[Bug target/38952] [4.4 Regression] EH does not work.

["dave dot korn dot cygwin at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #15 from dave dot korn dot cygwin at gmail dot com  2009-01-25 06:33 -------
(In reply to comment #14)
> And that is presumably the intention of this if clause in ix86_can_eliminate:
> 
>   if (stack_realign_fp)
>     return ((from == ARG_POINTER_REGNUM
>              && to == HARD_FRAME_POINTER_REGNUM)
>             || (from == FRAME_POINTER_REGNUM
>                 && to == STACK_POINTER_REGNUM));
>   else [ ... ]
> 

  I just looked twice at that and it seemed odd to me.  Testing this:

   if (stack_realign_fp)
     return ((from == ARG_POINTER_REGNUM
              && to == HARD_FRAME_POINTER_REGNUM)
             || (from == FRAME_POINTER_REGNUM
-                && to == STACK_POINTER_REGNUM));
+                && to == HARD_FRAME_POINTER_REGNUM));


gives me this:

(gdb) disass main
Dump of assembler code for function main:
0x00401070 <main+0>:    push   %ebp
0x00401071 <main+1>:    mov    %esp,%ebp
0x00401073 <main+3>:    and    $0xfffffff0,%esp
0x00401076 <main+6>:    push   %edi
0x00401077 <main+7>:    push   %esi
0x00401078 <main+8>:    push   %ebx
0x00401079 <main+9>:    sub    $0x54,%esp
0x0040107c <main+12>:   movl   $0x4075f0,-0x34(%ebp)
0x00401083 <main+19>:   movl   $0x407bc4,-0x30(%ebp)
0x0040108a <main+26>:   lea    -0x2c(%ebp),%eax
0x0040108d <main+29>:   lea    -0x18(%ebp),%edx
0x00401090 <main+32>:   mov    %edx,(%eax)
0x00401092 <main+34>:   mov    $0x4010e2,%edx
0x00401097 <main+39>:   mov    %edx,0x4(%eax)
0x0040109a <main+42>:   mov    %esp,0x8(%eax)
0x0040109d <main+45>:   lea    -0x4c(%ebp),%eax
0x004010a0 <main+48>:   mov    %eax,(%esp)
0x004010a3 <main+51>:   call   0x405058 <_Unwind_SjLj_Register>

Which works, but now I suspect the address calculations accessing the jmp_buf
are going to be wrong.  Argh.  It looks like we have to know whether there's
an offset in the mem rtx relating to FRAME_POINTER_REGNUM and if it's negative
we have to allow elimination against STACK_POINTER but if its zero or positive
we'd have to only allow elimination against HARD_FRAME_POINTER ... ouch.

Guess the next thing I'll try is simply not allowing elimination against
FRAME_POINTER at all in this context.  That'll mean we can't
-fomit-frame-pointer
in functions which have stack_realign_fp true, but it beats wrong code and
maybe there's a way of avoiding it that I haven't thought of.  Now testing:

   if (stack_realign_fp)
     return ((from == ARG_POINTER_REGNUM
              && to == HARD_FRAME_POINTER_REGNUM)
             || (from == FRAME_POINTER_REGNUM
-                && to == STACK_POINTER_REGNUM));
+                && 0));


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38952