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Re: how to check if target supports andnot instruction ?


On 12 October 2016 at 14:43, Richard Biener <rguenther@suse.de> wrote:
> On Wed, 12 Oct 2016, Marc Glisse wrote:
>
>> On Wed, 12 Oct 2016, Prathamesh Kulkarni wrote:
>>
>> > I was having a look at PR71636 and added the following pattern to match.pd:
>> > x & ((1U << b) - 1) -> x & ~(~0U << b)
>> > However the transform is useful only if the target supports "andnot"
>> > instruction.
>>
>> rth was selling the transformation as a canonicalization, which is beneficial
>> when there is an andnot instruction, and neutral otherwise, so it could be
>> done always.
>
> Well, its three instructions to three instructions and a more expensive
> constant(?).  ~0U might not be available as immediate for the shift
> instruction and 1U << b might be available as a bit-set instruction ...
> (vs. the andnot).
>
> So yes, we might decide to canonicalize to andnot (and decide that
> three binary to two binary and one unary op is "better").
>
> So no excuse to explore the target specific .pd fragment idea ... :/
Hi,
I have attached patch that adds the transform.
Does that look OK ?

I am not sure how to write test-cases for it though.
For the test-case:
unsigned f(unsigned x, unsigned b)
{
  unsigned t1 = 1U << b;
  unsigned t2 = t1 - 1;
  unsigned t3 = x & t2;
  return t3;
}

forwprop dump shows:
Applying pattern match.pd:523, gimple-match.c:47419
gimple_simplified to _6 = 4294967295 << b_1(D);
_8 = ~_6;
t3_5 = x_4(D) & _8;

I could scan for "_6 = 4294967295 << b_1(D);"  however I suppose
~0 would depend on width of int and not always be 4294967295 ?
Or should I scan for "_6 = 4294967295 << b_1(D);"
and add /* { dg-require-effective int32 } */  to the test-case ?

Thanks,
Prathamesh
>
> Richard.
>
>> > As pointed out by Marc in PR for -march=core2, lhs generates worse
>> > code than rhs,
>> > so we shouldn't do the transform if target doesn't support andnot insn.
>> > (perhaps we could do the reverse transform for target not supporting
>> > andnot?)
>>
>> Rereading my comment in the PR, I pointed out that instead of being neutral,
>> the transformation was very slightly detrimental in one case (one extra mov)
>> because of a RA issue. That doesn't mean we should avoid the transformation,
>> just that we should fix the RA issue (by the way, if you have time to file a
>> separate PR for the RA issue, that would be great, otherwise I'll try to do it
>> at some point...).
>>
>> > However it seems andnot isn't a standard pattern name, so am not sure how to
>> > check if target supports andnot insn ?
>

Attachment: pr71636-1.diff
Description: Text document


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