[Bug tree-optimization/66726] New: missed optimization, factor conversion out of COND_EXPR

["law at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66726

            Bug ID: 66726
           Summary: missed optimization, factor conversion out of
                    COND_EXPR
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: law at redhat dot com
  Target Milestone: ---

Given something like this:

   n = (short unsigned int) mode_size[(unsigned int) mode] * 8 <= 64 ? (int)
((short unsigned int) mode_size[(unsigned int) mode] * 8) : 64; 

Note the (int) cast on the true arm of the COND_EXPR.  As a result of that
cast, the transformations to turn a COND_EXPR into a MIN/MAX expression in
fold-const.c will not trigger.

This could be fixed by a patch to match.pd, but the pattern would only really
be applicable to GENERIC and thus isn't considered a good design match for
match.pd.

We could extend fold-const.c to catch this case, but that would really only
help GENERIC as well.

The best option it seems would be to catch this in phi-opt which would also
take us a step closer to handling pr45397.

Testcase:

/* { dg-do compile } */
/* { dg-options "-O2 -fdump-tree-original" } */


extern unsigned short mode_size[];
int
oof (int mode)
{
  return (64 < mode_size[mode] ? 64 : mode_size[mode]);
}

/* { dg-final { scan-tree-dump-times "MIN_EXPR" 1 "original" } } */