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* [Bug tree-optimization/102738] New: Failure to optimize right shift of 128-bit value after it's already been shifted by 127
@ 2021-10-14 0:36 gabravier at gmail dot com
2021-10-14 0:49 ` [Bug tree-optimization/102738] Failure to optimize right shift of 32-bit int after it's already been shifted by 31 pinskia at gcc dot gnu.org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: gabravier at gmail dot com @ 2021-10-14 0:36 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102738
Bug ID: 102738
Summary: Failure to optimize right shift of 128-bit value after
it's already been shifted by 127
Product: gcc
Version: 12.0
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 a(__int128 f, int g)
{
return (f >> 127) >> g;
}
This can be optimized to `return f >> 127;`. This optimization is done by LLVM,
but not by GCC.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/102738] Failure to optimize right shift of 32-bit int after it's already been shifted by 31
2021-10-14 0:36 [Bug tree-optimization/102738] New: Failure to optimize right shift of 128-bit value after it's already been shifted by 127 gabravier at gmail dot com
@ 2021-10-14 0:49 ` pinskia at gcc dot gnu.org
2021-10-14 0:53 ` [Bug tree-optimization/102738] Failure to optimize (signed) right shift if the range is already known to be [-1, 0] pinskia at gcc dot gnu.org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-10-14 0:49 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102738
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2021-10-14
Summary|Failure to optimize right |Failure to optimize right
|shift of 128-bit value |shift of 32-bit int after
|after it's already been |it's already been shifted
|shifted by 127 |by 31
Ever confirmed|0 |1
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
GCC does not even do the int case:
int a(int f, int g)
{
return (f >> 31) >> g;
}
Actually this can be simplified down to this really (which clang does not
handle but handles a simular case; see below):
int a(int f, int g)
{
if (f == 0 || f == -1)
return f >> g;
__builtin_unreachable();
}
This should just return f here :).
Basically if GCC knows f is already 0 or -1, then f shifted by any value is
still f.
clang is able to handle this case though
int a1(int f, int g)
{
if (f == 0 || f == 1)
return (-f) >> g;
__builtin_unreachable();
}
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/102738] Failure to optimize (signed) right shift if the range is already known to be [-1, 0]
2021-10-14 0:36 [Bug tree-optimization/102738] New: Failure to optimize right shift of 128-bit value after it's already been shifted by 127 gabravier at gmail dot com
2021-10-14 0:49 ` [Bug tree-optimization/102738] Failure to optimize right shift of 32-bit int after it's already been shifted by 31 pinskia at gcc dot gnu.org
@ 2021-10-14 0:53 ` pinskia at gcc dot gnu.org
2021-10-14 1:55 ` pinskia at gcc dot gnu.org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-10-14 0:53 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102738
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Summary|Failure to optimize right |Failure to optimize
|shift of 32-bit int after |(signed) right shift if the
|it's already been shifted |range is already known to
|by 31 |be [-1, 0]
Severity|normal |enhancement
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/102738] Failure to optimize (signed) right shift if the range is already known to be [-1, 0]
2021-10-14 0:36 [Bug tree-optimization/102738] New: Failure to optimize right shift of 128-bit value after it's already been shifted by 127 gabravier at gmail dot com
2021-10-14 0:49 ` [Bug tree-optimization/102738] Failure to optimize right shift of 32-bit int after it's already been shifted by 31 pinskia at gcc dot gnu.org
2021-10-14 0:53 ` [Bug tree-optimization/102738] Failure to optimize (signed) right shift if the range is already known to be [-1, 0] pinskia at gcc dot gnu.org
@ 2021-10-14 1:55 ` pinskia at gcc dot gnu.org
2021-10-14 18:02 ` cvs-commit at gcc dot gnu.org
2021-10-14 18:03 ` amacleod at redhat dot com
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-10-14 1:55 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102738
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
So this is interesting, I think clang/LLVM does this optimization late in their
pipeline take:
int a1(int f, int g)
{
if (f == 0 || f == 1)
return (f-1) >> g;
return 0;
}
They don't remove the shift at all. But if you change return 0 to
__builtin_unreachable(), they remove the shift.
Anyways we should be able to do this optimization even in the above case.
And even here where clang/LLVM misses too:
int a1(int f, int g)
{
if (f == 6 || f == 7)
return (f-7) >> g;
__builtin_unreachable();
}
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/102738] Failure to optimize (signed) right shift if the range is already known to be [-1, 0]
2021-10-14 0:36 [Bug tree-optimization/102738] New: Failure to optimize right shift of 128-bit value after it's already been shifted by 127 gabravier at gmail dot com
` (2 preceding siblings ...)
2021-10-14 1:55 ` pinskia at gcc dot gnu.org
@ 2021-10-14 18:02 ` cvs-commit at gcc dot gnu.org
2021-10-14 18:03 ` amacleod at redhat dot com
4 siblings, 0 replies; 6+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2021-10-14 18:02 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102738
--- Comment #3 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Andrew Macleod <amacleod@gcc.gnu.org>:
https://gcc.gnu.org/g:f0b7d4cc49ddb1c2c7474cc3f61e260aa93a96c0
commit r12-4413-gf0b7d4cc49ddb1c2c7474cc3f61e260aa93a96c0
Author: Andrew MacLeod <amacleod@redhat.com>
Date: Thu Oct 14 10:43:58 2021 -0400
Simplification for right shift.
When the first operand of a signed right shift is zero or negative one, the
RHS doesn't matter and the shift can be converted to a copy.
PR tree-optimization/102738
gcc/
* vr-values.c (simplify_using_ranges::simplify): Handle
RSHIFT_EXPR.
gcc/testsuite
* gcc.dg/pr102738.c: New.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/102738] Failure to optimize (signed) right shift if the range is already known to be [-1, 0]
2021-10-14 0:36 [Bug tree-optimization/102738] New: Failure to optimize right shift of 128-bit value after it's already been shifted by 127 gabravier at gmail dot com
` (3 preceding siblings ...)
2021-10-14 18:02 ` cvs-commit at gcc dot gnu.org
@ 2021-10-14 18:03 ` amacleod at redhat dot com
4 siblings, 0 replies; 6+ messages in thread
From: amacleod at redhat dot com @ 2021-10-14 18:03 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102738
Andrew Macleod <amacleod at redhat dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |amacleod at redhat dot com
Resolution|--- |FIXED
Status|NEW |RESOLVED
--- Comment #4 from Andrew Macleod <amacleod at redhat dot com> ---
I added this to the simplifier, so we now get all these cases right in EVRP.
I incorporated all 6 into one test.
^ permalink raw reply [flat|nested] 6+ messages in thread
end of thread, other threads:[~2021-10-14 18:03 UTC | newest]
Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
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2021-10-14 0:36 [Bug tree-optimization/102738] New: Failure to optimize right shift of 128-bit value after it's already been shifted by 127 gabravier at gmail dot com
2021-10-14 0:49 ` [Bug tree-optimization/102738] Failure to optimize right shift of 32-bit int after it's already been shifted by 31 pinskia at gcc dot gnu.org
2021-10-14 0:53 ` [Bug tree-optimization/102738] Failure to optimize (signed) right shift if the range is already known to be [-1, 0] pinskia at gcc dot gnu.org
2021-10-14 1:55 ` pinskia at gcc dot gnu.org
2021-10-14 18:02 ` cvs-commit at gcc dot gnu.org
2021-10-14 18:03 ` amacleod at redhat dot com
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