[PATCH] Add a flag to experiment with outlining optional branches.

[Daniel Jasper]

Hi chandlerc,

In a CFG with the edges A->B->C and A->C, B is an optional branch.

LLVM's default behavior is to lay the blocks out naturally, i.e. A, B, C, in order to improve code locality and fall throughs. However, if a function contains many of those optional branches only a few of which are taken, this leads to a lot of unnecessary icache misses. Moving B out of line can work around this.

http://reviews.llvm.org/D7719

Files:
  lib/CodeGen/MachineBlockPlacement.cpp
  test/CodeGen/X86/code_placement_outline_optional_branches.ll

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