Index: lib/Analysis/InlineCost.cpp
===================================================================
--- lib/Analysis/InlineCost.cpp
+++ lib/Analysis/InlineCost.cpp
@@ -331,10 +331,10 @@
   // Check whether inlining will turn a dynamic alloca into a static
   // alloca, and handle that case.
   if (I.isArrayAllocation()) {
-    if (Constant *Size = SimplifiedValues.lookup(I.getArraySize())) {
-      ConstantInt *AllocSize = dyn_cast<ConstantInt>(Size);
-      assert(AllocSize && "Allocation size not a constant int?");
+    if (ConstantInt *AllocSize = dyn_cast_or_null<ConstantInt>(
+            SimplifiedValues.lookup(I.getArraySize()))) {
       Type *Ty = I.getAllocatedType();
+      // FIXME: This can't be right? AllocatedSize is in *bytes*.
       AllocatedSize += Ty->getPrimitiveSizeInBits() * AllocSize->getZExtValue();
       return Base::visitAlloca(I);
     }
Index: test/Transforms/Inline/alloca_test.ll
===================================================================
--- test/Transforms/Inline/alloca_test.ll
+++ test/Transforms/Inline/alloca_test.ll
@@ -1,3 +1,4 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; This test ensures that alloca instructions in the entry block for an inlined
 ; function are moved to the top of the function they are inlined into.
 ;
@@ -23,3 +24,17 @@
   ret i32 %Y
 }
 
+; https://llvm.org/bugs/show_bug.cgi?id=27277
+; Don't assume that the size is a ConstantInt (an undef value is also considered a constant).
+
+define void @PR27277(i32 %p1) {
+; CHECK-LABEL: @PR27277(
+; CHECK-NEXT:    [[VLA:%.*]] = alloca double, i32 %p1
+; CHECK-NEXT:    call void @PR27277(i32 undef)
+; CHECK-NEXT:    ret void
+;
+  %vla = alloca double, i32 %p1
+  call void @PR27277(i32 undef)
+  ret void
+}
+