Using gcc-4.3.2-7.x86_64 (Fedora 10).
I have these functions:
void my_ntoh_float(float *f)
{
int p = (int*)f;
*p = ntohl(*p);
}
float my_get_float(const void * buffer)
{
float value;
memcpy(&val, buffer, sizeof(float);
my_ntoh_float(&value);
return value;
}
...which is compiled (-O2) like so:
<my_get_float+0>: push %ebp
<my_get_float+1>: mov %esp,%ebp
<my_get_float+3>: sub $0x10,%esp
<my_get_float+6>: mov 0x8(%ebp),%eax
<my_get_float+9>: mov (%eax),%eax
<my_get_float+11>: mov %eax,-0x4(%ebp)
<my_ntoh_float+0>: mov -0x4(%ebp),%eax
<my_get_float+17>: flds -0x4(%ebp)
<my_ntoh_float+6>: ror $0x8,%ax
<my_ntoh_float+10>: ror $0x10,%eax
<my_ntoh_float+13>: ror $0x8,%ax
<my_ntoh_float+17>: mov %eax,-0x4(%ebp)
<my_get_float+34>: leave
<my_get_float+35>: ret
Now, obviously this is not going to work correctly; the result of the
call to my_ntoh_float is effectively discarded. Is the bug in the code,
or in gcc? In either case, is there a way (besides dropping
optimizations) to fix this that doesn't involve non-portable code?
(Incidentally, there is at least one gcc "bug" here, in that the
instruction at my_ntoh_float+0 is superfluous.)
--
Matthew
Please do not quote my e-mail address unobfuscated in message bodies.
--
"It's not easy for [Microsoft] to accept [competing fairly]"
-- Dave Heiner (a Microsoft VP, paraphrased)
