https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64749
Bug ID: 64749
Summary: "truncating" instructions generated instead of a load
one using SSE & AVX2 intrinsics
Product: gcc
Version: 4.8.4
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: adrien at guinet dot me
Created attachment 34553
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34553&action=edit
test case
The code attached compiles and runs fine (that is the output of the program is
the good one) using GCC 4.9. When compiled with GCC 4.8, the output is
different and incorrect.
Indeed, when compiled with GCC 4.8, some kind of truncating is introduced at
the begginig of the loop (in f2). Here is the relevant assembly code (output of
GCC 4.8) :
xor eax, eax
mov rbp, rsp
and rsp, 0FFFFFFFFFFFFFFE0h
vbroadcastss ymm3, xmm6
add rsp, 10h
nop dword ptr [rax]
loc_400970:
vpmovzxwd ymm4, xmmword ptr [rdx+rax*4]
vpmovzxwd ymm2, xmmword ptr [rcx+rax*4]
vmovdqa [rsp-8+var_28], ymm4
; truncation here is done
vmovdqa xmm5, xmmword ptr [rsp-8+var_28]
vpmulld ymm0, ymm4, ymm2
; here it uses xmm5 which isn't thus the good value.
; xmm5 and ymm4 should be set like with something like this (like GCC 4.9
does):
; vmovqda xmm5, xmmword ptr [rdx+rax*4]
; vpmovzxwd ymm4, xmm5
vpmulhuw xmm1, xmm5, xmmword ptr [r8+rax*4]
vpmovzxwd ymm1, xmm1
vpmulld ymm1, ymm1, ymm3
vpsubd ymm0, ymm0, ymm1
vmovdqa xmmword ptr [rsi+rax*4], xmm0
add rax, 8
cmp rdi, rax
ja short loc_400970
GCC 4.9 indeed behaves correctly and generate this assembly code :
vbroadcastss ymm3, dword ptr [rbp-14h]
xor eax, eax
nop dword ptr [rax+00h]
loc_4009A8:
vmovdqa xmm0, xmmword ptr [rdx+rax*4] ; 128-bits load
vpmulhuw xmm2, xmm0, xmmword ptr [r8+rax*4] ; correctly uses xmm0
vpmovzxwd ymm2, xmm2 ; 16->32 bits conversion here
vpmulld ymm2, ymm2, ymm3
vpmovzxwd ymm1, xmm0
vpmovzxwd ymm0, xmmword ptr [rcx+rax*4]
vpmulld ymm0, ymm1, ymm0
vpsubd ymm0, ymm0, ymm2
vmovaps xmmword ptr [rsi+rax*4], xmm0
add rax, 8
cmp rdi, rax
ja short loc_4009A8
Thanks for any help about this!
P.S: sorry but I didn't manage to have a shorter test case :/
