Re: [PATCH 4/4] match.pd: Add x + ((-x) & m) -> (x + m) & ~m pattern

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Apr 30, 2015 at 1:44 PM, Marc Glisse <marc.glisse@inria.fr> wrote:
> On Thu, 30 Apr 2015, Richard Biener wrote:
>
>> On Wed, Jan 21, 2015 at 11:49 AM, Rasmus Villemoes
>> <rv@rasmusvillemoes.dk> wrote:
>>>
>>> Generalizing the x+(x&1) pattern, one can round up x to a multiple of
>>> a 2^k by adding the negative of x modulo 2^k. But it is fewer
>>> instructions, and presumably requires fewer registers, to do the more
>>> common (x+m)&~m where m=2^k-1.
>>>
>>> Signed-off-by: Rasmus Villemoes <rv@rasmusvillemoes.dk>
>>> ---
>>>  gcc/match.pd                      |  9 ++++++
>>>  gcc/testsuite/gcc.dg/20150120-4.c | 59
>>> +++++++++++++++++++++++++++++++++++++++
>>>  2 files changed, 68 insertions(+)
>>>  create mode 100644 gcc/testsuite/gcc.dg/20150120-4.c
>>>
>>> diff --git gcc/match.pd gcc/match.pd
>>> index 47865f1..93c2298 100644
>>> --- gcc/match.pd
>>> +++ gcc/match.pd
>>> @@ -273,6 +273,15 @@ along with GCC; see the file COPYING3.  If not see
>>>   (if (TREE_CODE (@2) != SSA_NAME || has_single_use (@2))
>>>    (bit_ior @0 (bit_not @1))))
>>>
>>> +/* x + ((-x) & m) -> (x + m) & ~m when m == 2^k-1.  */
>>> +(simplify
>>> + (plus:c @0 (bit_and@2 (negate @0) CONSTANT_CLASS_P@1))
>>
>>
>> I think you want to restrict this to INTEGER_CST@1
>
>
> Is this only to make the following test easier (a good enough reason for me)
> or is there some fundamental reason why this transformation would be wrong
> for vectors?

Good question - I suppose it also works for vectors (well, the predicates
don't).  for non-ingegers or complex ints we shouldn't arrive here as
we can't have bit_and for them.  for pointers we can't have plus on them.

So yes, it makes the following tests easier.  A TODO comment for vectors
might be appropriate (we'd simply need a predicate that can test for
all emlements being 2^k-1).

Richard.

>
>>> + (with { tree cst = fold_binary (PLUS_EXPR, TREE_TYPE (@1),
>>> +                                @1, build_one_cst (TREE_TYPE (@1))); }
>>
>>
>> We shouldn't dispatch to fold_binary in patterns.  int_const_binop would
>> be the appropriate function to use - but what happens for @1 == INT_MAX
>> where @1 + 1 overflows?  Similar, is this also valid for negative @1
>> and thus signed mask types?  IMHO we should check whether @1
>> is equal to wi::mask (TYPE_PRECISION (TREE_TYPE (@1)) - wi::clz (@1),
>> false, TYPE_PRECISION (TREE_TYPE (@1)).
>>
>> As with the other patch a ChangeLog entry is missing as well as stating
>> how you tested the patch.
>>
>> Thanks,
>> Richard.
>>
>>> +  (if ((TREE_CODE (@2) != SSA_NAME || has_single_use (@2))
>>> +       && cst && integer_pow2p (cst))
>>> +   (bit_and (plus @0 @1) (bit_not @1)))))
>
>
> --
> Marc Glisse