Index: lib/Transforms/Scalar/GVN.cpp
===================================================================
--- lib/Transforms/Scalar/GVN.cpp
+++ lib/Transforms/Scalar/GVN.cpp
@@ -1378,7 +1378,8 @@
   // We can replace assume value with true, which covers cases like this:
   // call void @llvm.assume(i1 %cmp)
   // br i1 %cmp, label %bb1, label %bb2 ; will change %cmp to true
-  ReplaceWithConstMap[V] = True;
+  if (!isa<Constant>(V))
+    ReplaceWithConstMap[V] = True;
 
   // If one of *cmp *eq operand is const, adding it to map will cover this:
   // %cmp = fcmp oeq float 3.000000e+00, %0 ; const on lhs could happen
Index: test/Transforms/GVN/no-crash-on-undef-assume.ll
===================================================================
--- /dev/null
+++ test/Transforms/GVN/no-crash-on-undef-assume.ll
@@ -0,0 +1,8 @@
+; RUN: opt < %s -gvn -S
+
+define i1 @foo() {
+    call void @llvm.assume(i1 undef)
+    ret i1 undef
+}
+
+declare void @llvm.assume(i1)