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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Wrong code generated with -fslp-vectorize"
href="https://bugs.llvm.org/show_bug.cgi?id=50338">50338</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Wrong code generated with -fslp-vectorize
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>kazu@google.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>I'm seeing a miscompilation triggered by -flp-vectorize, much like
PR50323. Unlike PR50323, this one is reproducible as of the latest
clang as of c82a0ae70e280c1c40b1af09ef275ddc7ed4254d.
Consider:
#include <immintrin.h>
#include <stdio.h>
struct Float64x8 {
__m256d ymm_lo;
__m256d ymm_hi;
};
static inline double ExtractFloat64x8(Float64x8 a, int index) {
return (index < 4) ? a.ymm_lo[index] : a.ymm_hi[index - 4];
}
// This function appears to be miscompiled.
__attribute__((noinline)) __m256d Permute(Float64x8 a, __m256i map) {
double result[4];
for (int i = 0; i < 4; ++i)
result[i] = ExtractFloat64x8(a, map[i] & 0x7);
return _mm256_loadu_pd(result);
}
int main() {
Float64x8 v;
v.ymm_lo = _mm256_set_pd(3.0, 2.0, 1.0, 0.0);
v.ymm_hi = _mm256_set_pd(7.0, 6.0, 5.0, 4.0);
__m256d r = Permute(v, _mm256_set_epi64x(1, 0, 0, 0));
fprintf(stderr, "%f %f %f %f\n", r[3], r[2], r[1], r[0]);
return 0;
}
With the latest clang (c82a0ae70e280c1c40b1af09ef275ddc7ed4254d), I see:
$ ./debug/bin/clang++ -O3 -mavx -fno-slp-vectorize permute2.cc ; ./a.out
1.000000 0.000000 0.000000 0.000000
$ ./debug/bin/clang++ -O3 -mavx -fslp-vectorize permute2.cc ; ./a.out
0.000000 2.000000 0.000000 0.000000
Notice that the top two lanes are different. The former output is
correct. The latter is not.
Here is the assembly output for Permute with -fslp-vectorize:
.text
.file "permute2.cc"
.section .rodata.cst16,"aM",@progbits,16
.p2align 4 # -- Begin function
_Z7Permute9Float64x8Dv4_x
.LCPI0_0:
.long 7 # 0x7
.long 7 # 0x7
.zero 4
.zero 4
.LCPI0_1:
.long 4 # 0x4
.long 4 # 0x4
.zero 4
.zero 4
.section .rodata.cst32,"aM",@progbits,32
.p2align 5
.LCPI0_2:
.quad 2 # 0x2
.quad 2 # 0x2
.quad 2 # 0x2
.quad 2 # 0x2
.text
.globl _Z7Permute9Float64x8Dv4_x
.p2align 4, 0x90
.type _Z7Permute9Float64x8Dv4_x,@function
_Z7Permute9Float64x8Dv4_x: # @_Z7Permute9Float64x8Dv4_x
.cfi_startproc
# %bb.0: # %entry
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset %rbp, -16
movq %rsp, %rbp
.cfi_def_cfa_register %rbp
andq $-32, %rsp
subq $96, %rsp
vpextrd $2, %xmm0, %eax
movl %eax, %edx
andl $7, %edx
leal -4(%rdx), %ecx
andl $3, %eax
vmovaps 16(%rbp), %ymm1
vmovaps 48(%rbp), %ymm2
vmovaps %ymm1, 32(%rsp)
vmovsd 32(%rsp,%rax,8), %xmm1 # xmm1 = mem[0],zero
vmovd %xmm0, %eax
vmovaps %ymm2, (%rsp)
cmpl $4, %edx
jb .LBB0_2
# %bb.1: # %entry
andl $3, %ecx
vmovsd (%rsp,%rcx,8), %xmm1 # xmm1 = mem[0],zero
.LBB0_2: # %entry
movl %eax, %ecx
andl $7, %ecx
subl $4, %ecx
jb .LBB0_3
# %bb.4: # %entry
andl $3, %ecx
vmovsd (%rsp,%rcx,8), %xmm2 # xmm2 = mem[0],zero
jmp .LBB0_5
.LBB0_3:
andl $3, %eax
vmovsd 32(%rsp,%rax,8), %xmm2 # xmm2 = mem[0],zero
.LBB0_5: # %entry
vmovlhps %xmm1, %xmm2, %xmm1 # xmm1 =
xmm2[0],xmm1[0]
vextractf128 $1, %ymm0, %xmm0
vpshufd $216, %xmm0, %xmm2 # xmm2 = xmm0[0,2,1,3]
vpand .LCPI0_0(%rip), %xmm2, %xmm2
vmovd %xmm0, %eax
addl $-4, %eax
vpmaxud .LCPI0_1(%rip), %xmm2, %xmm3
andl $3, %eax
vpcmpeqd %xmm3, %xmm2, %xmm3
vpcmpeqd %xmm4, %xmm4, %xmm4
vpxor %xmm4, %xmm3, %xmm3
vpextrd $2, %xmm0, %ecx
addl $-4, %ecx
vpaddq %xmm2, %xmm2, %xmm0
vmovapd 16(%rbp), %xmm2
vmovapd 32(%rbp), %xmm4
vpermilpd %xmm0, %xmm4, %xmm4
vpermilpd %xmm0, %xmm2, %xmm2
vpcmpgtq .LCPI0_2(%rip), %xmm0, %xmm0
vblendvpd %xmm0, %xmm4, %xmm2, %xmm0
andl $3, %ecx
vmovsd (%rsp,%rax,8), %xmm2 # xmm2 = mem[0],zero
vmovhpd (%rsp,%rcx,8), %xmm2, %xmm2 # xmm2 = xmm2[0],mem[0]
vpmovsxdq %xmm3, %xmm3
vblendvpd %xmm3, %xmm0, %xmm2, %xmm0
vinsertf128 $1, %xmm0, %ymm1, %ymm0
movq %rbp, %rsp
popq %rbp
.cfi_def_cfa %rsp, 8
retq</pre>
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