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* [Bug c/98658] New: Loop idiom recognization for memcpy/memmove
@ 2021-01-13 13:59 david.bolvansky at gmail dot com
2021-01-13 14:04 ` [Bug c/98658] " david.bolvansky at gmail dot com
` (2 more replies)
0 siblings, 3 replies; 4+ messages in thread
From: david.bolvansky at gmail dot com @ 2021-01-13 13:59 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98658
Bug ID: 98658
Summary: Loop idiom recognization for memcpy/memmove
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: david.bolvansky at gmail dot com
Target Milestone: ---
void copy(int *__restrict__ d, int * s, __SIZE_TYPE__ sz) {
__SIZE_TYPE__ i;
for (i = 0; i < sz; i++) {
*d++ = *s++;
}
}
gcc emits call to memcpy.
void copy(int * d, int * s, __SIZE_TYPE__ sz) {
__SIZE_TYPE__ i;
for (i = 0; i < sz; i++) {
*d++ = *s++;
}
}
gcc could emit memmove, but currently does not:
https://godbolt.org/z/5n1rnh
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug c/98658] Loop idiom recognization for memcpy/memmove
2021-01-13 13:59 [Bug c/98658] New: Loop idiom recognization for memcpy/memmove david.bolvansky at gmail dot com
@ 2021-01-13 14:04 ` david.bolvansky at gmail dot com
2021-01-13 15:37 ` rguenth at gcc dot gnu.org
2021-01-13 16:40 ` david.bolvansky at gmail dot com
2 siblings, 0 replies; 4+ messages in thread
From: david.bolvansky at gmail dot com @ 2021-01-13 14:04 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98658
--- Comment #1 from Dávid Bolvanský <david.bolvansky at gmail dot com> ---
ICC produces memcpy:
https://godbolt.org/z/oKxxTM
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug c/98658] Loop idiom recognization for memcpy/memmove
2021-01-13 13:59 [Bug c/98658] New: Loop idiom recognization for memcpy/memmove david.bolvansky at gmail dot com
2021-01-13 14:04 ` [Bug c/98658] " david.bolvansky at gmail dot com
@ 2021-01-13 15:37 ` rguenth at gcc dot gnu.org
2021-01-13 16:40 ` david.bolvansky at gmail dot com
2 siblings, 0 replies; 4+ messages in thread
From: rguenth at gcc dot gnu.org @ 2021-01-13 15:37 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98658
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Resolution|--- |INVALID
Status|UNCONFIRMED |RESOLVED
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
it's because
(gdb) p debug_data_dependence_relation (ddr)
(Data Dep:
#(Data Ref:
# bb: 3
# stmt: _1 = *s_19;
# ref: *s_19;
# base_object: *s_7(D);
# Access function 0: {0B, +, 4}_1
#)
#(Data Ref:
# bb: 3
# stmt: *d_18 = _1;
# ref: *d_18;
# base_object: *d_6(D);
# Access function 0: {0B, +, 4}_1
#)
(don't know)
)
and we say
/* Can't do memmove in case of unknown dependence or dependence without
classical distance vector. */
if (DDR_ARE_DEPENDENT (ddr) == chrec_dont_know
|| DDR_NUM_DIST_VECTS (ddr) == 0)
return;
which is because we need to check the depdenence is not backwards
(*s++ loads an earlier stored value). I don't see how that is
guaranteed in the 'copy' case and ICC either miscompiles this
or has a runtime check doing the memcpy only conditionally.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug c/98658] Loop idiom recognization for memcpy/memmove
2021-01-13 13:59 [Bug c/98658] New: Loop idiom recognization for memcpy/memmove david.bolvansky at gmail dot com
2021-01-13 14:04 ` [Bug c/98658] " david.bolvansky at gmail dot com
2021-01-13 15:37 ` rguenth at gcc dot gnu.org
@ 2021-01-13 16:40 ` david.bolvansky at gmail dot com
2 siblings, 0 replies; 4+ messages in thread
From: david.bolvansky at gmail dot com @ 2021-01-13 16:40 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98658
--- Comment #3 from Dávid Bolvanský <david.bolvansky at gmail dot com> ---
Yes, runtime check.
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2021-01-13 16:40 UTC | newest]
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2021-01-13 13:59 [Bug c/98658] New: Loop idiom recognization for memcpy/memmove david.bolvansky at gmail dot com
2021-01-13 14:04 ` [Bug c/98658] " david.bolvansky at gmail dot com
2021-01-13 15:37 ` rguenth at gcc dot gnu.org
2021-01-13 16:40 ` david.bolvansky at gmail dot com
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