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GPU: make Metal's cluster widths round as on the CPU #16065

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GPU: make Metal's cluster widths round as on the CPU #16065

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13 changes: 13 additions & 0 deletions GPU/GPUTracking/TPCClusterFinder/ClusterAccumulator.cxx
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Original file line number Diff line number Diff line change
Expand Up @@ -62,13 +62,26 @@ GPUd() void ClusterAccumulator::finalize(const CfChargePos& pos, const Charge q,
{
mQtot += q;

#ifdef __METAL__
// Metal's fast-math division and sqrt are not correctly rounded, which moves
// widths that sit within an ulp of a packing step. Do the operations the
// arm64 -ffast-math CPU build does here: reciprocal, products, fused variance.
const float invQ = metal::precise::divide(1.f, mQtot);
mPadMean *= invQ;
mTimeMean *= invQ;
const float padMean2 = mPadMean * mPadMean;
const float timeMean2 = mTimeMean * mTimeMean;
mPadSigma = metal::precise::sqrt(metal::fma(mPadSigma, invQ, -padMean2));
mTimeSigma = metal::precise::sqrt(metal::fma(mTimeSigma, invQ, -timeMean2));
#else
mPadMean /= mQtot;
mTimeMean /= mQtot;
mPadSigma /= mQtot;
mTimeSigma /= mQtot;

mPadSigma = CAMath::Sqrt(mPadSigma - mPadMean * mPadMean);
mTimeSigma = CAMath::Sqrt(mTimeSigma - mTimeMean * mTimeMean);
#endif

Pad pad = pos.pad();
mPadMean += pad;
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