Re: RFC: Two minor optimization patterns

[Andrew Pinski <pinskia at gmail dot com>]

On Tue, Jan 13, 2015 at 2:41 PM, Rasmus Villemoes <rv@rasmusvillemoes.dk> wrote:
> [My first attempt at submitting a patch for gcc, so please forgive me
> if I'm not following the right protocol.]

There are a few things missing.  For one, a testcase or two for the
added optimizations.

>
> Sometimes rounding a variable to the next even integer is written x += x
> & 1. This usually means using an extra register (and hence at least an
> extra mov instruction) compared to the equivalent x = (x + 1) & ~1. The
> first pattern below tries to do this transformation.
>
> While playing with various ways of rounding down, I noticed that gcc
> already optimizes all of x-(x&3), x^(x&3) and x&~(x&3) to simply
> x&~3. In fact, x&~(x&y) is rewritten as x&~y. However, the dual of this
> is not handled, so I included the second pattern below.
>
> I've tested the below in the sense that gcc compiles and that trivial
> test cases get compiled as expected.

The other thing you missed is a changelog entry for the change you did.
Also you mentioned you tested the patch below but did not mention
which target you tested it on and you should run the full GCC
testsuite.
https://gcc.gnu.org/contribute.html is a good page to start with how
to handle most of the items above.
https://gcc.gnu.org/wiki/HowToPrepareATestcase is a good page on how
to write the testcase for testing the added optimization.

Thanks,
Andrew Pinski

>
> Rasmus
>
>
>
> diff --git gcc/match.pd gcc/match.pd
> index 81c4ee6..04a0bc4 100644
> --- gcc/match.pd
> +++ gcc/match.pd
> @@ -262,6 +262,16 @@ along with GCC; see the file COPYING3.  If not see
>   (abs tree_expr_nonnegative_p@0)
>   @0)
>
> +/* x + (x & 1) -> (x + 1) & ~1 */
> +(simplify
> + (plus @0 (bit_and @0 integer_onep@1))
> + (bit_and (plus @0 @1) (bit_not @1)))
> +
> +/* x | ~(x | y) -> x | ~y */
> +(simplify
> + (bit_ior @0 (bit_not (bit_ior @0 @1)))
> + (bit_ior @0 (bit_not @1)))
> +
>
>  /* Try to fold (type) X op CST -> (type) (X op ((type-x) CST))
>     when profitable.