https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105968
Bug ID: 105968
Summary: GCC vectorizes but reports that it did not vectorize
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: steveire at gmail dot com
Target Milestone: ---
I'm looking for a way to know if GCC autovectorizes some code.
Starting with this testcase which I picked up somewhere:
```
#define N 10000
#define NTIMES 100000
double a[N] __attribute__ ((aligned (16)));
double b[N] __attribute__ ((aligned (16)));
double c[N] __attribute__ ((aligned (16)));
double r[N] __attribute__ ((aligned (16)));
int muladd (void) {
int i, times;
for (times = 0; times < NTIMES; times++) {
#if 1
// count up
for (i = 0; i < N; ++i)
r[i] = (a[i] + b[i]) * c[i];
#else
// count down (old gcc won't auto-vectorize)
for (i = N-1; i >= 0; --i)
r[i] = (a[i] + b[i]) * c[i];
#endif
}
return 0;
}
```
the command
```
g++ -O2 -ftree-vectorize -fno-verbose-asm -mavx2 -fopt-info-vec-all -c test.cpp
```
reports
```
test.cpp:9:5: note: vectorized 1 loops in function.
```
However, with -O3, GCC reports that it did not vectorize:
```
g++ -O3 -ftree-vectorize -fno-verbose-asm -mavx2 -fopt-info-vec-all -c test.cpp
```
output:
```
test.cpp:9:5: note: vectorized 0 loops in function.
```
even though vector instructions are generated.
Demo https://godbolt.org/z/3o41r7jWc
