Re: [PATCH] match.pd: Implement missed optimization (~X | Y) ^ X -> ~(X & Y) [PR109986]

[Jakub Jelinek <jakub@redhat.com>]

On Thu, Jul 06, 2023 at 03:00:28PM +0200, Richard Biener via Gcc-patches wrote:
> On Wed, Jul 5, 2023 at 3:42 PM Drew Ross via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> >     Adds a simplification for (~X | Y) ^ X to be folded into ~(X & Y).
> >     Tested successfully on x86_64 and x86 targets.
> >
> >             PR middle-end/109986
> >
> >     gcc/ChangeLog:
> >
> >             * match.pd ((~X | Y) ^ X -> ~(X & Y)): New simplification.
> >
> >     gcc/testsuite/ChangeLog:
> >
> >             * gcc.c-torture/execute/pr109986.c: New test.
> >             * gcc.dg/tree-ssa/pr109986.c: New test.
> > ---
> >  gcc/match.pd                                  |  11 ++
> >  .../gcc.c-torture/execute/pr109986.c          |  41 ++++
> >  gcc/testsuite/gcc.dg/tree-ssa/pr109986.c      | 177 ++++++++++++++++++
> >  3 files changed, 229 insertions(+)
> >  create mode 100644 gcc/testsuite/gcc.c-torture/execute/pr109986.c
> >  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr109986.c
> >
> > diff --git a/gcc/match.pd b/gcc/match.pd
> > index a17d6838c14..d9d7d932881 100644
> > --- a/gcc/match.pd
> > +++ b/gcc/match.pd
> > @@ -1627,6 +1627,17 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> >   (if (tree_nop_conversion_p (type, TREE_TYPE (@0)))
> >    (convert (bit_and @1 (bit_not @0)))))
> >
> > +/* (~X | Y) ^ X -> ~(X & Y).  */
> > +(simplify
> > + (bit_xor:c (nop_convert1?
> > +             (bit_ior:c (nop_convert2? (bit_not (nop_convert3? @0)))
> > +                        @1)) (nop_convert4? @0))
> 
> you want to reduce the number of nop_convert? - for example
> I wonder if we can canonicalize
> 
>  (T)~X and ~(T)X
> 
> for nop-conversions.  The same might apply to binary bitwise operations
> where we should push those to a direction where they are likely eliminated.
> Usually we'd push them outwards.
> 
> The issue with the above pattern is that nop_convertN? expands to 2^N
> separate patterns.  Together with the two :c you get 64 out of this.
> 
> I do not see that all of the combinations can happen when X has to
> match unless we fail to contract some of them like if we have
> (unsigned)(~(signed)X | Y) ^ X which we could rewrite like
> -> (unsigned)((signed)~X | Y) ^ X -> (~X | (unsigned) Y) ^ X
> with the last step being somewhat difficult unless we do
> (signed)~X | Y -> (signed)(~X | (unsigned)Y).  It feels like a
> propagation problem and less of a direct pattern matching one.

The nop_convert1? in the pattern might seem to be unnecessary
for cases like:
int i, j, k, l;
unsigned u, v, w, x;

void
foo (void)
{
  int t0 = i;
  int t1 = (~t0) | j;
  x = t1 ^ (unsigned) t0;
  unsigned t2 = u;
  unsigned t3 = (~t2) | v;
  i = ((int) t3) ^ (int) t2;
}
we actually optimize it with or without the nop_convert1? in place,
because we have the
/* Try to fold (type) X op CST -> (type) (X op ((type-x) CST))
   when profitable.
...
  (bitop (convert@2 @0) (convert?@3 @1))
...
   (convert (bitop @0 (convert @1)))))
simplification.
Except that on
void
bar (void)
{
  unsigned t0 = u;
  int t1 = (~(int) t0) | j;
  x = t1 ^ t0;
  int t2 = i;
  unsigned t3 = (~(unsigned) t2) | v;
  i = ((int) t3) ^ t2;
}
the optimization doesn't trigger without the nop_convert1? and does
with it.

Perhaps we could get rid of nop_convert3? and nop_convert4?
by introducing a macro/inline function predicate like:
bitwise_equal_p (expr1, expr2) and instead of using
(nop_convert3? @0) and (nop_convert4? @0) in the pattern
use @0 and @2 and then add
if (bitwise_equal_p (@0, @2))
to the condition.
For GENERIC (i.e. in generic-match-head.cc) it could be something like:
static inline bool
bitwise_equal_p (tree expr1, tree expr2)
{
  STRIP_NOPS (expr1);
  STRIP_NOPS (expr2);
  if (expr1 == expr2)
    return true;
  if (!tree_nop_conversion_p (TREE_TYPE (expr1), TREE_TYPE (expr2)))
    return false;
  if (TREE_CODE (expr1) == INTEGER_CST && TREE_CODE (expr2) == INTEGER_CST)
    return wi::to_wide (expr1) == wi::to_wide (expr2);
  return operand_equal_p (expr1, expr2, 0);
}
(the INTEGER_CST special case because operand_equal_p compares wi::to_widest
which could be different if one constant is signed and the other unsigned).
For GIMPLE, I wonder if it shouldn't be a macro that takes valueize into
account, and do something like:
#define bitwise_equal_p(expr1, expr2) gimple_bitwise_equal_p (expr1, expr2, valueize)

bool gimple_nop_convert (tree, tree *, tree (*)(tree));

static inline bool
gimple_bitwise_equal_p (tree expr1, tree expr2, tree (*valueize) (tree))
{
  if (expr1 == expr2)
    return true;
  if (!tree_nop_conversion_p (TREE_TYPE (expr1), TREE_TYPE (expr2)))
    return false;
  if (TREE_CODE (expr1) == INTEGER_CST && TREE_CODE (expr2) == INTEGER_CST)
    return wi::to_wide (expr1) == wi::to_wide (expr2);
  if (operand_equal_p (expr1, expr2, 0))
    return true;
  tree expr3, expr4;
  if (!gimple_nop_convert (expr1, &expr3, valueize))
    expr3 = expr1;
  if (!gimple_nop_convert (expr2, &expr4, valueize))
    expr4 = expr2;
  if (expr1 != expr3)
    {
      if (operand_equal_p (expr3, expr2, 0))
	return true;
      if (expr2 != expr4 && operand_equal_p (expr3, expr4, 0))
	return true;
    }
  if (expr2 != expr4 && operand_equal_p (expr1, expr4, 0))
    return true;
  return false;
}

Completely untested.  What do you think?
Though, that brings us only still to 16 cases of this.

	Jakub