https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90204
Bug ID: 90204
Summary: [8 Regression] C code is optimized worse than C++
Product: gcc
Version: 9.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: antoshkka at gmail dot com
Target Milestone: ---
Consider the example:
struct v {
int val[16];
};
struct v test(struct v a, struct v b) {
struct v res;
for (int i = 0; i < 16; i++)
res.val[i] = a.val[i] + b.val[i];
return res;
}
Compiling that snippet with `g++ -O3 -march=skylake-avx512` gives a short
assembly:
test(v, v):
push rbp
mov rax, rdi
mov rbp, rsp
vmovdqu32 ymm1, YMMWORD PTR [rbp+16]
vmovdqu32 ymm2, YMMWORD PTR [rbp+48]
vpaddd ymm0, ymm1, YMMWORD PTR [rbp+80]
vmovdqu32 YMMWORD PTR [rdi], ymm0
vpaddd ymm0, ymm2, YMMWORD PTR [rbp+112]
vmovdqu32 YMMWORD PTR [rdi+32], ymm0
vzeroupper
pop rbp
ret
Compiling the same sample with the C compiler and same flags produces a ~150
lines of assembly with a lot of jumps and comparisons. The regression appeared
after GCC-7.3
