I'd try using a union something like this:
void my_ntoh_float(float *f)
{
union {
int li;
float lf;
};
lf = *f;
li = ntohl(li);
*f = lf;
}
Brian
On Tue, Sep 29, 2009 at 9:58 AM, Matthew Woehlke
<mw_triad@xxxxxxxxxxxxxxxxxxxxx> wrote:
> 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 Please do not quote my e-mail address unobfuscated in message bodies.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)
>
