version of gcc and auto-vectorization

[Ian Lance Taylor]

ranjith kumar <ranjit_kumar_b4u@yahoo.co.uk> writes:

> What does auto-vectorization mean??
> I think it must be converting 'for' loops which does
> not exploit SIMD features of a processor(say Pentium
> 4) to 'for' loops which exploit SIMD features(of
> course, it could be at any intermediate
> representation).

Yes, that is what it means.

> I have taken vect-40.c in that directory and compiled
> as "gcc -march=pentium4 -S -O3 -ftree-vectorize
> vect-40.c". I looked at the assembly code. No
> MMX/SSE/SSE2 instructions were there.
> (I have gcc-4.1.2 installed in my P.C. and my
> processor is Pentium4.)
> 
> What can gcc do? Can it produce MMX/SSE/SSE2
> instructions even if the source file(.c) does not use
> any functions defined in 
> mmintrin.h/xmmintrin.h/emmintrin.h????

Yes, it can.

Here is an example which vectorizes for me with gcc 4.1.2 with -O2
-ftree-vectorize -std=c99 -march=pentium4

void
foo (int n, int * restrict c, const int * restrict a, const int * restrict b)
{
  int i;

  for (i = 0; i < n; ++i)
    c[i] = a[i] + b[i];
}

Ian