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From: "amacleod at redhat dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/101186] predictable comparison of integer
 variables not folded
Date: Thu, 24 Jun 2021 18:30:42 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101186

Andrew Macleod <amacleod at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |aldyh at redhat dot com,
                   |                            |amacleod at redhat dot com

--- Comment #3 from Andrew Macleod <amacleod at redhat dot com> ---
(In reply to Di Zhao from comment #0)
> GCC fail to remove dead codes for following cases:
>=20
> #include <stdio.h>
>=20
> void f (unsigned int a, unsigned int b, unsigned int c)
> // if a,b,c are signed, VRP can remove dead code
> {
>   if (a =3D=3D b)
>     {
>       printf ("a");
>       if (c < a)
> 	{
> 	  printf ("b");
> 	  if (c >=3D b)
> 	    printf ("Unreachable!");
> 	}
>     }
> }
>=20
I believe this commit fixes the first case, EVRP now eliminates the unreach=
able
print.


commit ce0b409f562cd09c67cc2dce74143a0f0647cde5 (origin/master, origin/HEAD)
Author: Andrew MacLeod <amacleod@redhat.com>
Date:   Thu Jun 24 11:13:47 2021 -0400

    Fix relation query of equivalences.

the second one requires some form of predication, or the desire to thread t=
hru
the top of the block.. It touches on some stuff Aldy is looking at (which i=
sn't
finished yet) .


<bb 2> :
  if (a_7(D) !=3D 0)
    goto <bb 3>; [INV]
  else
    goto <bb 9>; [INV]

  <bb 3> :
  goto <bb 9>; [INV]

  <bb 9> :
  # c_16 =3D PHI <x_10(D)(3), y_5(D)(2)>

  <bb 10> :
  # b_1 =3D PHI <b_8(D)(9), b_14(8)>
  if (b_1 <=3D 999)
    goto <bb 4>; [INV]
  else
    goto <bb 11>; [INV]

  <bb 4> :
  if (a_7(D) !=3D 0)
    goto <bb 5>; [INV]
  else
    goto <bb 7>; [INV]

  <bb 5> :
  if (x_10(D) < c_16)
    goto <bb 6>; [INV]
  else
    goto <bb 8>; [INV]

 I dont know what rules there are, but the path 2->3->9->10->4  would resul=
t in
bb4 becoming "goto bb5", and then BB5 could fold to "goto bb8" as c_16 =3D=
=3D x_10
on that path in BB9.=