Vectorize more work in reductions of vector operations #14254
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kronbichler
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| r0 += r1; | ||
| r2 += r3; | ||
| result = r0 + r2; | ||
| result = (r0 + r1) + (r2 + r3); |
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@kronbichler The following test fails on the serial configuration: |
| @@ -31,6 +31,7 @@ main() | |||
| { | |||
| initlog(); | |||
| deallog << std::setprecision(4); | |||
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Would it make sense to drop this std::setprecision or to set it to say 10 digits so that numdiff can do something about roundoff errors?
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The problem is that we print a residual that ought to be zero in exact arithmetic, but turns out to be 1e-3 or something (above the absolute/relative threshold of numdiff), because we've scaled by matrix by 1e10. I don't see a simple way to get this back to tolerance, apart from disabling the print of the residual to the logstream.
Yes, here it would be good. 👍 |
kronbichler
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Addresses one of the issues mentioned in #14251: The reductions for small sizes involve work on up to
n_lanes * 32 - 1vector items without vectorization. If the vector size is a few hundreds, this scalar work gets clearly visible. We can get this down to less thann_lanes. While in this code, I also restructured the reductions on the temporaryouter_resultsthat contain the result of 32 operations: We can the associated pairwise summation by a single loop (rather than two nested loops) by writing the temporary result to the end of a (now 2x larger) array and sum until we've reached the end.These two actions result in a considerable improvement. I tested for vector size 400 and found that around 20% fewer instructions get issued with this patch.
Note: This patch changes the order of additions, so I expect that we might need some updates to the result files that are sensitive to roundoff. I think I identified one, but let us wait for the CI machines.