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[release/3.1] Port dotnet/runtime#31904 fix for slow UTF-8 transcoding performance #28013
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Remove BMI2 from ASCII and UTF-16 processing hot paths, as not all processors have optimized implementations of pext/pdep
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Issue dotnet/runtime#2251 reports that on AMD processors, we severely regressed some UTF-8 transcoding code paths on AMD processors. These processors have unoptimized implementations of the
pdep/pextintrinsic. We didn't see this earlier because my benchmarking machine uses an Intel processor.This has already been fixed in .NET 5 (see dotnet/runtime#31904). This PR ports that fix down to release/3.1.
Customer Impact
Significant slowdowns in UTF-8 transcoding performance on 3.0 compared to 2.1 when using an AMD processor and where the input to transcode has mixed ASCII and non-ASCII data.
Regression?
Perf regression between 2.1 and 3.0. Not a functional regression.
Testing
Fix has been perf tested (see dotnet/runtime#31904 (comment)) and fuzzed (see dotnet/runtime#31904 (comment)).
Risk
Low. No functional changes; only performance changes. We have excellent unit test coverage and fuzzer coverage over this code.
Code Reviewer
@tannergooding