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

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

On Tue, Jul 11, 2023 at 3:08 PM Jakub Jelinek <jakub@redhat.com> wrote:
>
> 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.

I guess we can also not worry and hope for a better code generator ...

The obvious improvement there is to delay pattern expansion (with for and ?)
until we get two patterns on the same sub-tree so patterns that are the
only ones at some point during the sub-tree matching can then be expanded
with code generation optimized for code size (:c is the only difficult
case there).

Matching the shortest paths to leaf first might then improve things further.

But this is a complete rewrite of the decision tree builder, so ...

Richard.

>
>         Jakub
>