[PATCH] D60982: [SimplifyCFG] Use lookup tables when they are more space efficient or a huge speed win.

[Shawn Landden via Phabricator via llvm-commits]

shawnl created this revision.
shawnl added reviewers: nikic, hans, spatel, jmolloy, lebedev.ri.
Herald added subscribers: llvm-commits, kristof.beyls, javed.absar.
Herald added a project: LLVM.

I am trying to move towards much more space-efficient switch statements, using popcnt, as described in PR39013. This is the second patch towards that goal, and a continuation of D60673 <https://reviews.llvm.org/D60673>.

There are quite a few cases where the lookup table was smaller and it was still not used. I hard-coded values for x86_64, but arm64 was similar. Also, there was no consideration of how large the cases were in the calculation. These numbers will change in a later patch, (when sparse maps make switch much more compact) so we shouldn't argue too much about this cut-off.

When only an i8 is being switched over, the table is not large, and using a lookup table is a huge speed gain. (as can be seen in the test)

Eliminate and and xor when doing a fully covered table.


Repository:
  rL LLVM

https://reviews.llvm.org/D60982

Files:
  lib/Transforms/Utils/SimplifyCFG.cpp
  test/Transforms/SimplifyCFG/switch-genfori8.ll

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