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* [Bug tree-optimization/98236] New: x plus/minus y cmp 0 produces unoptimal code
@ 2020-12-11 10:18 denis.campredon at gmail dot com
2020-12-11 10:19 ` [Bug tree-optimization/98236] " denis.campredon at gmail dot com
` (4 more replies)
0 siblings, 5 replies; 6+ messages in thread
From: denis.campredon at gmail dot com @ 2020-12-11 10:18 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98236
Bug ID: 98236
Summary: x plus/minus y cmp 0 produces unoptimal code
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: denis.campredon at gmail dot com
Target Milestone: ---
Created attachment 49733
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49733&action=edit
input file
Compiling test.c on x86 with -O2 leads to unoptimal code generation.
f1 to f4 could be optimized to `add + set(g|ge|l|le)`
f5 to f8 could be optimized to `sud|cmp + set(g|ge|l|le)`
For f2, f4, f6 and f8 no pattern is recognized.
For f1, f3, f5 and f7, the optimizers are not aware that, at least on x86,
`add` and `sub` can set flags.
This can also be seen with the following function. gcc will produce `cmp + sub`
although only `sub` could be used
-----------
void foo();
int bar(int x, int y) {
if (x - y)
foo();
return x - y;
}
-----------
produces
-----------
bar(int, int):
push rbp
mov ebp, esi
push rbx
mov ebx, edi
sub rsp, 8
cmp edi, esi
je .L2
call foo()
.L2:
mov eax, ebx
add rsp, 8
sub eax, ebp
pop rbx
pop rbp
ret
-----------
expected
-----------
bar(int, int):
push rbx
mov ebx, edi
sub ebx, esi
je .L2
call foo()
.L2:
mov eax, ebx
pop rbx
ret
-----------
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/98236] x plus/minus y cmp 0 produces unoptimal code
2020-12-11 10:18 [Bug tree-optimization/98236] New: x plus/minus y cmp 0 produces unoptimal code denis.campredon at gmail dot com
@ 2020-12-11 10:19 ` denis.campredon at gmail dot com
2020-12-11 10:39 ` rguenth at gcc dot gnu.org
` (3 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: denis.campredon at gmail dot com @ 2020-12-11 10:19 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98236
--- Comment #1 from denis.campredon at gmail dot com ---
Created attachment 49734
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49734&action=edit
assemble generated
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug tree-optimization/98236] x plus/minus y cmp 0 produces unoptimal code
2020-12-11 10:18 [Bug tree-optimization/98236] New: x plus/minus y cmp 0 produces unoptimal code denis.campredon at gmail dot com
2020-12-11 10:19 ` [Bug tree-optimization/98236] " denis.campredon at gmail dot com
@ 2020-12-11 10:39 ` rguenth at gcc dot gnu.org
2020-12-11 10:40 ` [Bug middle-end/98236] " rguenth at gcc dot gnu.org
` (2 subsequent siblings)
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2020-12-11 10:39 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98236
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
This is likely because we expand from
int bar (int x, int y)
{
int _6;
<bb 2> [local count: 1073741824]:
if (x_2(D) != y_3(D))
goto <bb 3>; [48.88%]
else
goto <bb 4>; [51.12%]
<bb 3> [local count: 524845000]:
foo ();
<bb 4> [local count: 1073741824]:
_6 = x_2(D) - y_3(D);
return _6;
which means we simplified the condition to x != y. So related to PR91384
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/98236] x plus/minus y cmp 0 produces unoptimal code
2020-12-11 10:18 [Bug tree-optimization/98236] New: x plus/minus y cmp 0 produces unoptimal code denis.campredon at gmail dot com
2020-12-11 10:19 ` [Bug tree-optimization/98236] " denis.campredon at gmail dot com
2020-12-11 10:39 ` rguenth at gcc dot gnu.org
@ 2020-12-11 10:40 ` rguenth at gcc dot gnu.org
2021-08-01 18:14 ` pinskia at gcc dot gnu.org
2023-06-13 5:56 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: rguenth at gcc dot gnu.org @ 2020-12-11 10:40 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98236
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2020-12-11
Ever confirmed|0 |1
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/98236] x plus/minus y cmp 0 produces unoptimal code
2020-12-11 10:18 [Bug tree-optimization/98236] New: x plus/minus y cmp 0 produces unoptimal code denis.campredon at gmail dot com
` (2 preceding siblings ...)
2020-12-11 10:40 ` [Bug middle-end/98236] " rguenth at gcc dot gnu.org
@ 2021-08-01 18:14 ` pinskia at gcc dot gnu.org
2023-06-13 5:56 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2021-08-01 18:14 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98236
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Severity|normal |enhancement
Last reconfirmed|2020-12-11 00:00:00 |2021-8-1
^ permalink raw reply [flat|nested] 6+ messages in thread
* [Bug middle-end/98236] x plus/minus y cmp 0 produces unoptimal code
2020-12-11 10:18 [Bug tree-optimization/98236] New: x plus/minus y cmp 0 produces unoptimal code denis.campredon at gmail dot com
` (3 preceding siblings ...)
2021-08-01 18:14 ` pinskia at gcc dot gnu.org
@ 2023-06-13 5:56 ` pinskia at gcc dot gnu.org
4 siblings, 0 replies; 6+ messages in thread
From: pinskia at gcc dot gnu.org @ 2023-06-13 5:56 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98236
--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Note f4 right now produces the best code even though it is not add/setN.
leal (%rdi,%rsi), %eax
shrl $31, %eax
is better overall because it does not touch the flags register and all
especially if there is other code around it.
For f2, the biggest thing here is a cost model around doing the expansion as
not/shift or cmp/set.
f5, f6 and f7 are now caught in GCC 11+.
^ permalink raw reply [flat|nested] 6+ messages in thread
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2020-12-11 10:18 [Bug tree-optimization/98236] New: x plus/minus y cmp 0 produces unoptimal code denis.campredon at gmail dot com
2020-12-11 10:19 ` [Bug tree-optimization/98236] " denis.campredon at gmail dot com
2020-12-11 10:39 ` rguenth at gcc dot gnu.org
2020-12-11 10:40 ` [Bug middle-end/98236] " rguenth at gcc dot gnu.org
2021-08-01 18:14 ` pinskia at gcc dot gnu.org
2023-06-13 5:56 ` pinskia at gcc dot gnu.org
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