https://llvm.org/bugs/show_bug.cgi?id=27320
Bug ID: 27320
Summary: [AVX2] vpermd + vpshufb intrinsics merged into wrong
code at -O1 and higher
Product: new-bugs
Version: 3.8
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: pe...@cordes.ca
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
clang 3.8 (but not clang 3.7.1) mis-compiles a sequence of two shuffles.
(Background:
http://stackoverflow.com/questions/36327100/complex-data-reorganization-with-vector-instructions)
__m256i doubleshuff(__m256i in) {
const __m256i byteshuf = ...;
const __m256i lane_adjust_shuf = ...;
in = _mm256_permutevar8x32_epi32(in, lane_adjust_shuf);
return _mm256_shuffle_epi8(in, byteshuf);
}
clang 3.8.1-svn265380-1~exp1 (from llvm repo for Ubuntu 15.10) compiles that to
(with -O3 -march=haswell):
vmovdqa ymm0, ymmword ptr [rip + .LCPI3_0] # ymm0 = [0,1,2,2,3,4,5,5]
vpshufb ymm0, ymm0, ymmword ptr [rip + .LCPI3_1] # ymm0 =
ymm0[0,1,1,2,3,4,4,5,6,7,7,8,9,10,10,11,28,29,29,30,31,16,16,17,18,19,19,20,21,22,22,23]
ret
which is obviously wrong because it doesn't depend on the input. It's
shuffling the vpermd shuffle mask instead of shuffling the input.
gcc 5.2 emits:
vmovdqa ymm1, YMMWORD PTR .LC7[rip]
vpermd ymm0, ymm1, ymm0
vpshufb ymm0, ymm0, YMMWORD PTR .LC8[rip]
ret
I created a test-case that exits with true or false status, suitable for
testing with git bisect. (I don't have a checkout of the clang tree myself, or
a fast machine).
test-case:
#include <immintrin.h>
#include <assert.h>
#include <string.h>
__m256i doubleshuff(__m256i in) {
const __m256i byteshuf = _mm256_setr_epi8(0,1,1,2, 3,4,4,5, 6,7,7,8,
9,10,10,11,
0,1,1,2, 3,4,4,5, 6,7,7,8, 9,10,10,11);
// or broadcast128
const __m256i lane_adjust_shuf = _mm256_setr_epi32(0,1,2,2, 3,4,5,5);
// const __m256i lane_adjust_shuf =
_mm256_cvtepu8_epi32(_mm_setr_epi8(0,1,2,2, 3,4,5,5,
// /* unused padding that isn't optimized away :( */
0,0,0,0, 0,0,0,0));
in = _mm256_permutevar8x32_epi32(in, lane_adjust_shuf);
return _mm256_shuffle_epi8(in, byteshuf);
}
static unsigned char dst[33];
static const unsigned char src[33] =
"ABCDEFGHIJKL"
"abcdefghijkl"
"XXXXXXXX";
static const unsigned char good_output[33] =
"ABBCDEEFGHHIJKKL"
"abbcdeefghhijkkl";
// return false if result doesn't match
int shuffle_test(void) {
__m256i srcv = _mm256_loadu_si256((const __m256i*)src);
__m256i expanded = doubleshuff(srcv);
_mm256_storeu_si256((__m256i*)dst, expanded);
return !memcmp(dst, good_output, 32);
}
int main(int argc, char**argv) {
int test_passed = shuffle_test();
// assert(test_passed);
return !test_passed;
}
See also this code and a bigger loop using it on godbolt:
https://godbolt.org/g/nJqyJc
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