[LLVMdev] Packed instructions generaetd by LoopVectorize?

[Nadav Rotem]

Hi Tyler, 

Try adding -ffast-math.  We can only vectorize reduction variables if it is safe to reorder floating point operations. 

Thanks,
Nadav

On Apr 3, 2013, at 10:29 AM, "Nowicki, Tyler" <tyler.nowicki at intel.com> wrote:

> Hi,
>  
> I have a question about LoopVectorize. I wrote a simple test case, a dot product loop and found that packed instructions are generated when input arrays are integer, but not when they are float or double.
>  
> If I modify the float example in http://llvm.org/docs/Vectorizers.html by adding restrict to the input arrays packed instructions are generated. Although it should not be required I tried doing the same to the input arrays of my dot product example but it still doesn’t generate packed float or double instructions.
>  
> Is the loop vectorizer supposed to generate packed float and double instructions? Is this a bug, or am I doing something wrong?
>  
> Tyler
>  
> float dotproduct(float *A, float *B, int n) {
>   float sum = 0;
>   for(int i = 0; i < n; ++i) {
>     sum += A[i] * B[i];
>   }
>   return sum;
> }
>  
> clang dotproduct.cpp -O3 -fvectorize -march=atom -S -o -
>  
> <loop body>
> .LBB1_1:
>         movss   (%rdi), %xmm1
>         addq    $4, %rdi
>         mulss   (%rsi), %xmm1
>         addq    $4, %rsi
>         decl    %edx
>         addss   %xmm1, %xmm0
>         jne     .LBB1_1

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