https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110711
Bug ID: 110711
Summary: possible missed optimization for std::max with
-march=znver2
Product: gcc
Version: 13.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
Assignee: unassigned at gcc dot gnu.org
Reporter: mrks2023 at proton dot me
Target Milestone: ---
I think I found a missed optimization involving std::max() for -march=znver2
(sorry if it was already reported, but I didn't find anything related in the
bug tracker).
I have two functions that compute the maximum element of an array:
- function k_std_max uses std::max() and is never vectorized
- function k_max uses conditional assignment and is vectorized, when the
optimization flags allow for it
The code (also https://godbolt.org/z/hW49nbqMY):
#include <cassert>
#include <algorithm>
double k_std_max(size_t n_els, double * a)
{
assert(n_els > 0);
double m = a[0];
#ifdef _OPENMP
#pragma omp simd reduction(max:m)
#endif
for (size_t i = 1; i < n_els; ++i) {
m = std::max(m, a[i]);
}
return m;
}
double k_max(size_t n_els, double * a)
{
assert(n_els > 0);
double m = a[0];
#ifdef _OPENMP
#pragma omp simd reduction(max:m)
#endif
for (size_t i = 1; i < n_els; ++i) {
m = m < a[i] ? a[i] : m;
}
return m;
}
Compiling with "-O3 -fopenmp -march=znver2 -Wall -Wextra -DNDEBUG" vectorizes
k_max:
.L19:
vmovupd ymm3, YMMWORD PTR [rax+8]
add rax, 32
vmaxpd ymm1, ymm3, ymm1
cmp rax, rdx
jne .L19
but for k_std_max still scalar instructions are used:
.L3:
vmovsd xmm0, QWORD PTR [rax]
add rax, 8
vmaxsd xmm0, xmm0, xmm1
cmp rdx, rax
jne .L5
Note that I had to use -fopenmp as using only -fopenmp-simd did not vectorize
k_max.
Even when I use "-Ofast" or "-Ofast -fopenmp" instead of "-O3" k_std_max is not
vectorized:
.L3:
vmaxsd xmm0, xmm0, QWORD PTR [rax]
add rax, 8
cmp rdx, rax
jne .L3
