[PATCH] Select Elimination in InstCombine

[Gerolf Hoflehner]

Hi chandlerc, mcrosier, ributzka, majnemer,

Summary
In special cases select instructions can be eliminated by
replacing them with a cheaper bitwise operation even when the
select result is used outside its home block. The instances implemented
here are %x=icmp.eq; %y=select %x,%r, null; %z=icmp.eq|neq %y, null;
br %z,true, false ==> %x=icmp.ne; %y=icmp.eq %r,null; %z=or %x,%y; br
%z,true,false. The optimization is integrated into the instruction
combiner and performed only when all uses of the select result can
be replaced by the select operand proper. For this dominator information
is used. Integration into the combiner means that only the select-icmp is
transformed at first. Then the combiner iteratively works out the simpler
pattern. Note that the instruction combiner already handles the local pattern
when there is no use of the select result outside its home block.

http://reviews.llvm.org/D5258

Files:
  lib/Transforms/InstCombine/InstCombine.h
  lib/Transforms/InstCombine/InstCombineCompares.cpp
  lib/Transforms/InstCombine/InstructionCombining.cpp
  test/Transforms/InstCombine/select-cmp-br.ll
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