Re: [PATCH] match.pd: reassociate multiplications with constants

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

On Tue, Jul 18, 2017 at 7:07 PM, Alexander Monakov <amonakov@ispras.ru> wrote:
> On Mon, 17 Jul 2017, Alexander Monakov wrote:
>> On Mon, 17 Jul 2017, Marc Glisse wrote:
>> > > +/* Combine successive multiplications.  Similar to above, but handling
>> > > +   overflow is different.  */
>> > > +(simplify
>> > > + (mult (mult @0 INTEGER_CST@1) INTEGER_CST@2)
>> > > + (with {
>> > > +   bool overflow_p;
>> > > +   wide_int mul = wi::mul (@1, @2, TYPE_SIGN (type), &overflow_p);
>> > > +  }
>> > > +  (if (!overflow_p || TYPE_OVERFLOW_WRAPS (type))
>> >
>> > I wonder if there are cases where this would cause trouble for saturating
>> > integers. The only case I can think of is when @2 is -1, but that's likely
>> > simplified to NEGATE_EXPR first.
>>
>> Ah, yes, I think if @2 is -1 or 0 then we should not attempt this transform for
>> either saturating or sanitized types, just like in the first patch. I think
>> wrapping the 'with' with 'if (!integer_minus_onep (@2) && !integer_zerop (@2))'
>> works, since as you say it should become a negate/zero anyway?
>
> Updated patch:
>
>         * match.pd ((X * CST1) * CST2): Simplify to X * (CST1 * CST2).
> testsuite:
>         * gcc.dg/tree-ssa/assoc-2.c: Enhance.
>         * gcc.dg/tree-ssa/slsr-4.c: Adjust.
>
> diff --git a/gcc/match.pd b/gcc/match.pd
> index 36045f1..0bb5541 100644
> --- a/gcc/match.pd
> +++ b/gcc/match.pd
> @@ -283,6 +283,20 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
>          || mul != wi::min_value (TYPE_PRECISION (type), SIGNED))
>       { build_zero_cst (type); })))))
>
> +/* Combine successive multiplications.  Similar to above, but handling
> +   overflow is different.  */
> +(simplify
> + (mult (mult @0 INTEGER_CST@1) INTEGER_CST@2)
> + /* More specific rules can handle 0 and -1; skip them here to avoid
> +    wrong transformations for sanitized and saturating types.  */
> + (if (!integer_zerop (@2) && !integer_minus_onep (@2))

I think integer_zerop (@2) should never happen here if you order the
pattern after

(simplify
 (mult @0 integer_zerop@1)
 @1)

which I think you do.  Why's @1 == -1 ok when sanitizing but not @2 == -1?
If you rely on

/* Transform x * -1 into -x.  */
(simplify
 (mult @0 integer_minus_onep)
 (negate @0))

then you need to move that pattern above yours (there seem to be a bunch
of related ones like 0 - @1 -> -@1 to move earlier as well).

That would leave us with the case of saturating types (the above transforms
doesn't care for those).

So unless you can come up with a testcase that breaks I'd remove the
integer_zerop/integer_minus_onep checks.

> +  (with {
> +    bool overflow_p;
> +    wide_int mul = wi::mul (@1, @2, TYPE_SIGN (type), &overflow_p);
> +   }
> +   (if (!overflow_p || TYPE_OVERFLOW_WRAPS (type))

I think overflow in the constant multiplication is ok unless
TYPE_OVERFLOW_SANITIZED
(and can we have a ubsan testcase for that that would otherwise fail?).
It's not that we introduce new overflow here.

Ok with those changes.

Thanks,
Richard.

> +    (mult @0 { wide_int_to_tree (type, mul); })))))
> +
>  /* Optimize A / A to 1.0 if we don't care about
>     NaNs or Infinities.  */
>  (simplify
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/assoc-2.c b/gcc/testsuite/gcc.dg/tree-ssa/assoc-2.c
> index a92c882..cc0e9d4 100644
> --- a/gcc/testsuite/gcc.dg/tree-ssa/assoc-2.c
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/assoc-2.c
> @@ -5,4 +5,15 @@ int f0(int a, int b){
>    return a * 33 * b * 55;
>  }
>
> -/* { dg-final { scan-tree-dump-times "mult_expr" 2 "gimple" } } */
> +int f1(int a){
> +  a *= 33;
> +  return a * 55;
> +}
> +
> +int f2(int a, int b){
> +  a *= 33;
> +  return a * b * 55;
> +}
> +
> +/* { dg-final { scan-tree-dump-times "mult_expr" 7 "gimple" } } */
> +/* { dg-final { scan-tree-dump-times "mult_expr" 5 "optimized" } } */
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/slsr-4.c b/gcc/testsuite/gcc.dg/tree-ssa/slsr-4.c
> index 17d7b4c..1e943b7 100644
> --- a/gcc/testsuite/gcc.dg/tree-ssa/slsr-4.c
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/slsr-4.c
> @@ -23,13 +23,9 @@ f (int i)
>    foo (y);
>  }
>
> -/* { dg-final { scan-tree-dump-times "\\* 4" 1 "slsr" } } */
> -/* { dg-final { scan-tree-dump-times "\\* 10" 1 "slsr" } } */
> -/* { dg-final { scan-tree-dump-times "\\+ 20;" 1 "slsr" } } */
> +/* { dg-final { scan-tree-dump-times "\\* 40" 1 "slsr" } } */
>  /* { dg-final { scan-tree-dump-times "\\+ 200" 1 "slsr" } } */
> -/* { dg-final { scan-tree-dump-times "\\- 16;" 1 "slsr" } } */
>  /* { dg-final { scan-tree-dump-times "\\- 160" 1 "slsr" } } */
> -/* { dg-final { scan-tree-dump-times "\\* 4" 1 "optimized" } } */
> -/* { dg-final { scan-tree-dump-times "\\* 10" 1 "optimized" } } */
> +/* { dg-final { scan-tree-dump-times "\\* 40" 1 "optimized" } } */
>  /* { dg-final { scan-tree-dump-times "\\+ 200" 1 "optimized" } } */
>  /* { dg-final { scan-tree-dump-times "\\+ 40" 1 "optimized" } } */