Index: llvm/include/llvm/Transforms/IPO/FunctionAttrs.h
===================================================================
--- llvm/include/llvm/Transforms/IPO/FunctionAttrs.h
+++ llvm/include/llvm/Transforms/IPO/FunctionAttrs.h
@@ -20,6 +20,19 @@
 
 namespace llvm {
 
+class AAResults;
+
+/// The three kinds of memory access relevant to 'readonly' and
+/// 'readnone' attributes.
+enum MemoryAccessKind {
+  MAK_ReadNone = 0,
+  MAK_ReadOnly = 1,
+  MAK_MayWrite = 2
+};
+
+/// Returns the memory access properties of this copy of the function.
+MemoryAccessKind computeFunctionBodyMemoryAccess(Function &F, AAResults &AAR);
+
 /// Computes function attributes in post-order over the call graph.
 ///
 /// By operating in post-order, this pass computes precise attributes for
Index: llvm/lib/Transforms/IPO/FunctionAttrs.cpp
===================================================================
--- llvm/lib/Transforms/IPO/FunctionAttrs.cpp
+++ llvm/lib/Transforms/IPO/FunctionAttrs.cpp
@@ -53,27 +53,15 @@
 typedef SmallSetVector<Function *, 8> SCCNodeSet;
 }
 
-namespace {
-/// The three kinds of memory access relevant to 'readonly' and
-/// 'readnone' attributes.
-enum MemoryAccessKind {
-  MAK_ReadNone = 0,
-  MAK_ReadOnly = 1,
-  MAK_MayWrite = 2
-};
-}
-
-static MemoryAccessKind checkFunctionMemoryAccess(Function &F, AAResults &AAR,
+static MemoryAccessKind checkFunctionMemoryAccess(Function &F, bool ThisBody,
+                                                  AAResults &AAR,
                                                   const SCCNodeSet &SCCNodes) {
   FunctionModRefBehavior MRB = AAR.getModRefBehavior(&F);
   if (MRB == FMRB_DoesNotAccessMemory)
     // Already perfect!
     return MAK_ReadNone;
 
-  // Non-exact function definitions may not be selected at link time, and an
-  // alternative version that writes to memory may be selected.  See the comment
-  // on GlobalValue::isDefinitionExact for more details.
-  if (!F.hasExactDefinition()) {
+  if (!ThisBody) {
     if (AliasAnalysis::onlyReadsMemory(MRB))
       return MAK_ReadOnly;
 
@@ -172,6 +160,11 @@
   return ReadsMemory ? MAK_ReadOnly : MAK_ReadNone;
 }
 
+MemoryAccessKind llvm::computeFunctionBodyMemoryAccess(Function &F,
+                                                       AAResults &AAR) {
+  return checkFunctionMemoryAccess(F, /*ThisBody=*/true, AAR, {});
+}
+
 /// Deduce readonly/readnone attributes for the SCC.
 template <typename AARGetterT>
 static bool addReadAttrs(const SCCNodeSet &SCCNodes, AARGetterT &&AARGetter) {
@@ -182,7 +175,11 @@
     // Call the callable parameter to look up AA results for this function.
     AAResults &AAR = AARGetter(*F);
 
-    switch (checkFunctionMemoryAccess(*F, AAR, SCCNodes)) {
+    // Non-exact function definitions may not be selected at link time, and an
+    // alternative version that writes to memory may be selected.  See the
+    // comment on GlobalValue::isDefinitionExact for more details.
+    switch (checkFunctionMemoryAccess(*F, F->hasExactDefinition(),
+                                      AAR, SCCNodes)) {
     case MAK_MayWrite:
       return false;
     case MAK_ReadOnly: