SSE vectorization of math 'pow' function gcc

Question

I was trying to vectorize a loop that contains the use of the 'pow' function in the math library. I am aware intel compiler supports use of 'pow' for sse instructions - but I can't seem to get it to run with gcc ( I think ). This is the case I am working with:

int main(){
        int i=0;
        float a[256],
        b[256];

        float x= 2.3;


        for  (i =0 ; i<256; i++){
                a[i]=1.5;
        }

        for (i=0; i<256; i++){
                b[i]=pow(a[i],x);
        }

        for (i=0; i<256; i++){
                b[i]=a[i]*a[i];
        }
    return 0;

}

I'm compiling with the following:

gcc -O3 -Wall -ftree-vectorize -msse2 -ftree-vectorizer-verbose=5 code.c -o runthis

This is on os X 10.5.8 using gcc version 4.2 (I used 4.5 as well and couldn't tell if it had vectorized anything - as it didn't output anything at all). It appears that none of the loops vectorize - is there an allignment issue or some other issue that I need t use restrict? If I write one of the loops as a function I get slightly more verbose output(code):

void pow2(float *a, float * b, int n) {
        int i;
        for (i=0; i



output (using level 7 verbose output):

note: not vectorized: can't determine dependence between *D.2878_13 and *D.2877_8
bad data dependence.


I looked at the gcc auto-vectorization page but that didnt' help to much. If it is not possible to use pow in the gcc version what where could I find the resource to do a pow - equivalent function  (I'm mostly dealing with integer powers).

Edit so I was just digging into so other source- how did it vectorize this?!:

void array_op(double * d,int len,double value,void (*f)(double*,double*) ) { 
    for ( int i = 0; i < len; i++ ){
        f(&d[i],&value);
    }
};


The relevant gcc output:

note: Profitability threshold is 3 loop iterations.

note: LOOP VECTORIZED.


Well now I'm at a loss -- 'd' and 'value' are modified by a function that gcc doesn't know about - strange? Maybe I need to test this portion a little more thoroughly to make sure the results are correct for the vectorized portion. Still looking for a vectorized math library - why aren't there any open source ones?

SSE vectorization of math 'pow' function gcc

2 Answers