[PATCH] Short-term workaround for hidden x86 float precision in calculateSpillWeightAndHint()

[Hal Finkel]

----- Original Message -----
> From: "Chandler Carruth" <chandlerc at google.com>
> To: "Duncan P. N. Exon Smith" <dexonsmith at apple.com>
> Cc: "Commit Messages and Patches for LLVM" <llvm-commits at cs.uiuc.edu>
> Sent: Monday, April 21, 2014 1:27:09 PM
> Subject: Re: [PATCH] Short-term workaround for hidden x86 float precision in	calculateSpillWeightAndHint()
> 
> 
> 
> 
> For the long term...
> 
> 
> On Mon, Apr 21, 2014 at 9:39 AM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com > wrote:
> 
> 
> 
> 3. Redesign spill weights to use integers.
> 
> 
> 
> This is far and away my preferred approach.
> 
> 
> 
> 
> 4. Change spill weights to use a soft-float. I happen to have a
> soft-float implementation lying around here somewhere...
> With this as a somewhat distant second.
> 
> 
> 
> 
> While I'm sympathetic to Hal's comment, I just don't want to debug
> LLVM's behavior on systems with buggy floating point. This will keep
> coming up, time and again. What happens when we start forming FMAs
> in *just* the right way? No one writing LLVM code is attending to
> the contraction domains which avoid unwanted higher precision
> results, etc.

Yes, and having looked at this a bit more (see other e-mail), I agree. FP contraction we can turn off globally (and we already do for the test suite anyway), but *sigh* ;)

 -Hal

> 
> 
> 
> 
> To address why I prefer to just design spill weights to use integers
> from the beginning rather than leverage a soft-float implementation,
> I genuinely believe that the design will be simpler and faster as a
> result. And my memory is that Jakob had some clear idea of how to
> map the spill weights into integers anyways. I'm holding out hope
> that this is not another place where we actually *have* to have a
> floating point representation rather than a fixed point
> representation due to massively varying scales.
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory