Re: [PATCH] match.pd: rewrite select to branchless expression

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

On Wed, Nov 9, 2022 at 10:06 PM Michael Collison <collison@rivosinc.com> wrote:
>
> Richard,
>
> Thanks for your feedback. I want to make sure I am following what you
> are recommending. Are you suggesting changing:
>
> (for op (bit_xor bit_ior)
> (simplify
> (cond (eq (bit_and @0 integer_onep@1)
> integer_zerop)
> @2
> (op:c @3 @2))
> (if (INTEGRAL_TYPE_P (type)
> && (INTEGRAL_TYPE_P (TREE_TYPE (@0))))
> (op (bit_and (negate (convert:type (bit_and @0 @1))) @3) @2))))
>
>
> to
>
> (for op (bit_xor bit_ior)
>   (simplify
>    (cond (eq zero_one_valued_p@0
>              integer_zerop)
>          @1
>          (op:c @2 @1))
>    (if (INTEGRAL_TYPE_P (type)
>         && (INTEGRAL_TYPE_P (TREE_TYPE (@0))))
>         (op (bit_and (negate (convert:type (bit_and @0 { build_one_cst
> (type); }))) @2) @1))))

in the replacement you'd simply use

     (op (bit_and (negate (convert:type @0)) @2) @1)))

that is, convert the [0,1] valued @0 to 'type' directly.  At least I can't see
how that can go wrong?

>
>
> On 11/9/22 02:41, Richard Biener wrote:
> > On Tue, Nov 8, 2022 at 9:02 PM Michael Collison <collison@rivosinc.com> wrote:
> >> This patches transforms (cond (and (x , 0x1) == 0), y, (z op y)) into
> >> (-(and (x , 0x1)) & z ) op y, where op is a '^' or a '|'. It also
> >> transforms (cond (and (x , 0x1) != 0), (z op y), y ) into (-(and (x ,
> >> 0x1)) & z ) op y.
> >>
> >> Matching this patterns allows GCC to generate branchless code for one of
> >> the functions in coremark.
> >>
> >> Bootstrapped and tested on x86 and RISC-V. Okay?
> >>
> >> Michael.
> >>
> >> 2022-11-08  Michael Collison  <collison@rivosinc.com>
> >>
> >>       * match.pd ((cond (and (x , 0x1) == 0), y, (z op y) )
> >>       -> (-(and (x , 0x1)) & z ) op y)
> >>
> >> 2022-11-08  Michael Collison  <collison@rivosinc.com>
> >>
> >>       * gcc.dg/tree-ssa/branchless-cond.c: New test.
> >>
> >> ---
> >>    gcc/match.pd                                  | 22 ++++++++++++++++
> >>    .../gcc.dg/tree-ssa/branchless-cond.c         | 26 +++++++++++++++++++
> >>    2 files changed, 48 insertions(+)
> >>    create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/branchless-cond.c
> >>
> >> diff --git a/gcc/match.pd b/gcc/match.pd
> >> index 194ba8f5188..722f517ac6d 100644
> >> --- a/gcc/match.pd
> >> +++ b/gcc/match.pd
> >> @@ -3486,6 +3486,28 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> >>      (cond (le @0 integer_zerop@1) (negate@2 @0) integer_zerop@1)
> >>      (max @2 @1))
> >>
> >> +/* (cond (and (x , 0x1) == 0), y, (z ^ y) ) -> (-(and (x , 0x1)) & z )
> >> ^ y */
> > Please write the match as a C expression in the comment, as present
> > it's a weird mix.  So x & 0x1 == 0 ? y : z <op> y -> (-(typeof(y))(x &
> > 0x1) & z) <op> y
> >
> >> +(for op (bit_xor bit_ior)
> >> + (simplify
> >> +  (cond (eq (bit_and @0 integer_onep@1)
> >> +            integer_zerop)
> >> +        @2
> >> +        (op:c @3 @2))
> >> +  (if (INTEGRAL_TYPE_P (type)
> >> +       && (INTEGRAL_TYPE_P (TREE_TYPE (@0))))
> >> +       (op (bit_and (negate (convert:type (bit_and @0 @1))) @3) @2))))
> > Since you are literally keeping (bit_and @0 @1) and not matching @0 with
> > anything I suspect you could instead use
> >
> >   (simplify (cond (eq zero_one_valued_p@0 integer_zerop) ...
> >
> > eventually extending that to cover bit_and with one.  Do you need to guard
> > this against 'type' being a signed/unsigned 1-bit precision integer?
> >
> >> +
> >> +/* (cond (and (x , 0x1) != 0), (z ^ y), y ) -> (-(and (x , 0x1)) & z )
> >> ^ y */
> >> +(for op (bit_xor bit_ior)
> >> + (simplify
> >> +  (cond (ne (bit_and @0 integer_onep@1)
> >> +            integer_zerop)
> >> +    (op:c @3 @2)
> >> +        @2)
> >> +  (if (INTEGRAL_TYPE_P (type)
> >> +       && (INTEGRAL_TYPE_P (TREE_TYPE (@0))))
> >> +       (op (bit_and (negate (convert:type (bit_and @0 @1))) @3) @2))))
> >> +
> >>    /* Simplifications of shift and rotates.  */
> >>
> >>    (for rotate (lrotate rrotate)
> >> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/branchless-cond.c
> >> b/gcc/testsuite/gcc.dg/tree-ssa/branchless-cond.c
> >> new file mode 100644
> >> index 00000000000..68087ae6568
> >> --- /dev/null
> >> +++ b/gcc/testsuite/gcc.dg/tree-ssa/branchless-cond.c
> >> @@ -0,0 +1,26 @@
> >> +/* { dg-do compile } */
> >> +/* { dg-options "-O2 -fdump-tree-optimized" } */
> >> +
> >> +int f1(unsigned int x, unsigned int y, unsigned int z)
> >> +{
> >> +  return ((x & 1) == 0) ? y : z ^ y;
> >> +}
> >> +
> >> +int f2(unsigned int x, unsigned int y, unsigned int z)
> >> +{
> >> +  return ((x & 1) != 0) ? z ^ y : y;
> >> +}
> >> +
> >> +int f3(unsigned int x, unsigned int y, unsigned int z)
> >> +{
> >> +  return ((x & 1) == 0) ? y : z | y;
> >> +}
> >> +
> >> +int f4(unsigned int x, unsigned int y, unsigned int z)
> >> +{
> >> +  return ((x & 1) != 0) ? z | y : y;
> >> +}
> >> +
> >> +/* { dg-final { scan-tree-dump-times " -" 4 "optimized" } } */
> >> +/* { dg-final { scan-tree-dump-times " & " 8 "optimized" } } */
> >> +/* { dg-final { scan-tree-dump-not "if" "optimized" } } */
> >> --
> >> 2.34.1
> >>
> >>
> >>
> >>