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* [Bug tree-optimization/96696] New: Failure to optimize div+mul to mod+sub
@ 2020-08-19 0:11 gabravier at gmail dot com
2020-08-25 11:04 ` [Bug tree-optimization/96696] " rguenth at gcc dot gnu.org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: gabravier at gmail dot com @ 2020-08-19 0:11 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96696
Bug ID: 96696
Summary: Failure to optimize div+mul to mod+sub
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gabravier at gmail dot com
Target Milestone: ---
int f(int x, int y)
{
return (x / y) * y;
}
This can be optimized to `return x - (x % y);`. This transformation is done by
LLVM, but not by GCC.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/96696] Failure to optimize div+mul to mod+sub
2020-08-19 0:11 [Bug tree-optimization/96696] New: Failure to optimize div+mul to mod+sub gabravier at gmail dot com
@ 2020-08-25 11:04 ` rguenth at gcc dot gnu.org
2021-01-02 9:11 ` jakub at gcc dot gnu.org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2020-08-25 11:04 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96696
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
Last reconfirmed| |2020-08-25
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed. We have
/* Simplify (unsigned t / 2) * 2 -> unsigned t & ~1. */
(simplify
(mult (trunc_div @0 integer_pow2p@1) @1)
(if (TYPE_UNSIGNED (TREE_TYPE (@0)))
(bit_and @0 (negate @1))))
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/96696] Failure to optimize div+mul to mod+sub
2020-08-19 0:11 [Bug tree-optimization/96696] New: Failure to optimize div+mul to mod+sub gabravier at gmail dot com
2020-08-25 11:04 ` [Bug tree-optimization/96696] " rguenth at gcc dot gnu.org
@ 2021-01-02 9:11 ` jakub at gcc dot gnu.org
2021-01-15 10:51 ` rguenth at gcc dot gnu.org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: jakub at gcc dot gnu.org @ 2021-01-02 9:11 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96696
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |jakub at gcc dot gnu.org
--- Comment #2 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Do we want to do it in match.pd though, perhaps with :s or single_use check?
For GIMPLE it is 2 operations to 2 operations transformation, although
multiplication might often be more expensive than subtraction.
Or in backends only based on costs? I mean, x - (x % y) is actually longer so
might not the best idea e.g. for -Os.
But I'm more worried about the CSE possibilities if some code around does x /
y.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/96696] Failure to optimize div+mul to mod+sub
2020-08-19 0:11 [Bug tree-optimization/96696] New: Failure to optimize div+mul to mod+sub gabravier at gmail dot com
2020-08-25 11:04 ` [Bug tree-optimization/96696] " rguenth at gcc dot gnu.org
2021-01-02 9:11 ` jakub at gcc dot gnu.org
@ 2021-01-15 10:51 ` rguenth at gcc dot gnu.org
2021-01-16 11:35 ` jakub at gcc dot gnu.org
2021-04-27 12:46 ` cvs-commit at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2021-01-15 10:51 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96696
--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
I'd say doing sth at RTL expansion. For GIMPLE we should consider the cases
int g1, g2;
int f(int x, int y)
{
g1 = x / y;
return x - (x % y);
}
int g(int x, int y)
{
g2 = x % y;
return (x / y) * y;
}
in terms of CSE.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/96696] Failure to optimize div+mul to mod+sub
2020-08-19 0:11 [Bug tree-optimization/96696] New: Failure to optimize div+mul to mod+sub gabravier at gmail dot com
` (2 preceding siblings ...)
2021-01-15 10:51 ` rguenth at gcc dot gnu.org
@ 2021-01-16 11:35 ` jakub at gcc dot gnu.org
2021-04-27 12:46 ` cvs-commit at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: jakub at gcc dot gnu.org @ 2021-01-16 11:35 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96696
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |ASSIGNED
Assignee|unassigned at gcc dot gnu.org |jakub at gcc dot gnu.org
--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Created attachment 49982
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49982&action=edit
gcc11-pr96696.patch
Untested fix.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/96696] Failure to optimize div+mul to mod+sub
2020-08-19 0:11 [Bug tree-optimization/96696] New: Failure to optimize div+mul to mod+sub gabravier at gmail dot com
` (3 preceding siblings ...)
2021-01-16 11:35 ` jakub at gcc dot gnu.org
@ 2021-04-27 12:46 ` cvs-commit at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2021-04-27 12:46 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96696
--- Comment #5 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jakub Jelinek <jakub@gcc.gnu.org>:
https://gcc.gnu.org/g:3dcd1334b4f522352b80814513fdca902fc2a207
commit r12-150-g3dcd1334b4f522352b80814513fdca902fc2a207
Author: Jakub Jelinek <jakub@redhat.com>
Date: Tue Apr 27 14:45:45 2021 +0200
expand: Expand x / y * y as x - x % y if the latter is cheaper [PR96696]
The following patch tests both x / y * y and x - x % y expansion for the
former GIMPLE code and chooses the cheaper of those sequences.
2021-04-27 Jakub Jelinek <jakub@redhat.com>
PR tree-optimization/96696
* expr.c (expand_expr_divmod): New function.
(expand_expr_real_2) <case TRUNC_DIV_EXPR>: Use it for truncations
and
divisions. Formatting fixes.
<case MULT_EXPR>: Optimize x / y * y as x - x % y if the latter is
cheaper.
* gcc.target/i386/pr96696.c: New test.
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2021-04-27 12:46 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
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2020-08-19 0:11 [Bug tree-optimization/96696] New: Failure to optimize div+mul to mod+sub gabravier at gmail dot com
2020-08-25 11:04 ` [Bug tree-optimization/96696] " rguenth at gcc dot gnu.org
2021-01-02 9:11 ` jakub at gcc dot gnu.org
2021-01-15 10:51 ` rguenth at gcc dot gnu.org
2021-01-16 11:35 ` jakub at gcc dot gnu.org
2021-04-27 12:46 ` cvs-commit at gcc dot gnu.org
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