Re: RFC: Two minor optimization patterns

[Richard Biener <richard dot guenther at gmail dot com>]

On Tue, Jan 13, 2015 at 11:47 PM, Andrew Pinski <pinskia@gmail.com> wrote:
> 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.

Does it also handle x+(x&3)?  Where does it handle x - (x&3)?

That is, doesn't the pattern also work for constants other than 1?

Please put it before the abs simplifications after the last one handing
bit_and/bit_ior.

Thanks,
Richard.

> 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.