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

[Duncan P. N. Exon Smith]

On 2014-Apr-21, at 10:19, Hal Finkel <hfinkel at anl.gov> wrote:

> ----- Original Message -----
>> From: "Duncan P. N. Exon Smith" <dexonsmith at apple.com>
>> To: "Andrew Trick" <atrick at apple.com>, "Chandler Carruth" <chandlerc at google.com>
>> Cc: "Commit Messages and Patches for LLVM" <llvm-commits at cs.uiuc.edu>
>> Sent: Monday, April 21, 2014 11:39:48 AM
>> Subject: [PATCH] Short-term workaround for hidden x86 float precision in	calculateSpillWeightAndHint()
>> 
>> 
>> 
>> The attached patch is a workaround that prevents hidden precision
>> from
>> changing results in x86 floats in calculateSpillWeightAndHint().
>> 
>> Applying my BlockFrequencyInfo patch causes two test failures on i386
>> (see, e.g., r206704). My patch changes the output of the pass, and
>> somehow exposes this problem in calculateSpillWeightAndHint(). In
>> the affected code:
>> 
>> float hweight = Hint[hint] += weight;
>> if (TargetRegisterInfo::isPhysicalRegister(hint)) {
>> if (hweight > bestPhys && mri.isAllocatable(hint))
>> bestPhys = hweight, hintPhys = hint;
>> }
>> 
>> I converted `hweight` and `bestPhys` to APFloat and used
>> APFloat::toString() to dump them. In the problematic code path, they
>> are both exactly equal to 1. However, the comparison
>> `hweight > bestPHys` gives `true` due to hidden precision in x86.
>> Marking `hweight` as `volatile` forces it to be a stack variable
>> (preventing hidden precision), so this comparison correctly gives
>> `false` even on i386 (hack suggested by chapuni!).
> 
> To be honest, I don't understand why you're seeing this problem. I know this was a problem pre-SSE, but I think that once SSE became available, the cost of forcing IEEE strict math became reasonable. IEEE arithmetic (although not the transcendental functions) should be strictly deterministic across platforms, and I don't see any reason we should support building LLVM in any other mode.
> 
> -Hal

Frankly, neither do I, but I find floats *really* scary and hide under
the bed when I see them.

I've committed the workaround for now (r206765).  If want me to
investigate further (and have something you want me to test), let me
know.  I was kindly given access to the i386-freebsd [1] buildslave to
debug this and haven't given it up yet.

[1]: http://llvm-amd64.freebsd.your.org/b/builders/clang-i386-freebsd