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* [Bug tree-optimization/113265] New: [Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late
@ 2024-01-08 8:25 652023330028 at smail dot nju.edu.cn
2024-01-09 8:06 ` [Bug tree-optimization/113265] " rguenth at gcc dot gnu.org
` (7 more replies)
0 siblings, 8 replies; 9+ messages in thread
From: 652023330028 at smail dot nju.edu.cn @ 2024-01-08 8:25 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113265
Bug ID: 113265
Summary: [Regression] Missed optimization for redundancy
computation elimination may be due to constant
propagation about 0 too late
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: 652023330028 at smail dot nju.edu.cn
Target Milestone: ---
Hello, we noticed that maybe there is a missed optimization for redundancy
computation elimination.
https://godbolt.org/z/drfejE5cP
int x,y,z;
void func(int a, int b){
a=b-x;
b=a;
y=0;
z=(y+a)/(-b);
}
GCC -O3 :
func(int, int):
mov ecx, DWORD PTR x[rip]
mov eax, esi
mov DWORD PTR y[rip], 0
sub eax, ecx
sub ecx, esi
cdq
idiv ecx
mov DWORD PTR z[rip], eax
ret
Earlier GCC versions get the expected optimizations:
Expected code (GCC 7.5):
func(int, int):
mov DWORD PTR y[rip], 0
mov DWORD PTR z[rip], -1
ret
The following code is optimized by gcc as expected:
void func2(int a, int b){
a=b-x;
b=a;
y=0;
z=(0+a)/(-b);
}
Comparing the code for func and func2 and their ccp1(tree), we suspect that
this issue may be caused by too late propagation about y=0.
Thank you very much for your time and effort! We look forward to hearing from
you
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug tree-optimization/113265] [Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late
2024-01-08 8:25 [Bug tree-optimization/113265] New: [Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late 652023330028 at smail dot nju.edu.cn
@ 2024-01-09 8:06 ` rguenth at gcc dot gnu.org
2024-01-09 8:16 ` rguenth at gcc dot gnu.org
` (6 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: rguenth at gcc dot gnu.org @ 2024-01-09 8:06 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113265
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |pinskia at gcc dot gnu.org
Status|UNCONFIRMED |NEW
Ever confirmed|0 |1
Keywords| |missed-optimization
Last reconfirmed| |2024-01-09
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
We're somehow expanding -b as x - b. That's done by forwprop seeing
<bb 2> :
x.0_1 = x;
a_8 = b_7(D) - x.0_1;
y = 0;
y.1_2 = y;
_3 = a_8 + y.1_2;
_4 = -a_8;
_5 = _3 / _4;
z = _5;
this transform is done irrespectively of whether the non-negated expression
is still used (its a 1:1 replacement, though unary to binary). We're
thn missing folding of
a_8 = b_7(D) - x.0_1;
_4 = x.0_1 - b_7(D);
_5 = a_8 / _4;
failing to realize this is a_8 / -a_8.
Reduced testcase which isn't optimized by GCC 7 either:
int z;
void func(int a, int b){
z=(a-b)/-(a-b);
}
void func1(int a, int b){
z=(a-b)/(b-a);
}
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug tree-optimization/113265] [Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late
2024-01-08 8:25 [Bug tree-optimization/113265] New: [Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late 652023330028 at smail dot nju.edu.cn
2024-01-09 8:06 ` [Bug tree-optimization/113265] " rguenth at gcc dot gnu.org
@ 2024-01-09 8:16 ` rguenth at gcc dot gnu.org
2024-01-09 8:43 ` pinskia at gcc dot gnu.org
` (5 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: rguenth at gcc dot gnu.org @ 2024-01-09 8:16 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113265
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |rguenth at gcc dot gnu.org
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
We have
/* X / -X is -1. */
(simplify
(div:C @0 (negate @0))
...
which doesn't use (negate_expr_p @0) but that wouldn't help because
(match negate_expr_p
(minus @0 @1)
(if ((ANY_INTEGRAL_TYPE_P (type) && TYPE_OVERFLOW_WRAPS (type))
|| (FLOAT_TYPE_P (type)
&& !HONOR_SIGN_DEPENDENT_ROUNDING (type)
&& !HONOR_SIGNED_ZEROS (type)))))
as (a - b) might be INT_MIN which we can't negate w/o invoking undefined
behavior. We also don't have (negate_expr_p @0), a way to match the
then negated form.
VN might come to the rescue to turn b - a to -(a - b) if that is available.
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug tree-optimization/113265] [Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late
2024-01-08 8:25 [Bug tree-optimization/113265] New: [Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late 652023330028 at smail dot nju.edu.cn
2024-01-09 8:06 ` [Bug tree-optimization/113265] " rguenth at gcc dot gnu.org
2024-01-09 8:16 ` rguenth at gcc dot gnu.org
@ 2024-01-09 8:43 ` pinskia at gcc dot gnu.org
2024-01-09 22:55 ` [Bug tree-optimization/113265] [11/12/13/14 Regression] " pinskia at gcc dot gnu.org
` (4 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-01-09 8:43 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113265
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Thinking on how to solve this would be have a match which says
maybe_negative_expr which matches - and minus expressions and then have an
external function which checks if -@0 matches @1 .
