https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82108
Bug ID: 82108
Summary: [7.2 Regression] Wrong vectorized code generated for
x86_64
Product: gcc
Version: 7.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: ell_se at yahoo dot com
Target Milestone: ---
The following snippet generates wrong vectorized code with GCC 7.2 on x86_64
with -O3:
void downscale_2 (const float* src, int src_n, float* dst) {
int i;
for (i = 0; i < src_n; i += 2) {
const float* a = src;
const float* b = src + 4;
dst[0] = (a[0] + b[0]) / 2;
dst[1] = (a[1] + b[1]) / 2;
dst[2] = (a[2] + b[2]) / 2;
dst[3] = (a[3] + b[3]) / 2;
src += 2 * 4;
dst += 4;
}
The assembly for the vectorized version of the loop is:
.L5:
addl $1, %ecx
movups (%rdi,%rax), %xmm0
movups 16(%rdi,%rax,2), %xmm2
addps %xmm2, %xmm0
mulps %xmm1, %xmm0
movups %xmm0, (%rdx,%rax)
addq $16, %rax
cmpl %r8d, %ecx
jb .L5
Notice the missing ,2 on the first movups.
It can be tested with:
#include <stdio.h>
int main () {
const float in[4 * 4] = {
1, 2, 3, 4,
5, 6, 7, 8,
1, 2, 3, 4,
5, 6, 7, 8
};
float out[2 * 4];
downscale_2 (in, 4, out);
/* correct: 3, 4, 5, 6 */
printf ("%g, %g, %g, %g\n", out[0], out[1], out[2], out[3]);
/* incorrect: 5, 6, 7, 8; should also be 3, 4, 5, 6 */
printf ("%g, %g, %g, %g\n", out[4], out[5], out[6], out[7]);
}
This doesn't seem to happen with 7.1 or 6.3.
For a chuckle, this affects a recent build of GIMP, and has resulted in this
beauty:
https://bug787222.bugzilla-attachments.gnome.org/attachment.cgi?id=359042 :)
$gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/7.2.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib
--libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=https://bugs.archlinux.org/
--enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++ --enable-shared
--enable-threads=posix --enable-libmpx --with-system-zlib --with-isl
--enable-__cxa_atexit --disable-libunwind-exceptions --enable-clocale=gnu
--disable-libstdcxx-pch --disable-libssp --enable-gnu-unique-object
--enable-linker-build-id --enable-lto --enable-plugin
--enable-install-libiberty --with-linker-hash-style=gnu
--enable-gnu-indirect-function --disable-multilib --disable-werror
--enable-checking=release --enable-default-pie --enable-default-ssp
Thread model: posix
gcc version 7.2.0 (GCC)
