diff --git a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
--- a/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
+++ b/llvm/lib/Transforms/IPO/FunctionAttrs.cpp
@@ -255,12 +255,15 @@
     }
   }
 
+  // If the SCC contains both functions that read and functions that write, then
+  // we cannot add readonly attributes.
+  if (ReadsMemory && WritesMemory)
+    return false;
+
   // Success!  Functions in this SCC do not access memory, or only read memory.
   // Give them the appropriate attribute.
   bool MadeChange = false;
 
-  assert(!(ReadsMemory && WritesMemory) &&
-          "Function marked read-only and write-only");
   for (Function *F : SCCNodes) {
     if (F->doesNotAccessMemory())
       // Already perfect!
diff --git a/llvm/test/Transforms/FunctionAttrs/read-write-scc.ll b/llvm/test/Transforms/FunctionAttrs/read-write-scc.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/Transforms/FunctionAttrs/read-write-scc.ll
@@ -0,0 +1,20 @@
+; RUN: opt -S -functionattrs < %s | FileCheck %s
+; RUN: opt -S -passes=function-attrs < %s | FileCheck %s
+
+@i = global i32 0
+
+define void @foo() {
+; CHECK-LABEL: define void @foo() #0 {
+  store i32 1, i32* @i
+  call void @bar()
+  ret void
+}
+
+define void @bar() {
+; CHECK-LABEL: define void @bar() #0 {
+  %i = load i32, i32* @i
+  call void @foo()
+  ret void
+}
+
+; CHECK: attributes #0 = { nounwind }