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* [Bug c/38011] New: vectorizer ignores alignment, useless versioning
@ 2008-11-04 14:59 David dot Monniaux at ens dot fr
2008-11-04 15:50 ` [Bug tree-optimization/38011] " rguenth at gcc dot gnu dot org
0 siblings, 1 reply; 4+ messages in thread
From: David dot Monniaux at ens dot fr @ 2008-11-04 14:59 UTC (permalink / raw)
To: gcc-bugs
When compiling:
void assignMultiplyVec(double* restrict __attribute__ ((aligned (16))) a, const
double * restrict __attribute__ ((aligned (16))) b, double coef, unsigned
count) {
for(unsigned i=0; i<count; i++) {
a[i] = b[i]*coef;
}
}
Using: gcc -std=c99 -march=core2 -mtune=core2 -O3 -mfpmath=sse -ftree-vectorize
-ftree-vectorizer-verbose=9
The logs show:
essai_restrict_ref.c:2: note: Alignment of access forced using versioning.
essai_restrict_ref.c:2: note: Versioning for alignment will be applied.
essai_restrict_ref.c:2: note: Vectorizing an unaligned access.
and indeed the assembly code shows a test whether operands are 16-byte aligned.
This versioning is superfluous, since variable attributes guarantee 16-byte
alignment.
--
Summary: vectorizer ignores alignment, useless versioning
Product: gcc
Version: 4.3.2
Status: UNCONFIRMED
Severity: minor
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: David dot Monniaux at ens dot fr
GCC build triplet: i486-linux-gnu
GCC host triplet: i486-linux-gnu
GCC target triplet: i486-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38011
^ permalink raw reply [flat|nested] 4+ messages in thread
[parent not found: <bug-38011-4@http.gcc.gnu.org/bugzilla/>]
* [Bug tree-optimization/38011] vectorizer ignores alignment, useless versioning
[not found] <bug-38011-4@http.gcc.gnu.org/bugzilla/>
@ 2012-07-13 8:56 ` rguenth at gcc dot gnu.org
2013-03-27 12:35 ` rguenth at gcc dot gnu.org
1 sibling, 0 replies; 4+ messages in thread
From: rguenth at gcc dot gnu.org @ 2012-07-13 8:56 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38011
Richard Guenther <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Blocks| |53947
--- Comment #1 from Richard Guenther <rguenth at gcc dot gnu.org> 2012-07-13 08:56:00 UTC ---
Link to vectorizer missed-optimization meta-bug.
^ permalink raw reply [flat|nested] 4+ messages in thread
* [Bug tree-optimization/38011] vectorizer ignores alignment, useless versioning
[not found] <bug-38011-4@http.gcc.gnu.org/bugzilla/>
2012-07-13 8:56 ` rguenth at gcc dot gnu.org
@ 2013-03-27 12:35 ` rguenth at gcc dot gnu.org
1 sibling, 0 replies; 4+ messages in thread
From: rguenth at gcc dot gnu.org @ 2013-03-27 12:35 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38011
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |RESOLVED
Resolution| |INVALID
--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> 2013-03-27 12:35:44 UTC ---
Not true - you aligned the pointer, not the data it points to. There isn't
a good way to do that with an aligned attribute, the closest you can get at
is with
typedef double aligned_double __attribute__((aligned (16)));
void assignMultiplyVec(aligned_double* restrict a, aligned_double* restrict b,
double coef, unsigned count)
{
for(unsigned i=0; i<count; i++) {
a[i] = b[i]*coef;
}
}
but that has the issue that a[1] is not aligned but technically you
still say so (the issue is that the array has no gaps according to
the C standard but the alignment of the element type is bigger than
its size ...).
So instead we now have an assume_aligned builtin which you can use like
void assignMultiplyVec(double* restrict a_, const double * restrict b_,
double coef, unsigned count)
{
double* restrict a = __builtin_assume_aligned (a_, 16);
double* restrict b = __builtin_assume_aligned (b_, 16);
for(unsigned i=0; i<count; i++) {
a[i] = b[i]*coef;
}
}
which does not have this issue.
^ permalink raw reply [flat|nested] 4+ messages in thread
end of thread, other threads:[~2013-03-27 12:35 UTC | newest]
Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
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2008-11-04 14:59 [Bug c/38011] New: vectorizer ignores alignment, useless versioning David dot Monniaux at ens dot fr
2008-11-04 15:50 ` [Bug tree-optimization/38011] " rguenth at gcc dot gnu dot org
[not found] <bug-38011-4@http.gcc.gnu.org/bugzilla/>
2012-07-13 8:56 ` rguenth at gcc dot gnu.org
2013-03-27 12:35 ` rguenth at gcc dot gnu.org
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