Re: Multiply Optimization in match and Simplify

[Marc Glisse <marc.glisse@inria.fr>]

On Fri, 30 Oct 2015, Richard Biener wrote:

> +/* Convert (A + A) * C -> A * 2 * C.  */
> +(simplify
> + (mult:c (convert? (plus @0 @0)) (convert? @1))
> +  (if (tree_nop_conversion_p (TREE_TYPE (@0), type))
> +   (convert (mult @0 (mult { build_int_cst (TREE_TYPE (@1), 2); } @1)))))
> +(simplify
> + (mult:c (convert? (plus @0 @0)) INTEGER_CST@1)
> +  (if (tree_nop_conversion_p (TREE_TYPE (@0), type))
> +   (convert (mult @0 (mult { build_int_cst (TREE_TYPE (@1), 2); } @1)))))
>
> fold-const.c only handles constant C, so we only need to 2nd pattern.
> Also the :c on the mult in that is not needed due to canonicalization rules.
> Please build the result of the inner multiplication directly.
> I think the fold-const.c code has an overflow issue when the outer
> multiplication
> is signed and the inner addition unsigned.  (signed)((unsigned)INT_MAX
> + (unsigned)INT_MAX)*2
> is valid but INT_MAX * 4 is not as it overflows.  So I think we should
> _not_ allow
> nop conversions here (it's fine if all ops are signed or unsigned).

Is there a reason why the simple transformation A+A->2*A is not
generally a desirable canonicalization? We currently restrict it to
SCALAR_FLOAT_TYPE_P.

-- 
Marc Glisse