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* [Bug tree-optimization/95643] New: Optimizer fails to realize that a variable tested twice in a row is the same both times
@ 2020-06-11 16:32 josephcsible at gmail dot com
2020-06-11 18:27 ` [Bug tree-optimization/95643] " glisse at gcc dot gnu.org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: josephcsible at gmail dot com @ 2020-06-11 16:32 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95643
Bug ID: 95643
Summary: Optimizer fails to realize that a variable tested
twice in a row is the same both times
Product: gcc
Version: 10.1.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: josephcsible at gmail dot com
Target Milestone: ---
Consider this code, compiled at -O3:
extern int e;
void f(int x, int y) {
if(y) {
if(y && !x) __builtin_unreachable();
if(x) ++e;
}
}
GCC 10.1 on AMD64 produces the following assembly:
f:
testl %edi, %edi
je .L1
testl %esi, %esi
jne .L10
.L1:
ret
.L10:
addl $1, e(%rip)
ret
Godbolt link: https://godbolt.org/z/Z75QTM
The "y" in "if(y && !x)" is necessarily true, but GCC doesn't realize this,
since changing "if(y && !x)" to the equivalent "if(!x)" results in much better
assembly:
f:
testl %esi, %esi
je .L1
addl $1, e(%rip)
.L1:
ret
We should be able to generate this assembly even with the redundant check of
"y".
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/95643] Optimizer fails to realize that a variable tested twice in a row is the same both times
2020-06-11 16:32 [Bug tree-optimization/95643] New: Optimizer fails to realize that a variable tested twice in a row is the same both times josephcsible at gmail dot com
@ 2020-06-11 18:27 ` glisse at gcc dot gnu.org
2020-06-12 7:33 ` rguenth at gcc dot gnu.org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: glisse at gcc dot gnu.org @ 2020-06-11 18:27 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95643
--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
After FRE1 we have
_2 = x_9(D) == 0;
if (_2 != 0)
so we assert things for _2 and not x_9, and we lose the __builtin_unreachable
information in CCP2.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/95643] Optimizer fails to realize that a variable tested twice in a row is the same both times
2020-06-11 16:32 [Bug tree-optimization/95643] New: Optimizer fails to realize that a variable tested twice in a row is the same both times josephcsible at gmail dot com
2020-06-11 18:27 ` [Bug tree-optimization/95643] " glisse at gcc dot gnu.org
@ 2020-06-12 7:33 ` rguenth at gcc dot gnu.org
2021-05-30 22:48 ` pinskia at gcc dot gnu.org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2020-06-12 7:33 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95643
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |rguenth at gcc dot gnu.org
Ever confirmed|0 |1
Last reconfirmed| |2020-06-12
Status|UNCONFIRMED |NEW
Keywords| |easyhack
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Confirmed. value-numbering doesn't use the alternate assertion discovery
code and thus for the partly simplified condition does not record the
appropriate
expressions. Likewise EVRP lacks simple forward-propagation during this
assertion discovery (value-numbering would also need that) in
register_edge_assert_for (where it tests for defs like a & b and a | b
it needs to look for a simple boolean re-test/inversion as well).
Should be easy to amend for this case.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/95643] Optimizer fails to realize that a variable tested twice in a row is the same both times
2020-06-11 16:32 [Bug tree-optimization/95643] New: Optimizer fails to realize that a variable tested twice in a row is the same both times josephcsible at gmail dot com
2020-06-11 18:27 ` [Bug tree-optimization/95643] " glisse at gcc dot gnu.org
2020-06-12 7:33 ` rguenth at gcc dot gnu.org
@ 2021-05-30 22:48 ` pinskia at gcc dot gnu.org
2023-08-09 22:15 ` pinskia at gcc dot gnu.org
2023-08-09 22:16 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-05-30 22:48 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95643
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Severity|normal |enhancement
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/95643] Optimizer fails to realize that a variable tested twice in a row is the same both times
2020-06-11 16:32 [Bug tree-optimization/95643] New: Optimizer fails to realize that a variable tested twice in a row is the same both times josephcsible at gmail dot com
` (2 preceding siblings ...)
2021-05-30 22:48 ` pinskia at gcc dot gnu.org
@ 2023-08-09 22:15 ` pinskia at gcc dot gnu.org
2023-08-09 22:16 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-08-09 22:15 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95643
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target Milestone|--- |11.0
See Also| |https://gcc.gnu.org/bugzill
| |a/show_bug.cgi?id=95757
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/95643] Optimizer fails to realize that a variable tested twice in a row is the same both times
2020-06-11 16:32 [Bug tree-optimization/95643] New: Optimizer fails to realize that a variable tested twice in a row is the same both times josephcsible at gmail dot com
` (3 preceding siblings ...)
2023-08-09 22:15 ` pinskia at gcc dot gnu.org
@ 2023-08-09 22:16 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-08-09 22:16 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95643
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|NEW |RESOLVED
Resolution|--- |FIXED
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Fixed for GCC 11 by r11-7448-gff92ede8d269375f800e1 .
^ permalink raw reply [flat|nested] 6+ messages in thread
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2020-06-11 16:32 [Bug tree-optimization/95643] New: Optimizer fails to realize that a variable tested twice in a row is the same both times josephcsible at gmail dot com
2020-06-11 18:27 ` [Bug tree-optimization/95643] " glisse at gcc dot gnu.org
2020-06-12 7:33 ` rguenth at gcc dot gnu.org
2021-05-30 22:48 ` pinskia at gcc dot gnu.org
2023-08-09 22:15 ` pinskia at gcc dot gnu.org
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