[Bug rtl-optimization/56605] Redundant branch introduced during loop2 phases

[steven at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56605

Steven Bosscher <steven at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|steven at gcc dot gnu.org   |

--- Comment #3 from Steven Bosscher <steven at gcc dot gnu.org> 2013-03-12 23:14:39 UTC ---
GCC gets to implies_p with:

Breakpoint 11, implies_p (a=0x3fffb5fd1410, b=0x3fffb5fd15c0) at
../../trunk/gcc/loop-iv.c:1499
1499      if (GET_CODE (a) == EQ)
(gdb) p debug_rtx(a)
(eq:SI (subreg:SI (reg:DI 153 [ bnd.10D.2035+-4 ]) 4)
    (const_int 0 [0]))
$26 = void
(gdb) p debug_rtx(b)
(eq:SI (subreg:SI (reg:DI 153 [ bnd.10D.2035+-4 ]) 4)
    (const_int 0 [0]))
$27 = void
(gdb) 

But implies_p doesn't handle SUBREGs.  If it would allow REGs and SUBREGs
of REGs, it would fold the assumption away:

1504          if (REG_P (op0))
(gdb) p op0
$29 = (rtx) 0x3fffb5fd13e0
(gdb) p debug_rtx(op0)
(subreg:SI (reg:DI 153 [ bnd.10D.2035+-4 ]) 4)
$30 = void
(gdb) p debug_rtx(op1)
(const_int 0 [0])
$31 = void
(gdb) p debug_rtx(simplify_replace_rtx (b, op0, op1))
(const_int 1 [0x1])
$32 = void


Something like the following, perhaps?

Index: loop-iv.c
===================================================================
--- loop-iv.c   (revision 196575)
+++ loop-iv.c   (working copy)
@@ -1496,19 +1496,26 @@ implies_p (rtx a, rtx b)
   rtx op0, op1, opb0, opb1, r;
   enum machine_mode mode;

+  if (rtx_equal_p (a, b))
+    return true;
+
   if (GET_CODE (a) == EQ)
     {
       op0 = XEXP (a, 0);
       op1 = XEXP (a, 1);

-      if (REG_P (op0))
+      if (REG_P (op0)
+         || (GET_CODE (op0) == SUBREG
+             && REG_P (SUBREG_REG (op0))))
        {
          r = simplify_replace_rtx (b, op0, op1);
          if (r == const_true_rtx)
            return true;
        }

-      if (REG_P (op1))
+      if (REG_P (op1)
+         || (GET_CODE (op1) == SUBREG
+             && REG_P (SUBREG_REG (op1))))
        {
          r = simplify_replace_rtx (b, op1, op0);
          if (r == const_true_rtx)