InstCombine: Fix a crash in Descale for multiply-by-zero

Fix a crash in `InstCombiner::Descale()` when a multiply-by-zero gets
created as an argument to a GEP partway through an iteration, causing
-instcombine to optimize the GEP before the multiply.

rdar://problem/17615671

llvm-svn: 212742

1 files changed, 6 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index 46e3bfc7e44..08e24461a61 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1120,6 +1120,12 @@ Value *InstCombiner::Descale(Value *Val, APInt Scale, bool &NoSignedWrap) {
     return nullptr;
   }
 
+  // If Op is zero then Val = Op * Scale.
+  if (match(Op, m_Zero())) {
+    NoSignedWrap = true;
+    return Op;
+  }
+
   // We know that we can successfully descale, so from here on we can safely
   // modify the IR.  Op holds the descaled version of the deepest term in the
   // expression.  NoSignedWrap is 'true' if multiplying Op by Scale is known