rs6000 fused multiply-add patch

[Segher Boessenkool]

David Edelsohn wrote:

>         I would recommend that you focus on understanding why Geoff's
> patch is more correct.  The different patterns provide logically different
> building blocks for GCC to use instead of duplicating patterns to provide
> missing optimization transformations.  MD files should provide canonical
> forms of patterns and both patterns in the new patch are canonical,
> although they are the same machine instruction.  The original patch was
> not limited to canonical patterns.

Geoff's patch redefines canonical ;) :


Geoffrey Keating wrote:
> +@item
> +In combinations of @code{neg}, @code{mult}, @code{plus}, and
> +@code{minus}, the @code{neg} operations (if any) will be moved inside
> +the operations as far as possible.  For instance,
> +@code{(neg (mult A B))} is canonicalized as @code{(mult (neg A) B)}, but
> +@code{(plus (mult (neg A) B) C)} is canonicalized as
> +@code{(minus A (mult B C))}.

Why is it better to move the NEG's inward instead of outward?


> +void foo(double *a, double *b, double *c, double *d)
> +{
> +  a[0] =  b[0] + c[0] * d[0];          // fmadd
> +  a[1] =  b[1] - c[1] * d[1];          // fnmsub with fast-math
> +  a[2] = -b[2] + c[2] * d[2];          // fmsub
> +  a[3] = -b[3] - c[3] * d[3];          // fnmadd with fast-math
> +  a[4] = -( b[4] + c[4] * d[4]);       // fnmadd
> +  a[5] = -( b[5] - c[5] * d[5]);       // fmsub with fast-math
> +  a[6] = -(-b[6] + c[6] * d[6]);       // fnmsub
> +  a[7] = -(-b[7] - c[7] * d[7]);       // fmadd with fast-math
> +  a[10] =  b[10] - c[10] * -d[10];     // fmadd
> +  a[11] =  b[11] + c[11] * -d[11];     // fnmsub with fast-math
> +  a[12] = -b[12] - c[12] * -d[12];     // fmsub
> +  a[13] = -b[13] + c[13] * -d[13];     // fnmadd with fast-math
> +  a[14] = -( b[14] - c[14] * -d[14]);  // fnmadd
> +  a[15] = -( b[15] + c[15] * -d[15]);  // fmsub with fast-math
> +  a[16] = -(-b[16] - c[16] * -d[16]);  // fnmsub
> +  a[17] = -(-b[17] + c[17] * -d[17]);  // fmadd with fast-math
> +}

You are no tests that have the multiply first (to the left of the addition);
this mattered in my testing (with the non-patched gcc, too).


Segher