[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, 6 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 905f66a380f..6e4336ef875 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2140,6 +2140,12 @@ bool InstCombiner::OptimizeOverflowCheck(OverflowCheckFlavor OCF, Value *LHS,
     return true;
   };
 
+  // If the overflow check was an add followed by a compare, the insertion point
+  // may be pointing to the compare.  We want to insert the new instructions
+  // before the add in case there are uses of the add between the add and the
+  // compare.
+  Builder->SetInsertPoint(&OrigI);
+
   switch (OCF) {
   case OCF_INVALID:
     llvm_unreachable("bad overflow check kind!");