only subsequent additions, rather than
recomputing it for each pass
NOTES
- the classic gcse implementation is kept in for now for comparison
*/
/* References searched while implementing this.
This list will eventually be deleted but I wanted to have a record of it
for now.
Compilers Principles, Techniques and Tools
Aho, Sethi, Ullman
......@@ -49,12 +48,6 @@ Boston, MA 02111-1307, USA. */
Frederick Chow
Stanford Ph.D. thesis, Dec. 1983
xxx
Elimination Algorithms for Data Flow Analysis
B.G. Ryder, M.C. Paull
ACM Computing Surveys, Vol. 18, Num. 3, Sep. 1986
reread
A Fast Algorithm for Code Movement Optimization
D.M. Dhamdhere
SIGPLAN Notices, Vol. 23, Num. 10, Oct. 1988
......@@ -74,11 +67,6 @@ reread
R. Cytron, J. Ferrante, B.K. Rosen, M.N. Wegman, and F.K. Zadeck
ACM TOPLAS, Vol. 13, Num. 4, Oct. 1991
yyy
How to Analyze Large Programs Efficiently and Informatively
D.M. Dhamdhere, B.K. Rosen, F.K. Zadeck
ACM SIGPLAN Notices Vol. 27, Num. 7, Jul. 1992, '92 Conference on PLDI
Lazy Code Motion
J. Knoop, O. Ruthing, B. Steffen
ACM SIGPLAN Notices Vol. 27, Num. 7, Jul. 1992, '92 Conference on PLDI
......@@ -134,13 +122,24 @@ yyy
Michael Wolfe
Addison-Wesley, 1996
People wishing to speed up the code here should read xxx, yyy.
Advanced Compiler Design and Implementation
Steven Muchnick
Morgan Kaufmann, 1997
People wishing to speed up the code here should read:
Elimination Algorithms for Data Flow Analysis
B.G. Ryder, M.C. Paull
ACM Computing Surveys, Vol. 18, Num. 3, Sep. 1986
How to Analyze Large Programs Efficiently and Informatively
D.M. Dhamdhere, B.K. Rosen, F.K. Zadeck
ACM SIGPLAN Notices Vol. 27, Num. 7, Jul. 1992, '92 Conference on PLDI
People wishing to do something different can find various possibilities
in the above papers and elsewhere.
*/
#include "config.h"
/* Must precede rtl.h for FFS. */
#include "system.h"
#include "rtl.h"
......@@ -174,11 +173,10 @@ yyy
heuristics into gcse.c. */
#define FOLLOW_BACK_EDGES 1
/* We support two GCSE implementations: Classic GCSE (i.e. Dragon Book)
and PRE (Partial Redundancy Elimination) GCSE (based on Fred Chow's thesis).
The default is PRE.
/* We support GCSE via Partial Redundancy Elimination. PRE optimizations
are a superset of those done by GCSE.
In either case we perform the following steps:
We perform the following steps:
1) Compute basic block information.
......@@ -202,10 +200,7 @@ yyy
Function want_to_gcse_p says what these are.
PRE handles moving invariant expressions out of loops (by treating them as
partially redundant). This feature of PRE is disabled here (by not
propagating dataflow information along back edges) because loop.c has more
involved (and thus typically better) heuristics for what to move out of
loops.
partially redundant).
Eventually it would be nice to replace cse.c/gcse.c with SSA (static single
assignment) based GVN (global value numbering). L. T. Simpson's paper
......@@ -254,7 +249,7 @@ yyy
argue it is not. The number of iterations for the algorithm to converge
is typically 2-4 so I don't view it as that expensive (relatively speaking).
PRE GCSE depends heavily on the seconds CSE pass to clean up the copies
PRE GCSE depends heavily on the second CSE pass to clean up the copies
we create. To make an expression reach the place where it's redundant,
the result of the expression is copied to a new register, and the redundant
expression is deleted by replacing it with this new register. Classic GCSE
......@@ -263,35 +258,6 @@ yyy
**********************
When -fclassic-gcse, we perform a classic global CSE pass.
