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* [Bug tree-optimization/114545] New: [11/12/13/14 Regression] Missed optimization for CSE
@ 2024-04-01 9:20 652023330028 at smail dot nju.edu.cn
2024-04-01 21:11 ` [Bug tree-optimization/114545] " pinskia at gcc dot gnu.org
` (3 more replies)
0 siblings, 4 replies; 5+ messages in thread
From: 652023330028 at smail dot nju.edu.cn @ 2024-04-01 9:20 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114545
Bug ID: 114545
Summary: [11/12/13/14 Regression] Missed optimization for CSE
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 there may be a missed CSE (t+c). Here is the reduced
code:
https://godbolt.org/z/f68qWa89s
int a, b, c;
void func() {
int t=0;
t = -a;
b = -(t + c);
a = t + c;
}
GCC -O3 -fwrapv:
func():
mov edx, DWORD PTR a[rip]
mov eax, DWORD PTR c[rip]
mov ecx, edx
sub ecx, eax
sub eax, edx
mov DWORD PTR b[rip], ecx
mov DWORD PTR a[rip], eax
ret
Expected code:
GCC-7.5:
func():
mov eax, DWORD PTR c[rip]
sub eax, DWORD PTR a[rip]
mov edx, eax
mov DWORD PTR a[rip], eax
neg edx
mov DWORD PTR b[rip], edx
ret
Thank you very much for your time and effort! We look forward to hearing from
you.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/114545] [11/12/13/14 Regression] Missed optimization for CSE
2024-04-01 9:20 [Bug tree-optimization/114545] New: [11/12/13/14 Regression] Missed optimization for CSE 652023330028 at smail dot nju.edu.cn
@ 2024-04-01 21:11 ` pinskia at gcc dot gnu.org
2024-04-01 21:18 ` pinskia at gcc dot gnu.org
` (2 subsequent siblings)
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-04-01 21:11 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114545
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target Milestone|--- |11.5
Keywords| |missed-optimization
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/114545] [11/12/13/14 Regression] Missed optimization for CSE
2024-04-01 9:20 [Bug tree-optimization/114545] New: [11/12/13/14 Regression] Missed optimization for CSE 652023330028 at smail dot nju.edu.cn
2024-04-01 21:11 ` [Bug tree-optimization/114545] " pinskia at gcc dot gnu.org
@ 2024-04-01 21:18 ` pinskia at gcc dot gnu.org
2024-04-05 2:34 ` law at gcc dot gnu.org
2024-07-19 13:24 ` [Bug tree-optimization/114545] [12/13/14/15 " rguenth at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: pinskia at gcc dot gnu.org @ 2024-04-01 21:18 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114545
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I am not sure this is worse.
In the GCC 7 case we have:
```
sub eax, DWORD PTR a[rip]
mov edx, eax
...
neg edx
```
While in GCC 8+ we get:
```
movl %edx, %ecx
subl %eax, %ecx
subl %edx, %eax
```
In the case of GCC 8, we have 2 independent sub and still a move. In GCC 7 we
get one sub followed by a move an dependent neg. The latency for the GCC 8+
will be less than what was done for GCC 7 because both sub can happen at the
same time and the mov (which only happens on x86_64) is removed during rename.
aarch64 produces for GCC 8+:
```
adrp x1, a
adrp x2, c
adrp x3, b
ldr w0, [x1, #:lo12:a]
ldr w2, [x2, #:lo12:c]
sub w4, w2, w0
sub w0, w0, w2
str w4, [x1, #:lo12:a]
str w0, [x3, #:lo12:b]
ret
```
While before:
```
adrp x1, a
adrp x0, c
adrp x2, b
ldr w3, [x1, #:lo12:a]
ldr w0, [x0, #:lo12:c]
sub w0, w0, w3
str w0, [x1, #:lo12:a]
neg w0, w0
str w0, [x2, #:lo12:b]
ret
```
So the neg will issue with the first str but if you have 2 store units and 2
ALUs, the GCC 8+ is better.
So for superscalars, what GCC 8+ is doing is better and even in order cores,
GCC 8+ will still be better due to the 2 independent instructions
I think only at -Os/-Oz it might make a difference for x86_64 really.
^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug tree-optimization/114545] [11/12/13/14 Regression] Missed optimization for CSE
2024-04-01 9:20 [Bug tree-optimization/114545] New: [11/12/13/14 Regression] Missed optimization for CSE 652023330028 at smail dot nju.edu.cn
2024-04-01 21:11 ` [Bug tree-optimization/114545] " pinskia at gcc dot gnu.org
2024-04-01 21:18 ` pinskia at gcc dot gnu.org
@ 2024-04-05 2:34 ` law at gcc dot gnu.org
2024-07-19 13:24 ` [Bug tree-optimization/114545] [12/13/14/15 " rguenth at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: law at gcc dot gnu.org @ 2024-04-05 2:34 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114545
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] 5+ messages in thread
* [Bug tree-optimization/114545] [12/13/14/15 Regression] Missed optimization for CSE
2024-04-01 9:20 [Bug tree-optimization/114545] New: [11/12/13/14 Regression] Missed optimization for CSE 652023330028 at smail dot nju.edu.cn
` (2 preceding siblings ...)
2024-04-05 2:34 ` law at gcc dot gnu.org
@ 2024-07-19 13:24 ` rguenth at gcc dot gnu.org
3 siblings, 0 replies; 5+ messages in thread
From: rguenth at gcc dot gnu.org @ 2024-07-19 13:24 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114545
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target Milestone|11.5 |12.5
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
GCC 11 branch is being closed.
^ permalink raw reply [flat|nested] 5+ messages in thread
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2024-04-01 9:20 [Bug tree-optimization/114545] New: [11/12/13/14 Regression] Missed optimization for CSE 652023330028 at smail dot nju.edu.cn
2024-04-01 21:11 ` [Bug tree-optimization/114545] " pinskia at gcc dot gnu.org
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2024-07-19 13:24 ` [Bug tree-optimization/114545] [12/13/14/15 " rguenth at gcc dot gnu.org
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