Question about the loop vectorizer

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1

Hi all,

I have a question about the loop vectorizer.

For the following code:

void example1 (float a, float b, float c, int n) {
    int i;
    for (i=0; i<n; i++){
        a[i] = c[i];
    }
    for (i=0; i<n; i++){
        a[i] += b[i];
    }
}

void example2 (float a, float b, float c, int n) {
    int i;
    for (i=0; i<n; i++){
        a[i] = b[i] + c[i];
    }
}

The two functions are doing the same thing. But only example1 is
vectorized. Is there a reason for not vectorizing example2? Or, is
it simply a feature not available in llvm3.2?

Regards,
Siu

2

Hi Michael,

Thanks for trying the Loop Vectorizer. All of the loops in the examples you sent are vectorized in LLVM ToT. On Sanybridge, example2 is vectorized to 8 x float (YMM registers) and is unrolled twice. I imagine that 3.2 is too old.

Thanks,
Nadav