On Tue, May 26, 2009 at 09:42:40PM -0700, me22 wrote:
> 2009/5/26 Florian Weimer <fw@xxxxxxxxxxxxx>:
> > * Michael Meissner:
> >
> >> On Sat, May 23, 2009 at 03:55:34PM +0200, Florian Weimer wrote:
> >>> It seems to me that x86_64 supports direct moves from XMM to
> >>> general-purpose registers, so that it's possible to access the
> >>> representation of a floating point value without going through memory:
> >>>
> >>> long getbits(double src)
> >>> {
> >>> long result;
> >>> __asm__ ("movq %1, %0" : "=q" (result) : "x" (src));
> >>> return result;
> >>> }
> >>>
> >>> Is this functionally available as some sort of built-in?
> >>
> >> Well you typically can use a union to get the bits.
> >
> > Okay. I didn't realize that GCC was smart enough to avoid the
> > round-trip through memory.
> >
>
> In my experience compilers are often even smart enough to have this
> avoid the memory roundtrip, and it's undeniably legal:
>
> int ftoi(float x) {
> int i;
> memcpy(&i, &x, 4);
> return i;
> }
Well from a pedantic point of view, it is only legal if sizeof (i) == sizeof
(x) == 4, but you presumably already know that.
--
Michael Meissner, IBM
4 Technology Place Drive, MS 2203A, Westford, MA, 01886, USA
meissner@xxxxxxxxxxxxxxxxxx
