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* [Bug tree-optimization/111724] New: [Regression] Missed optimizations probably because of too early arithmetic optimization
@ 2023-10-08 7:55 652023330028 at smail dot nju.edu.cn
2023-10-09 9:53 ` [Bug tree-optimization/111724] " rguenth at gcc dot gnu.org
2023-10-09 10:34 ` 652023330028 at smail dot nju.edu.cn
0 siblings, 2 replies; 3+ messages in thread
From: 652023330028 at smail dot nju.edu.cn @ 2023-10-08 7:55 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111724
Bug ID: 111724
Summary: [Regression] Missed optimizations probably because of
too early arithmetic optimization
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 found some optimizations (probably because of too early arithmetic
optimization) that GCC may have missed. We would greatly appreicate if you can
take a look and let us know what you think.
Given the following code:
https://godbolt.org/z/8EW8fx78K
int n;
void func(int w, int a, int b){
for(int i=0;i<w;i++){
n += (a+a)+(b+b);
b += b;
}
}
`a+a` is a loop invariant. But gcc-trunk -O3:
func(int, int, int):
push {r4, lr}
subs lr, r0, #0
ble .L1
movw r4, #:lower16:.LANCHOR0
movt r4, #:upper16:.LANCHOR0
mov ip, #0
ldr r0, [r4]
.L3:
adds r3, r1, r2
add ip, ip, #1
lsls r2, r2, #1
cmp lr, ip
add r0, r0, r3, lsl #1
bne .L3
str r0, [r4]
From original(tree):
(void) (n = (a + b) * 2 + n) >>>>>;
This leads to:
_1 = a_11(D) + b_18;
_2 = _1 * 2;
_4 = _2 + n_lsm.4_7;
So, it looks like the missed LICM is due to too early arithmetic optimization.
On gcc-6.4, it works as expected.
We found that this also affects other optimizations, such as common
subexpression elimination. We can provide examples if required.
Thank you very much for your time and effort! We look forward to hearing from
you.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/111724] [Regression] Missed optimizations probably because of too early arithmetic optimization
2023-10-08 7:55 [Bug tree-optimization/111724] New: [Regression] Missed optimizations probably because of too early arithmetic optimization 652023330028 at smail dot nju.edu.cn
@ 2023-10-09 9:53 ` rguenth at gcc dot gnu.org
2023-10-09 10:34 ` 652023330028 at smail dot nju.edu.cn
1 sibling, 0 replies; 3+ messages in thread
From: rguenth at gcc dot gnu.org @ 2023-10-09 9:53 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111724
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Last reconfirmed| |2023-10-09
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Yup, it's difficult. reassoc doesn't handle signed arithmetic, that's usually
the pass that optimizes association for invariant motion.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/111724] [Regression] Missed optimizations probably because of too early arithmetic optimization
2023-10-08 7:55 [Bug tree-optimization/111724] New: [Regression] Missed optimizations probably because of too early arithmetic optimization 652023330028 at smail dot nju.edu.cn
2023-10-09 9:53 ` [Bug tree-optimization/111724] " rguenth at gcc dot gnu.org
@ 2023-10-09 10:34 ` 652023330028 at smail dot nju.edu.cn
1 sibling, 0 replies; 3+ messages in thread
From: 652023330028 at smail dot nju.edu.cn @ 2023-10-09 10:34 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111724
--- Comment #2 from Yi <652023330028 at smail dot nju.edu.cn> ---
(In reply to Richard Biener from comment #1)
> Yup, it's difficult. reassoc doesn't handle signed arithmetic, that's
> usually the pass that optimizes association for invariant motion.
Yes. This issue has the same result for 'unsigned'.
^ permalink raw reply [flat|nested] 3+ messages in thread
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