It is based on the algorithms in the Dragon book, and is based on code
written by Devor Tevi at Intel.
The steps for Classic GCSE are:
1) Build the hash table of expressions we wish to GCSE (expr_hash_table).
Also recorded are reaching definition "gen" statements (rd_gen)
2) Compute the reaching definitions (reaching_defs).
This is a bitmap for each basic block indexed by INSN_CUID that is 1
for each statement containing a definition that reaches the block.
3) Compute the available expressions (ae_in).
This is a bitmap for each basic block indexed by expression number
that is 1 for each expression that is available at the beginning of
the block.
4) Perform GCSE.
This is done by scanning each instruction looking for sets of the form
(set (pseudo-reg) (expression)) and checking if `expression' is in the
hash table. If it is, and if the expression is available, and if only
one computation of the expression reaches the instruction, we substitute
the expression for a register containing its value. If there is no
such register, but the expression is expensive enough we create an
instruction to copy the result of the expression into and use that.
**********************
A fair bit of simplicity is created by creating small functions for simple
tasks, even when the function is only called in one place. This may
measurably slow things down [or may not] by creating more function call
......@@ -307,6 +273,23 @@ yyy
/* -dG dump file. */
static FILE *gcse_file;
/* Note whether or not we should run jump optimization after gcse. We
want to do this for two cases.
* If we changed any jumps via cprop.
* If we added any labels via edge splitting. */
static int run_jump_opt_after_gcse;
/* Element I is a list of I's predecessors/successors. */
static int_list_ptr *s_preds;
static int_list_ptr *s_succs;
/* Element I is the number of predecessors/successors of basic block I. */
static int *num_preds;
static int *num_succs;
/* Bitmaps are normally not included in debugging dumps.
However it's useful to be able to print them from GDB.
We could create special functions for this, but it's simpler to
......@@ -325,23 +308,6 @@ static char can_copy_p[(int) NUM_MACHINE_MODES];
/* Non-zero if can_copy_p has been initialized. */
static int can_copy_init_p;
/* Element I is a list of I's predecessors/successors. */
static int_list_ptr *s_preds;
static int_list_ptr *s_succs;
/* Element I is the number of predecessors/successors of basic block I. */
static int *num_preds;
static int *num_succs;
/* Note whether or not we should run jump optimization after gcse. We
want to do this for two cases.
* If we changed any jumps via cprop.
* If we added any labels via edge splitting. */
static int run_jump_opt_after_gcse;
/* Hash table of expressions. */
struct expr
......@@ -354,8 +320,9 @@ struct expr
struct expr *next_same_hash;
/* List of anticipatable occurrences in basic blocks in the function.
An "anticipatable occurrence" is one that is the first occurrence in the
basic block and the operands are not modified in the basic block prior
to the occurrence. */
basic block, the operands are not modified in the basic block prior
to the occurrence and the output is not used between the start of
the block and the occurrence. */
struct occr *antic_occr;
/* List of available occurrence in basic blocks in the function.
An "available occurrence" is one that is the last occurrence in the
......@@ -444,11 +411,10 @@ static int n_sets;
For simplicity, GCSE is done on sets of pseudo-regs only. PRE GCSE only
requires knowledge of which blocks kill which regs [and thus could use
a bitmap instead of the lists `reg_set_table' uses]. The classic GCSE
uses the information in lists.
a bitmap instead of the lists `reg_set_table' uses].
If the classic GCSE pass is deleted `reg_set_table' and could be turned
into an array of bitmaps (num-bbs x num-regs)
`reg_set_table' and could be turned into an array of bitmaps
(num-bbs x num-regs)
[however perhaps it may be useful to keep the data as is].
One advantage of recording things this way is that `reg_set_table' is
fairly sparse with respect to pseudo regs but for hard regs could be
......@@ -515,7 +481,6 @@ static int copy_prop_count;
extern char *current_function_name;
extern int current_function_calls_setjmp;
extern int current_function_calls_longjmp;
/* These variables are used by classic GCSE.
Normally they'd be defined a bit later, but `rd_gen' needs to
......
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