https://bugs.llvm.org/show_bug.cgi?id=34665
Bug ID: 34665
Summary: [AVX] wrong answer after SLP Vectorizer
Product: new-bugs
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: ilia.tara...@intel.com
CC: llvm-bugs@lists.llvm.org
This test gives wrong answer after SLP Vectorizer:
================= nice.c ==============
void init (unsigned int a [], unsigned int n)
{
unsigned int j = 0;
for (j = 0; j < n; j++)
a[j] = (45 + j);
}
int main ()
{
unsigned int i = 0, k = 0, j = 0, res = 0;
unsigned int z [64] = {0}, x [64] = {0};
init((unsigned int * )x, 64);
scanf(0);
for (i = 1; i < 6; ++i)
{
for (k = 9; k > 1; --k)
x[k] -= k;
for (j = 2; j < 10; ++j)
z[j] = x[j];
}
printf("res = %u\n", z[8]);
return 0;
}
============================================
>>> clang -v
clang version 6.0.0 (trunk 313612)
Target: x86_64-unknown-linux-gnu
Thread model: posix
...
>>> clang -march=skylake -O0 -o nice.exe nice.c
>>> ./nice.exe
res = 13
>>> clang -march=skylake -O2 -o nice.exe nice.c
>>> ./nice.exe
res = 5
If we look at generated asm, we'll see that after SLP vectorization
storage to ymm2 register is in wrong place, so it result gets an additional
subtraction:
================= nice.s ==================
...
callq scanf
vmovdqu 8(%rsp), %xmm0 # $rsp = [45, 46, 47, 48, 49, 50, 51,
52, 53, 54, ...]
vmovdqu 24(%rsp), %xmm1
vpextrd $3, %xmm1, %r8d
leal -9(%r8), %ecx
movl %ecx, 36(%rsp)
vpextrd $2, %xmm1, %r9d
leal -8(%r9), %edx
movl %edx, 32(%rsp)
vpextrd $1, %xmm1, %esi
leal -7(%rsi), %edx
movl %edx, 28(%rsp)
vpextrd $3, %xmm0, %edi
leal -5(%rdi), %edx
vmovdqu 8(%rsp), %ymm2 # $ymm2 = [47, 48, 49, 50, 51, 45, 45, 45]
...
vpaddd .LCPI1_12(%rip), %ymm2, %ymm0 # .LCPI1_12(%rip) =
[-10, -15, -20, -25, -30, -35, -40, -45]
vmovdqu %ymm0, 8(%rsp) # $rsp = [37, 33, 29, 25, 21,
10, 5, 0] - wrong answer
vmovups 8(%rsp), %ymm0 # right answer is [37, 33,
29, 25, 21, 17, 13, 9]
...
===========================================
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