Index: lib/Transforms/Scalar/LICM.cpp
===================================================================
--- lib/Transforms/Scalar/LICM.cpp
+++ lib/Transforms/Scalar/LICM.cpp
@@ -687,35 +687,56 @@
     if (match(CI, m_Intrinsic<Intrinsic::assume>()))
       // Assumes don't actually alias anything or throw
       return true;
-    
+
     // Handle simple cases by querying alias analysis.
     FunctionModRefBehavior Behavior = AA->getModRefBehavior(CI);
     if (Behavior == FMRB_DoesNotAccessMemory)
       return true;
-    if (AliasAnalysis::onlyReadsMemory(Behavior)) {
-      // A readonly argmemonly function only reads from memory pointed to by
-      // it's arguments with arbitrary offsets.  If we can prove there are no
-      // writes to this memory in the loop, we can hoist or sink.
-      if (AliasAnalysis::onlyAccessesArgPointees(Behavior)) {
-        // TODO: expand to writeable arguments
-        for (Value *Op : CI->arg_operands())
-          if (Op->getType()->isPointerTy() &&
-              pointerInvalidatedByLoop(
-                  MemoryLocation(Op, MemoryLocation::UnknownSize, AAMDNodes()),
-                  CurAST, CurLoop, AA))
+    if (AliasAnalysis::onlyAccessesArgPointees(Behavior)) {
+      CallSite CS(CI);
+      ModRefInfo CallMask = createModRefInfo(Behavior);
+      
+      // Some intrinsics are marked as modifying memory for control flow
+      // modelling purposes, but don't actually modify any specific memory
+      // location. 
+      using namespace PatternMatch;
+      if (CI->use_empty() &&
+          match(CI, m_Intrinsic<Intrinsic::invariant_start>()))
+        CallMask = clearMod(CallMask);
+      
+      for (auto AI = CS.arg_begin(), AE = CS.arg_end(); AI != AE; ++AI) {
+        const Value *Arg = *AI;
+        if (!Arg->getType()->isPointerTy())
+          continue;
+        unsigned ArgIdx = std::distance(CS.arg_begin(), AI);
+        MemoryLocation ArgLoc = MemoryLocation::getForArgument(CS, ArgIdx,
+                                                               nullptr);
+        ModRefInfo ArgMask = AA->getArgModRefInfo(CS, ArgIdx);
+        ArgMask = intersectModRef(CallMask, ArgMask);
+        if (isRefSet(ArgMask) &&
+            pointerInvalidatedByLoop(ArgLoc, CurAST, CurLoop, AA))
+          return false;
+        if (isModSet(ArgMask)) {
+          auto &AS = CurAST->getAliasSetFor(ArgLoc);
+          if (AS.isRef() || !AS.isMustAlias())
+            // Quick exit test, handled by the full path below as well.
             return false;
-        return true;
+          auto *UniqueI = AS.getUniqueInstruction();
+          if (!UniqueI)
+            // other memory op, give up
+            return false;
+          assert(UniqueI == CI && "AS must contain SI");
+          // falthrough, success
+        }
       }
-
-      // If this call only reads from memory and there are no writes to memory
-      // in the loop, we can hoist or sink the call as appropriate.
-      if (isReadOnly(CurAST))
-        return true;
+      return true;
     }
-
-    // FIXME: This should use mod/ref information to see if we can hoist or
-    // sink the call.
-
+    
+    // If this call only reads from memory and there are no writes to memory
+    // in the loop, we can hoist or sink the call as appropriate.
+    if (AliasAnalysis::onlyReadsMemory(Behavior) && isReadOnly(CurAST))
+      return true;
+    // TODO: Handle a WriteOnly call analogously to a fence
     return false;
   } else if (auto *FI = dyn_cast<FenceInst>(&I)) {
     // Fences alias (most) everything to provide ordering.  For the moment,