[PATCH] Switch lowering: extract jump tables and bit tests before building binary tree (PR22262)

[Hans Wennborg]

Anton asked me at Euro-LLVM if it was possible to construct a switch that exhibits the worst-case properties of the old switch lowering. I believe the attached file does that: F476439: bad.c.gz <http://reviews.llvm.org/F476439>. It's 10k-case switch where the distance between cases increases linearly.

(I used perf stat -r5 for the timings except for slow run which I only ran once.)

Without my patch:
clang -O0: 3.26 s ( +-  2.34% )
clang -O3: 7.43 s ( +-  3.49% )
./a.out: ~40 s

With my patch:
clang -O0: 0.76 s ( +-  3.04% )
clang -O3:  6.24 s ( +-  3.33% )
./a.out: 0.32 s ( +-  0.23% )


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http://reviews.llvm.org/D8649

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