Re: compilers using MMX instructions in the generated code

bcombee@metrowerks.com (Ben Combee) writes:


> There are limitations... I used global arrays on purpose here -- we do
> not yet support the ISO C 1999 keyword "restrict" that would let you
> give the compiler enough information to know that the b and c arrays
> did not alias with array a, so if you passed the arrays in as
> parameters, we would not attempt the vectorization.


You also didn't mention the alignment restrictions. Isn't it the case
that the inputs need to be 128-bit aligned? So if you don't know the
alignment at compile-time, or the alignment happens to be unfortunate,
you're out of luck. Consider:


short int a1[50], a2[50], b[50], c[50];


void foo(void)
{
        int i;
        for (i = 0; i < 50; i++) a1[i] = b[i] + c[i];
        for (i = 0; i < 49; i++) a2[i] = b[i+1] + c[i];
}


In this example, I don't think you can vectorize both loops.


-- g

Return to the comp.compilers page.
Search the comp.compilers archives again.