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* [Bug tree-optimization/82885] memcpy does not propagate aliasing knowledge
[not found] <bug-82885-4@http.gcc.gnu.org/bugzilla/>
@ 2021-08-01 17:07 ` pinskia at gcc dot gnu.org
0 siblings, 0 replies; only message in thread
From: pinskia at gcc dot gnu.org @ 2021-08-01 17:07 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82885
Andrew Pinski <pinskia at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Component|middle-end |tree-optimization
Severity|normal |enhancement
--- Comment #4 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
The original testcase is fixed in GCC 8+:
<bb 2> [local count: 1073741825]:
__builtin_memcpy (ch1_3(D), ch2_4(D), 1048576);
_1 = MEM[(const char *)ch2_4(D) + 2B];
*ch1_3(D) = _1;
MEM[(char *)ch1_3(D) + 1B] = _1;
fre1 is able to figure this out but this is not exactly propagating aliasing
info, just that the value of ch2[2] cannot change if ch1[0] is stored with the
same value of what was in ch2[2]; memcpy is not needed to show that :).
A better testcase which shows there is still a missing aliasing case:
void foo3(char* ch1, char* ch2) {
__builtin_memcpy(ch1, ch2, 1024*1024);
ch1[0] = ch2[2] + 1;
ch1[1] = ch2[2];
}
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2021-08-01 17:07 ` [Bug tree-optimization/82885] memcpy does not propagate aliasing knowledge pinskia at gcc dot gnu.org
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