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* [Bug tree-optimization/110327] New: [12/13/14 Regression] Missed Dead Code Elimination when using __builtin_unreachable since r12-4790-g4b3a325f07a
@ 2023-06-20 17:11 theodort at inf dot ethz.ch
2023-06-21 8:33 ` [Bug tree-optimization/110327] " rguenth at gcc dot gnu.org
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: theodort at inf dot ethz.ch @ 2023-06-20 17:11 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110327
Bug ID: 110327
Summary: [12/13/14 Regression] Missed Dead Code Elimination
when using __builtin_unreachable since
r12-4790-g4b3a325f07a
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: theodort at inf dot ethz.ch
Target Milestone: ---
https://godbolt.org/z/eT1jqzrT5
Given the following code:
void foo(void);
static int a, c;
static int *d, *e = &a;
static int **f = &d, **g = &e;
static int ***h = &g;
static int ****i = &h;
static short j;
static char(k)(char b) {
if (!(((b) >= -28) && ((b) <= -28))) {
__builtin_unreachable();
}
return 0;
}
static int l() {
a = -28;
for (; a != 2; a++) {
j = c;
c = 0;
for (; c != 2; c = j)
if (a) break;
k(****i);
if (****i >= 5 ^ 1) {
***i = *f;
if (c < 1) return 1;
if (a) foo();
;
}
}
return a;
}
int main() { l(); }
gcc-trunk -O2 does not eliminate the call to foo:
main:
subq $8, %rsp
movl $-28, %eax
movl $-28, a(%rip)
.p2align 4,,10
.p2align 3
.L10:
movl c(%rip), %edx
testl %eax, %eax
jne .L19
cmpw $2, %dx
je .L4
.L5:
jmp .L5
.p2align 4,,10
.p2align 3
.L4:
movl $2, c(%rip)
movl $2, %esi
.L3:
movq h(%rip), %rdx
movq (%rdx), %rdx
movq (%rdx), %rcx
cmpl $4, (%rcx)
jg .L7
movq d(%rip), %rcx
movq %rcx, (%rdx)
testl %esi, %esi
je .L16
testl %eax, %eax
jne .L20
.L7:
movl a(%rip), %eax
addl $1, %eax
movl %eax, a(%rip)
cmpl $2, %eax
jne .L10
.L16:
xorl %eax, %eax
addq $8, %rsp
ret
.p2align 4,,10
.p2align 3
.L20:
call foo
jmp .L7
.L19:
xorl %edx, %edx
xorl %esi, %esi
movl %edx, c(%rip)
jmp .L3
gcc-11.4.0 -O2 eliminates the call to foo:
main:
movl $-28, a(%rip)
movq h(%rip), %rcx
movl $-28, %edx
.p2align 4,,10
.p2align 3
.L8:
movzwl c(%rip), %eax
movl $0, c(%rip)
testl %edx, %edx
jne .L2
.p2align 4,,10
.p2align 3
.L3:
cmpw $2, %ax
jne .L3
movl $2, c(%rip)
movq (%rcx), %rax
movq (%rax), %rdx
cmpl $4, (%rdx)
jg .L5
movq d(%rip), %rdx
movq %rdx, (%rax)
.L5:
movl a(%rip), %eax
leal 1(%rax), %edx
movl %edx, a(%rip)
cmpl $2, %edx
jne .L8
.L13:
xorl %eax, %eax
ret
.L2:
movq (%rcx), %rax
movq (%rax), %rdx
cmpl $4, (%rdx)
jg .L5
movq d(%rip), %rdx
movq %rdx, (%rax)
jmp .L13
Bisects to r12-4790-g4b3a325f07a
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/110327] [12/13/14 Regression] Missed Dead Code Elimination when using __builtin_unreachable since r12-4790-g4b3a325f07a
2023-06-20 17:11 [Bug tree-optimization/110327] New: [12/13/14 Regression] Missed Dead Code Elimination when using __builtin_unreachable since r12-4790-g4b3a325f07a theodort at inf dot ethz.ch
@ 2023-06-21 8:33 ` rguenth at gcc dot gnu.org
2023-06-24 8:09 ` pinskia at gcc dot gnu.org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2023-06-21 8:33 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110327
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Target Milestone|--- |12.4
Keywords| |missed-optimization
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/110327] [12/13/14 Regression] Missed Dead Code Elimination when using __builtin_unreachable since r12-4790-g4b3a325f07a
2023-06-20 17:11 [Bug tree-optimization/110327] New: [12/13/14 Regression] Missed Dead Code Elimination when using __builtin_unreachable since r12-4790-g4b3a325f07a theodort at inf dot ethz.ch
2023-06-21 8:33 ` [Bug tree-optimization/110327] " rguenth at gcc dot gnu.org
@ 2023-06-24 8:09 ` pinskia at gcc dot gnu.org
2023-11-24 3:55 ` pinskia at gcc dot gnu.org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-06-24 8:09 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110327
--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
What is interesting is that the call to foo is still there on the gimple level
in GCC 11, it is only on the RTL level it is able to be removed ....
