https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68483
Bug ID: 68483
Summary: gcc 5.2: suboptimal code compared to 4.9
Product: gcc
Version: 5.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: other
Assignee: unassigned at gcc dot gnu.org
Reporter: lvqcl.mail at gmail dot com
Target Milestone: ---
#include <stdint.h>
void test(int32_t* input, int32_t* out, unsigned x1, unsigned x2)
{
unsigned i, j;
unsigned end = x1;
for(i = j = 0; i < 1000; i++) {
int32_t sum = 0;
end += x2;
for( ; j < end; j++)
sum += input[j];
out[i] = sum;
}
}
options used: -S -O2 -ftree-vectorize -msse2
GCC 5.2 generates the following code:
...
movdqa %xmm0, %xmm1
movl 8(%esp), %ebx
psrldq $8, %xmm1
paddd %xmm1, %xmm0
movdqa %xmm0, %xmm3
pshufd $255, %xmm0, %xmm2
addl %ebx, %eax
cmpl %ebx, %esi
pshufd $85, %xmm0, %xmm1
punpckhdq %xmm0, %xmm3
movd %xmm2, %ecx
punpckldq %xmm3, %xmm1
movd %ecx, %xmm2
punpcklqdq %xmm2, %xmm1
paddd %xmm1, %xmm0
movd %xmm0, %ecx
...
while GCC 4.9.2 generates this:
...
movdqa %xmm0, %xmm1
movl 8(%esp), %ebx
psrldq $8, %xmm1
paddd %xmm1, %xmm0
movdqa %xmm0, %xmm1
addl %ebx, %eax
cmpl %ebx, %esi
psrldq $4, %xmm1
paddd %xmm1, %xmm0
movd %xmm0, %ecx
...
GCC 4.9.2: 1 psrldq instruction
GCC 5.2.0: 2 pshufd, 2 movd, 2 punpckldq, 1 punpcklqdq instructions.
Also, GCC 5.2.0 can generate the same code as GCC 4.9.2, but it requires
-mssse3 option for this. It's strange that -mssse3 is necessary to generate
more efficient SSE2 code.
