Autovectorization help

[Nicklas Bo Jensen]

Thanks for your response Tim,

I'm not familiar with the term "simulated gather" and google doesn't
help much. Would it be something like performing the memory operations
with scalar instructions first and then perform the reduction with
vector instructions? Does gcc have such capabilities?

Thanks,
Nicklas



On Thu, Oct 4, 2012 at 3:04 PM, Tim Prince <n8tm@aol.com> wrote:
> On 10/4/2012 8:20 AM, Nicklas Bo Jensen wrote:
>>
>> Hi,
>>
>> I'm trying to use the autovectorizer in gcc 4.7.2. However I'm getting
>> bad data references in the .vect comments generated using
>> -ftree-vectorizer-verbose=, even though I have used restrict on all
>> arrays. Its a reference with two arrays where one array is used to
>> index the other: array1[array2[index]]. I don't see how this should
>> not be vectorizable as we only read from the arrays.
>>
>> Is this not supported or is there some clever way to rewrite this?
>>
>> Example:
>>
>> int foo(int * restrict array1, int * restrict array2) {
>>    int res = 0;
>>    for (int i = 0; i < 50000; i++) {
>>      int v = array2[array1[i]]; //This gives bad data reference comment
>> in .vect file.
>>      res += v * v;
>>    }
>>    return res;
>> }
>>
>>
> You would require "simulated gather" to take advantage of SSE4 or AVX scalar
> to vector register moves (so your -march setting enters in).  restrict has
> no bearing here, as you modify only res and v which have segregated scope
> from the data regions accessed by pointer.
> Intel compilers tend to require pragma stuff such as #pragma simd
> reduction(+: res) to promote "vectorization" using simulated gather.
> Evidently, such idioms are typically used with floating point data types and
> -ffast-math or equivalent options to enable associative-math.
>
> --
> Tim Prince
>