Re: Autovectorization help

[Tim Prince <n8tm at aol dot com>]

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