What I see missing on the gimple level on the trunk is a jump threading
opportunity (but might be too complex to do):
<bb 8> [local count: 12528922893]:
# c.3_16 = PHI <0(4), 2(7)>
_9 = h;
_10 = *_9;
_11 = *_10;
_12 = *_11;
if (_12 <= 4)
goto <bb 10>; [50.00%]
else
goto <bb 9>; [50.00%]
...
<bb 10> [local count: 6264461501]:
_15 = d;
*_10 = _15;
if (c.3_16 == 0)
goto <bb 11>; [5.50%]
else
goto <bb 12>; [94.50%]
<bb 12> [local count: 5919916023]:
if (a.11_30 != 0)
goto <bb 13>; [33.00%]
else
goto <bb 9>; [67.00%]
The threading is:
4->8->10->11
7->8->10->12
This requires copying bb 8 and 10. I am not saying it is the right thing to do
but rather that is how to optimize this out I think.
There is also this one too:
<bb 3> [local count: 12528922784]:
# a.11_30 = PHI <_18(14), -28(2)>
c.0_3 = c;
if (a.11_30 != 0)
goto <bb 4>; [5.50%]
else
goto <bb 5>; [94.50%]
2->3->4
14->3
But as I said I have not looked into more than that.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/110327] [12/13/14 Regression] Missed Dead Code Elimination when using __builtin_unreachable since r12-4790-g4b3a325f07a
2023-06-20 17:11 [Bug tree-optimization/110327] New: [12/13/14 Regression] Missed Dead Code Elimination when using __builtin_unreachable since r12-4790-g4b3a325f07a theodort at inf dot ethz.ch
2023-06-21 8:33 ` [Bug tree-optimization/110327] " rguenth at gcc dot gnu.org
2023-06-24 8:09 ` pinskia at gcc dot gnu.org
@ 2023-11-24 3:55 ` pinskia at gcc dot gnu.org
2024-03-08 15:31 ` law at gcc dot gnu.org
2024-03-09 7:32 ` law at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-11-24 3:55 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110327
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
CC| |pinskia at gcc dot gnu.org
Last reconfirmed| |2023-11-24
--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/110327] [12/13/14 Regression] Missed Dead Code Elimination when using __builtin_unreachable since r12-4790-g4b3a325f07a
2023-06-20 17:11 [Bug tree-optimization/110327] New: [12/13/14 Regression] Missed Dead Code Elimination when using __builtin_unreachable since r12-4790-g4b3a325f07a theodort at inf dot ethz.ch
` (2 preceding siblings ...)
2023-11-24 3:55 ` pinskia at gcc dot gnu.org
@ 2024-03-08 15:31 ` law at gcc dot gnu.org
2024-03-09 7:32 ` law at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: law at gcc dot gnu.org @ 2024-03-08 15:31 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110327
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] 6+ messages in thread
* [Bug tree-optimization/110327] [12/13/14 Regression] Missed Dead Code Elimination when using __builtin_unreachable since r12-4790-g4b3a325f07a
2023-06-20 17:11 [Bug tree-optimization/110327] New: [12/13/14 Regression] Missed Dead Code Elimination when using __builtin_unreachable since r12-4790-g4b3a325f07a theodort at inf dot ethz.ch
` (3 preceding siblings ...)
2024-03-08 15:31 ` law at gcc dot gnu.org
@ 2024-03-09 7:32 ` law at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: law at gcc dot gnu.org @ 2024-03-09 7:32 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110327
--- Comment #3 from Jeffrey A. Law <law at gcc dot gnu.org> ---
Two block copies isn't fatal when the second one is the one with the actual
jump thread. But costing does get more complex.
Basically we copy 8 so that we can isolate its two incoming paths which thread
differently in bb10. That's pretty standard stuff.
It looks like that particular threading possibility is hidden until after DOM3
is complete. Prior to and during DOM3, there's another block in the way.
# c.3_16 = PHI <0(3), 2(5)>
_9 = h;
_10 = *_9;
_11 = *_10;
_12 = *_11;
_13 = (char) _12;
if (_13 != -28)
goto <bb 8>; [0.00%]
else
goto <bb 9>; [100.00%]
;; succ: 8
;; 9
;; basic block 8, loop depth 0
;; pred: 7
__builtin_unreachable ();
;; succ:
;; basic block 9, loop depth 1
;; pred: 7
if (_12 <= 4)
goto <bb 10>; [50.00%]
else
goto <bb 15>; [50.00%]
;; succ: 10
;; 15
;; basic block 10, loop depth 1
;; pred: 9
_15 = d;
*_10 = _15;
if (c.3_16 <= 0)
goto <bb 11>; [5.50%]
else
goto <bb 12>; [94.50%]
Note bb9.
^ permalink raw reply [flat|nested] 6+ messages in thread
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2023-06-20 17:11 [Bug tree-optimization/110327] New: [12/13/14 Regression] Missed Dead Code Elimination when using __builtin_unreachable since r12-4790-g4b3a325f07a theodort at inf dot ethz.ch
2023-06-21 8:33 ` [Bug tree-optimization/110327] " rguenth at gcc dot gnu.org
2023-06-24 8:09 ` pinskia at gcc dot gnu.org
2023-11-24 3:55 ` pinskia at gcc dot gnu.org
2024-03-08 15:31 ` law at gcc dot gnu.org
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