* Melchior FRANZ -- Friday 02 December 2005 01:43:
> But ... we weren't really returning the address of an auto var.
Is it a gcc 4.0.2 (SuSE 10.0) compiler bug? tiny_xdr.cxx contains
this function;
float
XDR_decode_float ( const xdr_data_t & f_Val )
{
float* tmp;
xdr_data_t dummy;
dummy = XDR_decode_int32 (f_Val);
tmp = (float*) &dummy;
return (*tmp);
}
And it turned out that when compiled with gcc 4.0.2 the return
value wasn't safe. When called three times in a row with different
values, we would get three times the same result. None of those
correct. This placed MP aircraft somewhere around the middle
of our Earth. For those understanding x86 assembler, here is
the resulting code (why does it not call _Z16XDR_decode_int32RKj?
"Optimized" away?):
non-static "dummy" (-O2) --> doesn't work
(gdb) disass XDR_decode_float
Dump of assembler code for function _Z16XDR_decode_floatRKj:
0x08310816 <_Z16XDR_decode_floatRKj+0>: push %ebp
0x08310817 <_Z16XDR_decode_floatRKj+1>: mov %esp,%ebp
0x08310819 <_Z16XDR_decode_floatRKj+3>: sub $0x10,%esp
0x0831081c <_Z16XDR_decode_floatRKj+6>: flds 0xfffffffc(%ebp)
0x0831081f <_Z16XDR_decode_floatRKj+9>: leave
0x08310820 <_Z16XDR_decode_floatRKj+10>: ret
End of assembler dump.
The following was compiled without optimization. The resulting code
works.
non-static "dummy" (-O0) --> works
(gdb) disass XDR_decode_float
Dump of assembler code for function _Z16XDR_decode_floatRKj:
0x083be33a <_Z16XDR_decode_floatRKj+0>: push %ebp
0x083be33b <_Z16XDR_decode_floatRKj+1>: mov %esp,%ebp
0x083be33d <_Z16XDR_decode_floatRKj+3>: sub $0x18,%esp
0x083be340 <_Z16XDR_decode_floatRKj+6>: mov 0x8(%ebp),%eax
0x083be343 <_Z16XDR_decode_floatRKj+9>: mov %eax,(%esp)
0x083be346 <_Z16XDR_decode_floatRKj+12>: call 0x83be1b4
<_Z16XDR_decode_int32RKj>
0x083be34b <_Z16XDR_decode_floatRKj+17>: mov %eax,0xfffffff8(%ebp)
0x083be34e <_Z16XDR_decode_floatRKj+20>: lea 0xfffffff8(%ebp),%eax
0x083be351 <_Z16XDR_decode_floatRKj+23>: mov %eax,0xfffffffc(%ebp)
0x083be354 <_Z16XDR_decode_floatRKj+26>: mov 0xfffffffc(%ebp),%eax
0x083be357 <_Z16XDR_decode_floatRKj+29>: mov (%eax),%eax
0x083be359 <_Z16XDR_decode_floatRKj+31>: mov %eax,0xffffffec(%ebp)
0x083be35c <_Z16XDR_decode_floatRKj+34>: flds 0xffffffec(%ebp)
0x083be35f <_Z16XDR_decode_floatRKj+37>: leave
0x083be360 <_Z16XDR_decode_floatRKj+38>: ret
End of assembler dump.
and this hack (committed to cvs) works with and without optimization.
Making "dummy" static shouldn't be necessary, but
static "dummy" --> works
(gdb) disass XDR_decode_float
Dump of assembler code for function _Z16XDR_decode_floatRKj:
0x08310816 <_Z16XDR_decode_floatRKj+0>: push %ebp
0x08310817 <_Z16XDR_decode_floatRKj+1>: mov %esp,%ebp
0x08310819 <_Z16XDR_decode_floatRKj+3>: sub $0x4,%esp
0x0831081c <_Z16XDR_decode_floatRKj+6>: mov 0x8(%ebp),%eax
0x0831081f <_Z16XDR_decode_floatRKj+9>: mov %eax,(%esp)
0x08310822 <_Z16XDR_decode_floatRKj+12>: call 0x83107e2
<_Z16XDR_decode_int32RKj>
0x08310827 <_Z16XDR_decode_floatRKj+17>: mov %eax,0x8560e00
0x0831082c <_Z16XDR_decode_floatRKj+22>: flds 0x8560e00
0x08310832 <_Z16XDR_decode_floatRKj+28>: leave
0x08310833 <_Z16XDR_decode_floatRKj+29>: ret
End of assembler dump.
clueless
m.
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