Re: [PATCH] tree-optimization/106379 - add missing ~(a ^ b) folding for _Bool

[Richard Biener <rguenther@suse.de>]

On Mon, 25 Jul 2022, H.J. Lu wrote:

> On Fri, Jul 22, 2022 at 11:10 PM Richard Biener via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> >
> >
> > > Am 22.07.2022 um 22:17 schrieb H.J. Lu via Gcc-patches <gcc-patches@gcc.gnu.org>:
> > >
> > > On Thu, Jul 21, 2022 at 4:24 AM Richard Biener via Gcc-patches
> > > <gcc-patches@gcc.gnu.org> wrote:
> > >>
> > >> The following makes sure to fold ~(a ^ b) to a == b for truth
> > >> values (but not vectors, we'd have to check for vector support of
> > >> equality).  That turns the PR106379 testcase into a ranger one.
> > >>
> > >> Note that while we arrive at ~(a ^ b) in a convoluted way from
> > >> original !a == !b one can eventually write the expression this
> > >> way directly as well.
> > >>
> > >> Bootstrapped and tested on x86_64-unknown-linux-gnu, pushed.
> > >>
> > >>        PR tree-optimization/106379
> > >>        * match.pd (~(a ^ b) -> a == b): New pattern.
> > >>
> > >>        * gcc.dg/pr106379-1.c: New testcase.
> > >> ---
> > >> gcc/match.pd                      | 6 ++++++
> > >> gcc/testsuite/gcc.dg/pr106379-1.c | 9 +++++++++
> > >> 2 files changed, 15 insertions(+)
> > >> create mode 100644 gcc/testsuite/gcc.dg/pr106379-1.c
> > >>
> > >> diff --git a/gcc/match.pd b/gcc/match.pd
> > >> index 8bbc0dbd5cd..88a1a5aa9cc 100644
> > >> --- a/gcc/match.pd
> > >> +++ b/gcc/match.pd
> > >> @@ -1938,6 +1938,12 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
> > >>  (if (tree_nop_conversion_p (type, TREE_TYPE (@0)))
> > >>   (bit_not (bit_xor (view_convert @0) @1))))
> > >>
> > >> +/* ~(a ^ b) is a == b for truth valued a and b.  */
> > >> +(simplify
> > >> + (bit_not (bit_xor:s truth_valued_p@0 truth_valued_p@1))
> > >> + (if (!VECTOR_TYPE_P (type))
> > >> +  (convert (eq @0 @1))))
> > >
> > > For integers, isn't it wrong to convert ~(boolean exp) to boolean exp?
> >
> > That’s what the (convert. …) should compensate for?
> 
> Is ~(boolean exp) == ~((int) (boolean exp)) or (int) (~(boolean exp))?

Depends on what 'boolean exp' is, and as PR106414 shows I was wrong
because in C a == b has type 'int' and not type bool but we consider
it "boolean".  There of course ~(a == b) is not equal to a != b.

Richard.