Index: lib/Transforms/IPO/ArgumentPromotion.cpp
===================================================================
--- lib/Transforms/IPO/ArgumentPromotion.cpp
+++ lib/Transforms/IPO/ArgumentPromotion.cpp
@@ -145,7 +145,19 @@
     if (CS.getInstruction()->getParent()->getParent() == F)
       isSelfRecursive = true;
   }
-  
+
+  // Third check: See if this function contains any musttail call sites.  If so,
+  // we can't hack on the calling convention without breaking musttail ABI
+  // guarantees.
+  for (auto CR : *CGN) {
+    Value *Instr = CR.first;
+    CallInst *CS = dyn_cast_or_null<CallInst>(Instr);
+    if (!CS)
+      continue;
+    if (CS->isMustTailCall())
+      return nullptr;
+  }
+
   // Check to see which arguments are promotable.  If an argument is promotable,
   // add it to ArgsToPromote.
   SmallPtrSet<Argument*, 8> ArgsToPromote;
Index: test/Transforms/ArgumentPromotion/tail.ll
===================================================================
--- test/Transforms/ArgumentPromotion/tail.ll
+++ test/Transforms/ArgumentPromotion/tail.ll
@@ -18,3 +18,18 @@
   call void @bar(%pair* byval %Data)
   ret void
 }
+
+; Don't fire on musttail calls, since the calling convention can't be changed.
+declare i8* @musttail_inner(%pair* byval)
+
+define internal i8* @musttail_middle(%pair* byval %Data) {
+; CHECK: define internal i8* @musttail_middle(%pair* byval %Data)
+; CHECK: musttail call i8* @musttail_inner(%pair* byval %Data)
+  %r = musttail call i8* @musttail_inner(%pair* byval %Data)
+  ret i8* %r
+}
+
+define i8* @musttail_outer(%pair* byval %Data) {
+  %r = call i8* @musttail_middle(%pair* byval %Data)
+  ret i8* %r
+}