https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107099
Bug ID: 107099
Summary: uncprop a bit
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: amonakov at gcc dot gnu.org
Target Milestone: ---
For the following testcase
#include <immintrin.h>
__attribute__((target("avx")))
int f(__m128i a[], long n)
{
for (long i = 0; i < n; i++)
if (!_mm_testz_si128(a[i], a[i]))
return 0;
return 1;
}
gcc -O2 generates
f:
test rsi, rsi
jle .L4
xor eax, eax
jmp .L3
.L10:
add rax, 1
cmp rsi, rax
je .L4
.L3:
mov rdx, rax
sal rdx, 4
vmovdqa xmm0, XMMWORD PTR [rdi+rdx]
xor edx, edx
vptest xmm0, xmm0
sete dl
je .L10
mov eax, edx
ret
.L4:
mov edx, 1
mov eax, edx
ret
Note the redundant assignments to edx in the loop and compare with gcc -O2
-fdisable-tree-uncprop1
Also note that generally uncprop adds a data dependency where only a control
dependency existed, hurting speculative execution (hence more appropriate for
-Os than -O2).
