Re: gas/config/tc-alpha.c

[Alan Modra <alan at linuxcare dot com dot au>]

On Mon, 15 May 2000, Michael Meissner wrote:

> On Sun, May 14, 2000 at 09:42:15PM +1000, Alan Modra wrote:
> > Is there any real reason to prefer the dangerous first form of these
> > macros?
> > 
> > extract from tc-alpha.c:
> > 
> > /* Macros for sign extending from 16- and 32-bits.  */
> > /* XXX: The cast macros will work on all the systems that I care about,
> >    but really a predicate should be found to use the non-cast forms.  */
> > 
> > #if 1
> > #define sign_extend_16(x)	((short)(x))
> > #define sign_extend_32(x)	((int)(x))
> > #else
> > #define sign_extend_16(x)	((offsetT)(((x) & 0xFFFF) ^ 0x8000) - 0x8000)
> > #define sign_extend_32(x)	((offsetT)(((x) & 0xFFFFFFFF) \
> > 					   ^ 0x80000000) - 0x80000000)
> > #endif
> 
> If you are worried about micro-optimizations, the first form is faster than the
> second on some platforms (in particular, if your machine does not support
> and/xor/sub of the constants in place, GCC will not re-optimize the expression
> into an appropriate sign extension).

Sure.  That's why I said "any real reason" rather than "any reason".  :-)

I guess it doesn't matter a great deal as binutils is riddled with this
sort of assumption about sizes of integral types.  I did have some idea of
attacking the problem, but it seems a whole lot of work for little
immediate benefit.

-- 
Linuxcare.  Support for the Revolution.