http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53645
Bug #: 53645
Summary: Missed optimization for division of vector types
Classification: Unclassified
Product: gcc
Version: 4.7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: andrii.riabushe...@barclays.com
for the following code
v4si ttt(v4si x) {
return x / (v4si) {3,3,3,3};
}
GCC generates the following assembler
ttt:
movdqa (%rcx), %xmm0
movl $1431655766, %ecx
movd %xmm0, %r8d
pextrd $1, %xmm0, %r10d
pextrd $2, %xmm0, %r11d
movl %r8d, %eax
sarl $31, %r8d
imull %ecx
movl %r10d, %eax
sarl $31, %r10d
movl %edx, %r9d
imull %ecx
movl %r11d, %eax
subl %r8d, %r9d
sarl $31, %r11d
movl %edx, %r8d
imull %ecx
subl %r10d, %r8d
movl %edx, %r10d
subl %r11d, %r10d
pextrd $3, %xmm0, %r11d
movl %r11d, %eax
imull %ecx
sarl $31, %r11d
movd %r10d, %xmm1
movd %r9d, %xmm0
pinsrd $0x1, %r8d, %xmm0
subl %r11d, %edx
pinsrd $0x1, %edx, %xmm1
punpcklqdq %xmm1, %xmm0
ret
Thus gcc DOES optimize the division to be done through High Multiplication, but
it is applied to each value separately instead of vectorized ones. Assember
should look like
movdqa .LC190(%rip), %xmm0
pmulld (%rcx), %xmm0
pslld $31, %xmm0
ret
