[PATCH] D39383: [InstCombine] Simplify selects that test cmpxchg instructions

[Matthew Simpson via Phabricator via llvm-commits]

mssimpso created this revision.

If a select instruction tests the returned flag of a cmpxchg instruction and selects between the returned value of the cmpxchg instruction and its compare operand, the result of the select will always be equal to its false value.

For example, in the code below `%2` is the original value located at `%ptr` prior to any potential update. If the cmpxchg operation succeeds (`%1` is true), `%2` and `%compare` must be equal. Thus, the result of the select is always equal to `%2`.

  %0 = cmpxchg i64* %ptr, i64 %compare, i64 %new_value seq_cst seq_cst
  %1 = extractvalue { i64, i1 } %0, 1
  %2 = extractvalue { i64, i1 } %0, 0
  %3 = select i1 %1, i64 %compare, i64 %2


https://reviews.llvm.org/D39383

Files:
  lib/Transforms/InstCombine/InstCombineSelect.cpp
  test/Transforms/InstCombine/select-cmpxchg.ll

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