[PATCH] D21291: [SimplifyCFG] Range reduce switches

[James Molloy via llvm-commits]

jmolloy created this revision.
jmolloy added reviewers: mcrosier, sbaranga, sanjoy.
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If a switch is sparse and all the cases (once sorted) are in arithmetic progression, we can extract the common factor out of the switch and create a dense switch. For example:

    switch (i) {
    case 5: ...
    case 9: ...
    case 13: ...
    case 17: ...
    }

can become:

    if ( (i - 5) % 4 ) goto default;
    switch ((i - 5) / 4) {
    case 0: ...
    case 1: ...
    case 2: ...
    case 3: ...
    }

The division and remainder operations could be costly so we only do this if the factor is a power of two. Dense switches can be lowered significantly better than sparse switches and can even be transformed into lookup tables.

Repository:
  rL LLVM

http://reviews.llvm.org/D21291

Files:
  lib/Transforms/Utils/SimplifyCFG.cpp
  test/Transforms/SimplifyCFG/rangereduce.ll

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