diff --git a/lib/Transforms/Utils/InlineFunction.cpp b/lib/Transforms/Utils/InlineFunction.cpp
index 5525830..acd2b10 100644
--- a/lib/Transforms/Utils/InlineFunction.cpp
+++ b/lib/Transforms/Utils/InlineFunction.cpp
@@ -240,12 +240,15 @@
          E = CalledFunc->arg_end(); I != E; ++I, ++AI, ++ArgNo) {
       Value *ActualArg = *AI;
       
-      // When byval arguments actually inlined, we need to make the copy implied
-      // by them explicit.  However, we don't do this if the callee is readonly
-      // or readnone, because the copy would be unneeded: the callee doesn't
-      // modify the struct.
-      if (CalledFunc->paramHasAttr(ArgNo+1, ParamAttr::ByVal) &&
-          !CalledFunc->onlyReadsMemory()) {
+      // When byval arguments are inlined, we need to make the copy implied
+      // by them explicit.  It is tempting to think that this is not needed if
+      // the callee is readonly, because the callee doesn't modify the struct.
+      // However this would be wrong: readonly means that any writes the callee
+      // performs are not visible to the caller.  But writes by the callee to
+      // an argument passed byval are by definition not visible to the caller!
+      // Since we allow this kind of readonly function, there needs to be an
+      // explicit copy in order to keep the writes invisible after inlining.
+      if (CalledFunc->paramHasAttr(ArgNo+1, ParamAttr::ByVal)) {
         const Type *AggTy = cast<PointerType>(I->getType())->getElementType();
         const Type *VoidPtrTy = PointerType::getUnqual(Type::Int8Ty);
         
diff --git a/test/CFrontend/2008-01-26-ReadOnlyByVal.c b/test/CFrontend/2008-01-26-ReadOnlyByVal.c
new file mode 100644
index 0000000..cea6d1e
--- /dev/null
+++ b/test/CFrontend/2008-01-26-ReadOnlyByVal.c
@@ -0,0 +1,19 @@
+// RUN: %llvmgcc %s -S -O1 -o - | llvm-as | opt -std-compile-opts | llvm-dis | not grep add
+
+struct S { int A; int B; int C; int D; };
+
+int f(struct S x) __attribute__ ((const));
+
+static int __attribute__ ((const)) g(struct S x) {
+	x.A = x.B;
+	return f(x);
+}
+
+int h(void) {
+	struct S x;
+	int r;
+	x.A = 0;
+	x.B = 9;
+	r = g(x);
+	return r + x.A;
+}