Re: [PATCH] Simplify mult where both arguments are 0 or 1 (PR tree-optimization/87954).

[Martin Liška]

On 6/5/19 1:13 PM, Richard Biener wrote:
> On Wed, Jun 5, 2019 at 12:56 PM Martin Liška <mli...@suse.cz> wrote:
>>
>> Hi.
>>
>> I'm suggesting one multiplication simplification pattern.
>>
>> Patch can bootstrap on x86_64-linux-gnu and survives regression tests.
>>
>> Ready to be installed?
> 
> +  (if (INTEGRAL_TYPE_P (type)
> +       && wi::eq_p (get_nonzero_bits (@1), wi::one (TYPE_PRECISION (type)))
> +       && wi::eq_p (get_nonzero_bits (@2), wi::one (TYPE_PRECISION (type))))
> 
>   && wi::eq_p (wi::bit_or (get_nonzero_bits (@1), get_nonzero_bits  (@2)), 1))
> 
> (I think literal 1 still works)? 

Yep, I can confirm that.

> How does it behave for  singed/unsigned 1-bit
> bitfields?  A gimple testcase maybe necessary to see.

Can we really have a mult that will have a bitfield type?

$ cat gcc/testsuite/gcc.dg/pr87954-2.c
#define __GFP_DMA 1u
#define __GFP_RECLAIM 0x10u

struct bt
{
  unsigned int v:1;
};

unsigned int
imul(unsigned int flags)
{
  struct bt is_dma, is_rec;

  is_dma.v = !!(flags & __GFP_DMA);
  is_rec.v = !!(flags & __GFP_RECLAIM);

  return is_rec.v * !is_dma.v;
}

$ ./xgcc -B. /home/marxin/Programming/gcc/gcc/testsuite/gcc.dg/pr87954-2.c 
-fdump-tree-optimized=/dev/stdout -O2

;; Function imul (imul, funcdef_no=0, decl_uid=1909, cgraph_uid=1, 
symbol_order=0)

imul (unsigned int flags)
{
  struct bt is_dma;
  _Bool _1;
  unsigned int _2;
  _Bool _3;
  unsigned char _4;
  _Bool _6;
  unsigned int _9;
  <unnamed-unsigned:1> _11;
  unsigned char _14;

  <bb 2> [local count: 1073741824]:
  _1 = (_Bool) flags_7(D);
  _2 = flags_7(D) & 16;
  _3 = _2 != 0;
  is_dma.v = _1;
  _4 = BIT_FIELD_REF <is_dma, 8, 0>;
  _14 = ~_4;
  _6 = (_Bool) _14;
  _11 = _3 & _6;
  _9 = (unsigned int) _11;
  is_dma ={v} {CLOBBER};
  return _9;
}

> 
> Does this mean we want to turn plus into bit_ior when
> get_nonzero_bits() & get_nonzero_bits() is zero?

That's quite interesting transformation, I'll add it as a follow up patch.

> 
> X * [0, 1] -> X & sign-extend-from-bit-1 also works I guess, but 
> multiplication
> looks more canonical.
Ok here.

Martin

> 
> Thanks,
> Richard.
> 
>> Thanks,
>> Martin
>>
>> gcc/ChangeLog:
>>
>> 2019-06-05  Martin Liska  <mli...@suse.cz>
>>
>>         PR tree-optimization/87954
>>         * match.pd: Simplify mult where both arguments are 0 or 1.
>>
>> gcc/testsuite/ChangeLog:
>>
>> 2019-06-05  Martin Liska  <mli...@suse.cz>
>>
>>         PR tree-optimization/87954
>>         * gcc.dg/pr87954.c: New test.
>> ---
>>  gcc/match.pd                   |  8 ++++++++
>>  gcc/testsuite/gcc.dg/pr87954.c | 21 +++++++++++++++++++++
>>  2 files changed, 29 insertions(+)
>>  create mode 100644 gcc/testsuite/gcc.dg/pr87954.c
>>
>>