svn commit: r232721 - head/sys/x86/include

[John Baldwin]

On Tuesday, March 20, 2012 10:19:07 am Tijl Coosemans wrote:
> On Tuesday 20 March 2012 13:34:10 John Baldwin wrote:
> > On Friday, March 09, 2012 6:48:56 am Tijl Coosemans wrote:
> >> Author: tijl
> >> Date: Fri Mar  9 11:48:56 2012
> >> New Revision: 232721
> >> URL: http://svn.freebsd.org/changeset/base/232721
> >> 
> >> Log:
> >>   Clean up x86 endian.h:
> >>   - Remove extern "C". There are no functions with external linkage here. 
[1]
> >>   - Rename bswapNN_const(x) to bswapNN_gen(x) to indicate that these 
macros
> >>     are generic implementations that can take non-constant arguments. [1]
> >>   - Split up __GNUCLIKE_ASM && __GNUCLIKE_BUILTIN_CONSTANT_P and deal 
with
> >>     each separately.
> >>   - Replace _LP64 with __amd64__ because asm instructions are machine
> >>     dependent, not ABI dependent.
> >>   
> >>   Submitted by:	bde [1]
> >>   Reviewed by:	bde
> > 
> > BTW, I think I found an old "bug" in this file.  The _gen() variants
> > should only use the _gen() variants of smaller types rather than using
> > the version that re-checks __build_constant_p() I think.  That is:
> > 
> > Index: x86/include/endian.h
> > ===================================================================
> > --- endian.h	(revision 233184)
> > +++ endian.h	(working copy)
> > @@ -65,9 +65,9 @@
> >  
> >  #define	__bswap16_gen(x)	(__uint16_t)((x) << 8 | (x) >> 8)
> >  #define	__bswap32_gen(x)		\
> > -	(((__uint32_t)__bswap16(x) << 16) | __bswap16((x) >> 16))
> > +	(((__uint32_t)__bswap16_gen(x) << 16) | __bswap16_gen((x) >> 16))
> >  #define	__bswap64_gen(x)		\
> > -	(((__uint64_t)__bswap32(x) << 32) | __bswap32((x) >> 32))
> > +	(((__uint64_t)__bswap32_gen(x) << 32) | __bswap32_gen((x) >> 32))
> >  
> >  #ifdef __GNUCLIKE_BUILTIN_CONSTANT_P
> >  #define	__bswap16(x)				\
> > 
> > I ran into this while porting the __builtin_constant_p() functionality
> > over to ia64.
> 
> No, on i386 bswap64 with a variable argument currently expands to two
> bswap instructions. With your change it would be many shifts and logical
> operations. The _gen variants are more like fallback implementations.
> If bswapNN cannot be implemented directly it is split up. If those
> smaller problems can be implemented directly, good, if not split it up
> again and so on.

Oh, I now parse the comment in __bswap64_var() correctly.  That's fugly.

Still, it seems that if I keep the patch to port this to ia64 (so it
can do constants as constants), then it will need to use this approach
since it won't have the i386 problem (in its case the _gen variants
are only used for constants).

-- 
John Baldwin