Index: lib/Transforms/Utils/LoopUnrollRuntime.cpp
===================================================================
--- lib/Transforms/Utils/LoopUnrollRuntime.cpp
+++ lib/Transforms/Utils/LoopUnrollRuntime.cpp
@@ -295,10 +295,13 @@
   if (isa<SCEVCouldNotCompute>(BECount) || !BECount->getType()->isIntegerTy())
     return false;
 
-  // Add 1 since the backedge count doesn't include the first loop iteration
+  // Add 1 since the backedge count doesn't include the first loop iteration.
   const SCEV *TripCountSC =
     SE->getAddExpr(BECount, SE->getConstant(BECount->getType(), 1));
-  if (isa<SCEVCouldNotCompute>(TripCountSC))
+
+  // If the trip count is not computable, zero, or the preceding add caused an
+  // overflow of the SCEV (causing a zero trip count), then don't unroll.
+  if (isa<SCEVCouldNotCompute>(TripCountSC) || TripCountSC->isZero())
     return false;
 
   // We only handle cases when the unroll factor is a power of 2.
Index: test/Transforms/LoopUnroll/runtime-zero-tripcount.ll
===================================================================
--- test/Transforms/LoopUnroll/runtime-zero-tripcount.ll
+++ test/Transforms/LoopUnroll/runtime-zero-tripcount.ll
@@ -0,0 +1,33 @@
+; RUN: opt < %s -S -loop-unroll -unroll-runtime | FileCheck %s
+
+; PR21409 ( http://llvm.org/bugs/show_bug.cgi?id=21409 )
+; Don't unroll a loop with a zero trip count.
+; Either the loop is never entered, or the calculation of the
+; trip count overflowed.
+
+define i32 @zero_trip_count() {
+entry:
+  br label %while.body
+
+while.body:
+  %d = phi i32 [ 0, %entry ], [ %inc, %while.body ]
+  %inc = add i32 %d, 1
+  %cmp = icmp eq i32 %inc, 0
+  br i1 %cmp, label %while.end, label %while.body
+
+while.end:
+  ret i32 0
+
+; CHECK-LABEL: zero_trip_count(
+; CHECK: entry:
+; CHECK-NEXT: br label %while.body
+; CHECK: while.body:
+; CHECK-NEXT: %d = phi i32 [ 0, %entry ], [ %inc, %while.body ]
+; CHECK-NEXT: %inc = add i32 %d, 1
+; CHECK-NEXT: %cmp = icmp eq i32 %inc, 0
+; CHECK-NEXT: br i1 %cmp, label %while.end, label %while.body
+; CHECK: while.end:
+; CHECK-NEXT: ret i32 0
+}
+
+