[InstCombine] Fix PR24605.

PR24605 is caused due to an incorrect insert point in instcombine's IR
builder.  When simplifying

  %t = add X Y
  ...
  %m = icmp ... %t

the replacement for %t should be placed before %t, not before %m, as
there could be a use of %t between %t and %m.

llvm-svn: 246315

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
index ac934f1bd85..9f5cdcbb329 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -360,6 +360,11 @@ private:
   /// \brief Try to optimize a sequence of instructions checking if an operation
   /// on LHS and RHS overflows.
   ///
+  /// If this overflow check is done via one of the overflow check intrinsics,
+  /// then CtxI has to be the call instruction calling that intrinsic.  If this
+  /// overflow check is done by arithmetic followed by a compare, then CtxI has
+  /// to be the arithmetic instruction.
+  ///
   /// If a simplification is possible, stores the simplified result of the
   /// operation in OperationResult and result of the overflow check in
   /// OverflowResult, and return true.  If no simplification is possible,