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Re: [patch][rfc] Hookize SMALL_REGISTER_CLASSES, make SSE register classes not small


On Tue, Dec 09, 2008 at 11:30:58PM +0100, Steven Bosscher wrote:
> Hi,
> 
> Attached is a patch to hookize the SMALL_REGISTER_CLASSES target
> macro.  But the actual purpose of the few hours I've spent on this,
> was to make it possible to have SMALL_REGISTER_CLASSES be false for
> some register classes but not for all.
> 
> In the current situation, SMALL_REGISTER_CLASSES is true or false, but
> targets are not black and white.  For x86-64, I've noticed we often
> get better results with floating point code if I disable
> SMALL_REGISTER_CLASSES, but it also results in lots of trouble with
> INTEGRAL_MODE_P modes (similar to what happens if you enable
> scheduling for x86-64).
> 
> With the patch, a target can choose to allow more optimizations when
> it has good reason to believe that registers in some machine modes
> will not be allocated to small register classes.  I didn't test this
> with a recent trunk, but before the IRA merge this gave me a ~1.3%
> bonus (average) for the Fortran Polyhedron benchmarks.
> 
> As with the code-size aware PRE patch, I'm looking for opinions on
> this approach.  Does this look like a reasonable approach?  Would it
> be useful for other targets? (I've now simply converted the existing
> SMALL_REGISTER_CLASSES definitions to hooks).

Back when I was at AMD, I often times thought about making
SMALL_REGISTER_CLASSES false for x86_64 bit (where you have more registers),
but I never got around to doing it.

-- 
Michael Meissner, IBM
4 Technology Place Drive, MS 2203A, Westford, MA, 01886, USA
meissner@linux.vnet.ibm.com


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