https://llvm.org/bugs/show_bug.cgi?id=25543
Bug ID: 25543
Summary: [x86] bad codegen while extracting vector elements
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Backend: X86
Assignee: unassignedb...@nondot.org
Reporter: spatel+l...@rotateright.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
It may be possible to further reduce this, but here's a small C example that
shows the problem(s):
$ cat shifty.c
#include <immintrin.h>
void foo(__m128i v, int bits, short *s0, short *s1) {
int i[4];
_mm_storeu_si128((__m128i *)i, v); // get vector bits as ints
if (bits & 1) { *s0 = i[0]; } // conditionally store part of elt 0
if (bits & 2) { *s1 = i[1]; } // conditionally store part of elt 1
}
Testing with:
$ ./clang -v
clang version 3.8.0 (trunk 253043)
Target: x86_64-apple-darwin15.0.0
$ ./clang -fomit-frame-pointer -O2 shifty.c -S -o -
_foo:
movd %xmm0, %rax # Wrong disasm? That's a 64-bit move (movq).
testb $1, %dil
je LBB0_2
movw %ax, (%rsi)
LBB0_2:
testb $2, %dil
je LBB0_4
# Things get weird...why are we messing with elements 2 and 3?
pshufd $78, %xmm0, %xmm0 ## xmm0 = xmm0[2,3,0,1]
movd %xmm0, %rcx
shldq $32, %rax, %rcx # Could have just shifted %rax left?
movw %cx, (%rdx)
LBB0_4:
retq
--
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