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[Bug c/40141] New: accessing aliased __m128 miscompiles
- From: "kretz at kde dot org" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: 14 May 2009 09:40:24 -0000
- Subject: [Bug c/40141] New: accessing aliased __m128 miscompiles
- Reply-to: gcc-bugzilla at gcc dot gnu dot org
Testcase:
#include <emmintrin.h>
#include <stdio.h>
typedef float floatA __attribute__((__may_alias__));
int main()
{
__m128 x = _mm_setzero_ps();
int i;
for (i = 0; i < 4; ++i) {
const float xx = ((floatA*)&x)[i];
if (xx != 0.f) {
printf("%d: %f\n", i, xx);
return -1;
}
}
return 0;
}
This fails with -O2, as well as with -O2 -fno-strict-aliasing. It works with
-O1 and -O0.
The generated assembly shows that the stack is created but not initialized and
then read from and checked whether it's 0.
If instead of
const float xx = ((floatA*)&x)[i];
you write
const float xx = ((floatA*)&x)[0];
(or any other constant between 0 and 3) then the testcase doesn't fail.
Tested versions:
4.2.4: doesn't fail
4.3.2: fails
4.3.3: fails
4.4.0: doesn't fail
--
Summary: accessing aliased __m128 miscompiles
Product: gcc
Version: 4.3.3
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: kretz at kde dot org
GCC build triplet: x86_64-unknown-linux-gnu
GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40141