Re: [PATCH] Move X - (X / Y) * Y folding to match.pd

[Marc Glisse <marc dot glisse at inria dot fr>]

On Fri, 26 Jun 2015, Marek Polacek wrote:

> This is an attempt to move one pattern from fold-const.c to match.pd.
> It ought to be 1:1, but is not, e.g. with this patch we won't fold e.g.
>
> int
> f (int a, int b)
> {
>  return a - (unsigned) ((a / b) * b)
> }
>
> anymore, but we're able to fold
>
> int
> ff (int a, unsigned int b)
> {
>  return a - ((a / b) * b);
> }
>
> and fold-const.c is not.  I played around with converts, but didn't find
> anything that would work well.  Any suggestions how to make this pattern
> better?

Anything wrong with this?

+/* X - (X / Y) * Y is the same as X % Y.  */
+(simplify
+ (minus (convert? @0) (convert? (mult (trunc_div @0 @1) @1)))
+ (if (INTEGRAL_TYPE_P (type) || VECTOR_INTEGER_TYPE_P (type))
+  (convert (trunc_mod @0 @1))))

(the other div/mod pairs could benefit from the same transformation as 
long as there are no conversions, but the conversion seems easier to 
handle with trunc_)

> diff --git gcc/match.pd gcc/match.pd
> index b2f8429..2bc158b 100644
> --- gcc/match.pd
> +++ gcc/match.pd
> @@ -238,6 +238,12 @@ along with GCC; see the file COPYING3.  If not see
>       && tree_nop_conversion_p (type, TREE_TYPE (@1)))
>   (trunc_mod @0 (convert @1))))
>
> +/* X - (X / Y) * Y is the same as X % Y.  */
> +(simplify
> + (minus @0 (mult (trunc_div @0 @1) @1))
> + (if (INTEGRAL_TYPE_P (type) || VECTOR_INTEGER_TYPE_P (type))
> +  (trunc_mod @0 @1)))
> +
> /* Optimize TRUNC_MOD_EXPR by a power of two into a BIT_AND_EXPR,
>    i.e. "X % C" into "X & (C - 1)", if X and C are positive.
>    Also optimize A % (C << N)  where C is a power of 2,
>
> 	Marek

--
Marc Glisse