Though that would be gcc 15 material and should be replacing where we already
match (a-b) and (b-a) manually and that might simplify things too ...
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug tree-optimization/113265] [11/12/13/14 Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late
2024-01-08 8:25 [Bug tree-optimization/113265] New: [Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late 652023330028 at smail dot nju.edu.cn
` (2 preceding siblings ...)
2024-01-09 8:43 ` pinskia at gcc dot gnu.org
@ 2024-01-09 22:55 ` pinskia at gcc dot gnu.org
2024-01-09 23:01 ` pinskia at gcc dot gnu.org
` (3 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-01-09 22:55 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113265
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Known to fail| |8.1.0
Summary|[Regression] Missed |[11/12/13/14 Regression]
|optimization for redundancy |Missed optimization for
|computation elimination may |redundancy computation
|be due to constant |elimination may be due to
|propagation about 0 too |constant propagation about
|late |0 too late
Target Milestone|--- |11.5
Known to work| |7.5.0
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug tree-optimization/113265] [11/12/13/14 Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late
2024-01-08 8:25 [Bug tree-optimization/113265] New: [Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late 652023330028 at smail dot nju.edu.cn
` (3 preceding siblings ...)
2024-01-09 22:55 ` [Bug tree-optimization/113265] [11/12/13/14 Regression] " pinskia at gcc dot gnu.org
@ 2024-01-09 23:01 ` pinskia at gcc dot gnu.org
2024-03-07 20:52 ` law at gcc dot gnu.org
` (2 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-01-09 23:01 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113265
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Assignee|unassigned at gcc dot gnu.org |pinskia at gcc dot gnu.org
Status|NEW |ASSIGNED
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Started with r8-4394-g81bd903a6aa903 which added the `-(a - b)` -> `b - a`
simplification to match.
Anyways I am going to fix this for GCC 15.
Note
/* (A - B) == 0 ? (A - B) : (B - A) same as (B - A) */
Could be represented via:
@0 == 0 ? @0 : (maybe_neg @1)
if (expr_is_negative (@0, @1))
...
Too.
This would be merged into:
/* A == 0 ? A : -A same as -A */
even.
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug tree-optimization/113265] [11/12/13/14 Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late
2024-01-08 8:25 [Bug tree-optimization/113265] New: [Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late 652023330028 at smail dot nju.edu.cn
` (4 preceding siblings ...)
2024-01-09 23:01 ` pinskia at gcc dot gnu.org
@ 2024-03-07 20:52 ` law at gcc dot gnu.org
2024-05-26 5:43 ` [Bug tree-optimization/113265] [11/12/13/14/15 " pinskia at gcc dot gnu.org
2024-05-26 5:44 ` pinskia at gcc dot gnu.org
7 siblings, 0 replies; 9+ messages in thread
From: law at gcc dot gnu.org @ 2024-03-07 20:52 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113265
Jeffrey A. Law <law at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Priority|P3 |P2
CC| |law at gcc dot gnu.org
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug tree-optimization/113265] [11/12/13/14/15 Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late
2024-01-08 8:25 [Bug tree-optimization/113265] New: [Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late 652023330028 at smail dot nju.edu.cn
` (5 preceding siblings ...)
2024-03-07 20:52 ` law at gcc dot gnu.org
@ 2024-05-26 5:43 ` pinskia at gcc dot gnu.org
2024-05-26 5:44 ` pinskia at gcc dot gnu.org
7 siblings, 0 replies; 9+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-05-26 5:43 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113265
--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
*** Bug 113433 has been marked as a duplicate of this bug. ***
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug tree-optimization/113265] [11/12/13/14/15 Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late
2024-01-08 8:25 [Bug tree-optimization/113265] New: [Regression] Missed optimization for redundancy computation elimination may be due to constant propagation about 0 too late 652023330028 at smail dot nju.edu.cn
` (6 preceding siblings ...)
2024-05-26 5:43 ` [Bug tree-optimization/113265] [11/12/13/14/15 " pinskia at gcc dot gnu.org
@ 2024-05-26 5:44 ` pinskia at gcc dot gnu.org
7 siblings, 0 replies; 9+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-05-26 5:44 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113265
--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Created attachment 58289
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58289&action=edit
Patch without testcase
Just need to add testcases but the patch was able to bootstrap/test without any
regressions.
^ permalink raw reply [flat|nested] 9+ messages in thread
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2024-01-09 8:06 ` [Bug tree-optimization/113265] " rguenth at gcc dot gnu.org
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