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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/85390] [9/10/11/12 Regression] possible
 missed optimisation / regression from 6.3 with conditional expression
Date: Thu, 16 Dec 2021 07:01:08 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D85390

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to fail|                            |7.1.0
   Last reconfirmed|                            |2021-12-16
   Target Milestone|---                         |9.5
           Keywords|                            |needs-bisection
     Ever confirmed|0                           |1
            Summary|possible missed             |[9/10/11/12 Regression]
                   |optimisation / regression   |possible missed
                   |from 6.3 with conditional   |optimisation / regression
                   |expression                  |from 6.3 with conditional
                   |                            |expression
                 CC|                            |pinskia at gcc dot gnu.org
             Status|UNCONFIRMED                 |NEW
          Component|rtl-optimization            |tree-optimization
      Known to work|                            |6.4.0

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Ok, the issue is just dumb luck:

Take:
extern int a, b, c;

int f(int x)
{
    __builtin_prefetch((void *) (__SIZE_TYPE__)(x ? a : b));
    return c;
}

int f1(int x)
{
    int t =3D x ? a :b;
    __builtin_prefetch((void *)(__SIZE_TYPE__)t);
    return c;
}

The f1 uses cmov while f does not.

The gimple level difference is there is two cast (one in each branch) for f
while only one cast outside of the PHI node.
But that was that way even in GCC 6.
I wonder what changed in GCC 7 which caused this issue to appear.=