function with side effects ignored

[Brian Budge]

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@users.sourceforge.net> 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 Please do not quote my e-mail address unobfuscated in message bodies.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)
>