https://llvm.org/bugs/show_bug.cgi?id=27625
Bug ID: 27625
Summary: clang loop vectorizer doesn't work when reduction
variable is a slot in an array
Product: clang
Version: 3.8
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: -New Bugs
Assignee: unassignedclangb...@nondot.org
Reporter: albertnet...@gmail.com
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
```
#define SIZE (1 << 15)
#define XMM_ALIGNMENT_BYTES 16
float *vec_a __attribute__((aligned (XMM_ALIGNMENT_BYTES)));
float *vec_b __attribute__((aligned (XMM_ALIGNMENT_BYTES)));
float *vec_c __attribute__((aligned (XMM_ALIGNMENT_BYTES)));
void matvec_autovec()
{
int i, j;
for (i = 0; i < SIZE; i++) {
for (j = 0; j < SIZE; j++) {
vec_c[i] += vec_a[j] * vec_b[j];
}
}
}
```
Command used: `clang-3.8 -c -S -Ofast -Rpass-analysis=loop-vectorize test.c`
```
test.c:17:22: remark: loop not vectorized: value that could not be identified
as reduction is used outside the loop [-Rpass-analysis=loop-vectorize]
vec_c[i] += vec_a[j] * vec_b[j];```
```
I was expecting that `vec_c[i]` to be treated as inner loop invariant and the
vectorization would work. The auto-vectorization works fine if I use a tmp
variable for accumulation. Could someone explain why the vectorizer bails out
in this case?
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