Imrpoved costing for equalsByteString and equalsString - #7804
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kwxm
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zliu41
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May 29, 2026
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This fixes the FIXME comments in the costing function inference code here and here.
The costing functions for
equalsByteStringandequalsStringare linear on the diagonal (ie, when we're comparing two inputs of the same size) and constant off the diagonal (when the inputs have different sizes, in which case they can't possibly be equal). Previously we just ran benchmarks for inputs which were actually equal (which is the worst case) and took the off-diagonal cost to be equal to the minimum of the on-diagonal cost. That was a little inelegant, so this PR adds some benchmark inputs that really are of different sizes, and the R code uses the results for those to determine the off-diagonal cost. Graphs of a test run of the benchmarks are shown below and show that our assumption of constant costs away from the diagonal is reasonable.. The off-diagonal costs determined from the new benchmarks aren't significantly different from the old ones, so I haven't changed the cost models.Benchmark results for
equalsByteString:Benchmark results for
equalsString: