Andrew Haley wrote:
Matthew Woehlke 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);
}
[snip snip]
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?
It's in your code. You're accessing an object (p) as an incompatible type.
See C99, Section 6.3.2.3 for all the details.
That's what I suspected.
Do this:
float my_ntoh_float(float f)
{
That should be 'float * f'.
union
{
int i;
float f;
} u;
D'oh, should have thought of that. Brain must not be working :-).
Anyway, I saw Brian's reply first, and already got it working with a
variant of this (i.e. not changing the signature of my_ntoh_float, which
I can't do for compatibility reasons!!). Thanks both for the quick replies!
Which generates this:
my_get_float:
pushl %ebp
movl %esp, %ebp
subl $4, %esp
movl 8(%ebp), %eax
movl (%eax), %eax
bswap %eax
movl %eax, -4(%ebp)
flds -4(%ebp)
leave
ret
Hmm, I still get this:
ror $0x8,%ax
ror $0x10,%eax
ror $0x8,%ax
...which is interesting. Are you building 32-bit code? Looks like I
maybe need to see if there is a -march that we aren't using that might
help our performance (not just here, but in general).
(I do get bswap with 64-bit code.)
--
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Please do not quote my e-mail address unobfuscated in message bodies.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)
