soft-fp patch: Use __builtin_clz*

[Joseph S. Myers]

On Thu, 5 Jan 2006, Roland McGrath wrote:

> > The soft-fp code has an inefficient generic default implementation of 
> > __FP_CLZ.  GCC 3.4 and greater provide __builtin_clz which can be used for 
> > a better and simpler implementation.  This patch makes soft-fp use these 
> > built-in functions.
> > 
> > In fact the only non-default definitions of __FP_CLZ are those in 
> > sysdeps/i386/soft-fp/sfp-machine.h and 
> > sysdeps/x86_64/soft-fp/sfp-machine.h.  GCC knows how to expand 
> > __builtin_clz/__builtin_clzl properly for both platforms, though on x86_64 
> > it seems this support isn't there until GCC 4.0.
> 
> From these two statements it's not clear whether __builtin_clz{,l,ll}
> actually work on all machines in GCC 3.4 or not.  If they don't, then we
> can't use them unconditionally here.

They work on all machines in 3.4.  If the target-specific expansion isn't 
there then they call __clzsi2 and __clzdi2 which are defined in libgcc.  
The definitions in libgcc in turn come from longlong.h, either 
target-specific or a generic fallback.  By "expand properly" I simply mean 
using the proper instruction inline instead of calling __clz*.

-- 
Joseph S. Myers
joseph@codesourcery.com