[Bug rtl-optimization/47918] New: [4.6 regression] noreturn discovery broke non local gotos on m68k

["mikpe at it dot uu.se" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47918

           Summary: [4.6 regression] noreturn discovery broke non local
                    gotos on m68k
           Product: gcc
           Version: 4.6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: mikpe@it.uu.se


gcc.dg/torture/stackalign/non-local-goto-4.c and several similar test cases
fail with current 4.6 on m68k-linux.  Bisection identified r160124 as the
cause:

Author: hubicka
Date: Tue Jun  1 21:55:49 2010
New Revision: 160124

URL: http://gcc.gnu.org/viewcvs?root=gcc&view=rev&rev=160124
Log:
    * ipa-pure-const.c (local_pure_const): Do NORETURN discovery.

The essence of non-local-goto-4.c is a function x() with a local self-recursive
function y() and a non-local goto from y() to x():

x(a)
{
  __label__ xlab;
  void y(a)
    {
      if (a==0)
        goto xlab;
      y (a-1);
    }
  y (a);
 xlab:;
  return a;
}

With r160123, gcc -O3 -fomit-frame-pointer -S generates this for x() and y():

        .text
        .align  2
        .type   y.1201, @function
y.1201:
        link.w %fp,#0
        move.l #.L2,%a1
        move.l (%a0),%fp
        move.l 4(%a0),%sp
        jmp (%a1)
        .size   y.1201, .-y.1201
        .align  2
        .globl  x
        .type   x, @function
x:
        lea (-12,%sp),%sp
        fmovem #252,-(%sp)
        movem.l #16190,-(%sp)
        lea (128,%sp),%a0
        move.l %a0,116(%sp)
        move.l %sp,120(%sp)
        move.l 132(%sp),-(%sp)
        lea (120,%sp),%a0
        jsr y.1201
        addq.l #4,%sp
.L4:
        move.l 132(%sp),%d0
        movem.l (%sp)+,#31996
        fmovem (%sp)+,#63
        lea (12,%sp),%sp
        rts
.L2:
        move.l 132(%sp),%d0
        movem.l (%sp)+,#31996
        fmovem (%sp)+,#63
        lea (12,%sp),%sp
        rts
        .size   x, .-x

This works.  But r160124 changes the code to:

        .align  2
        .type   y.1201, @function
y.1201:
        link.w %fp,#0
        move.l #.L2,%a1
        move.l (%a0),%fp
        move.l 4(%a0),%sp
        jmp (%a1)
        .size   y.1201, .-y.1201
        .align  2
        .globl  x
        .type   x, @function
x:
        lea (-12,%sp),%sp
        fmovem #252,-(%sp)
        movem.l #16190,-(%sp)
        lea (128,%sp),%a0
        move.l %a0,116(%sp)
        move.l %sp,120(%sp)
        move.l 132(%sp),-(%sp)
        lea (120,%sp),%a0
        jsr y.1201
.L2:
.L4:
        move.l 136(%sp),%d0
        movem.l (%sp)+,#31996
        fmovem (%sp)+,#63
        lea (12,%sp),%sp
        rts
        .size   x, .-x

A diff shows that L2 and L4 have been merged, a stack increment before L4 has
been removed and the stack offset for loading %d0 in L4 has been adjusted for
that.  However, the path leading up to L2 is the same as before, but L2 is now
the same as L4 so uses an incorrect (off-by-4) stack offset when fetching %d2.

--- non-local-goto-4.s-r160123  2011-02-27 23:44:28.000000000 +0100
+++ non-local-goto-4.s-r160124  2011-02-27 23:36:27.000000000 +0100
@@ -23,15 +23,9 @@
        move.l 132(%sp),-(%sp)
        lea (120,%sp),%a0
        jsr y.1201
-       addq.l #4,%sp
-.L4:
-       move.l 132(%sp),%d0
-       movem.l (%sp)+,#31996
-       fmovem (%sp)+,#63
-       lea (12,%sp),%sp
-       rts
 .L2:
-       move.l 132(%sp),%d0
+.L4:
+       move.l 136(%sp),%d0
        movem.l (%sp)+,#31996
        fmovem (%sp)+,#63
        lea (12,%sp),%sp