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[clang][dataflow] Add an early-out to
flowConditionImplies()
/ `flo…
…wConditionAllows()`. This saves having to assemble the set of constraints and run the SAT solver in the trivial case where `F` is true. This is a performance win on the benchmarks for the Crubit nullability checker: ``` name old cpu/op new cpu/op delta BM_PointerAnalysisCopyPointer 64.1µs ± 5% 63.1µs ± 0% -1.56% (p=0.000 n=20+17) BM_PointerAnalysisIntLoop 172µs ± 2% 171µs ± 0% ~ (p=0.752 n=20+17) BM_PointerAnalysisPointerLoop 408µs ± 3% 355µs ± 0% -12.99% (p=0.000 n=20+17) BM_PointerAnalysisBranch 201µs ± 2% 184µs ± 0% -8.28% (p=0.000 n=20+19) BM_PointerAnalysisLoopAndBranch 684µs ± 2% 613µs ± 2% -10.38% (p=0.000 n=20+19) BM_PointerAnalysisTwoLoops 309µs ± 2% 308µs ± 2% ~ (p=0.728 n=20+19) BM_PointerAnalysisJoinFilePath 37.9ms ± 2% 37.9ms ± 2% +0.06% (p=0.041 n=20+19) BM_PointerAnalysisCallInLoop 26.5ms ± 2% 26.4ms ± 4% -0.59% (p=0.024 n=20+20) ``` When running clang-tidy on real-world code, the results are less clear. In three runs, averaged, on an arbitrarily chosen input file, I get 11.91 s of user time without this patch and 11.81 s with it, though with considerable measurement noise (I'm seeing up to 0.2 s of variation between runs). Still, this is a very simple change, and it is a clear win in benchmarks, so I think it is worth making.
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