------- 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
