Index: include/llvm/IR/Instructions.h
===================================================================
--- include/llvm/IR/Instructions.h
+++ include/llvm/IR/Instructions.h
@@ -2413,6 +2413,10 @@
     return getNumOperands() != 0 ? getOperand(0) : nullptr;
   }
 
+  /// Returns a musttail call instruction if one immediately precedes the given
+  /// return instruction with an optional bitcast instruction between them.
+  CallInst *getPrecedingMustTailCallIfPresent();
+
   unsigned getNumSuccessors() const { return 0; }
 
   // Methods for support type inquiry through isa, cast, and dyn_cast:
Index: lib/IR/Instructions.cpp
===================================================================
--- lib/IR/Instructions.cpp
+++ lib/IR/Instructions.cpp
@@ -658,6 +658,34 @@
 ReturnInst::~ReturnInst() {
 }
 
+CallInst *ReturnInst::getPrecedingMustTailCallIfPresent() {
+  if (this == getParent()->begin())
+    return nullptr;
+
+  Instruction *Prev = getPrevNode();
+  if (!Prev)
+    return nullptr;
+
+  if (Value *RV = getReturnValue()) {
+    if (RV != Prev)
+      return nullptr;
+
+    // Look through the optional bitcast.
+    if (auto *BI = dyn_cast<BitCastInst>(Prev)) {
+      RV = BI->getOperand(0);
+      Prev = BI->getPrevNode();
+      if (!Prev || RV != Prev)
+        return nullptr;
+    }
+  }
+
+  if (auto *CI = dyn_cast<CallInst>(Prev)) {
+    if (CI->isMustTailCall())
+      return CI;
+  }
+  return nullptr;
+}
+
 //===----------------------------------------------------------------------===//
 //                        ResumeInst Implementation
 //===----------------------------------------------------------------------===//
Index: lib/Transforms/Instrumentation/MemorySanitizer.cpp
===================================================================
--- lib/Transforms/Instrumentation/MemorySanitizer.cpp
+++ lib/Transforms/Instrumentation/MemorySanitizer.cpp
@@ -2342,6 +2342,12 @@
       VAHelper->visitCallSite(CS, IRB);
     }
 
+    // If this is a musttail call site, we can't insert propagation code here.
+    // The return type of the caller must match the callee, so the shadow should
+    // already be set up for an immediate return.
+    if (CS.isMustTailCall())
+      return;
+
     // Now, get the shadow for the RetVal.
     if (!I.getType()->isSized()) return;
     IRBuilder<> IRBBefore(&I);
@@ -2375,6 +2381,10 @@
   }
 
   void visitReturnInst(ReturnInst &I) {
+    // Don't propagate shadow between musttail calls and the return.
+    if (I.getPrecedingMustTailCallIfPresent())
+      return;
+
     IRBuilder<> IRB(&I);
     Value *RetVal = I.getReturnValue();
     if (!RetVal) return;
Index: lib/Transforms/Utils/InlineFunction.cpp
===================================================================
--- lib/Transforms/Utils/InlineFunction.cpp
+++ lib/Transforms/Utils/InlineFunction.cpp
@@ -486,33 +486,6 @@
   }
 }
 
-/// Returns a musttail call instruction if one immediately precedes the given
-/// return instruction with an optional bitcast instruction between them.
-static CallInst *getPrecedingMustTailCall(ReturnInst *RI) {
-  Instruction *Prev = RI->getPrevNode();
-  if (!Prev)
-    return nullptr;
-
-  if (Value *RV = RI->getReturnValue()) {
-    if (RV != Prev)
-      return nullptr;
-
-    // Look through the optional bitcast.
-    if (auto *BI = dyn_cast<BitCastInst>(Prev)) {
-      RV = BI->getOperand(0);
-      Prev = BI->getPrevNode();
-      if (!Prev || RV != Prev)
-        return nullptr;
-    }
-  }
-
-  if (auto *CI = dyn_cast<CallInst>(Prev)) {
-    if (CI->isMustTailCall())
-      return CI;
-  }
-  return nullptr;
-}
-
 /// InlineFunction - This function inlines the called function into the basic
 /// block of the caller.  This returns false if it is not possible to inline
 /// this call.  The program is still in a well defined state if this occurs
@@ -765,7 +738,7 @@
       for (ReturnInst *RI : Returns) {
         // Don't insert llvm.lifetime.end calls between a musttail call and a
         // return.  The return kills all local allocas.
-        if (InlinedMustTailCalls && getPrecedingMustTailCall(RI))
+        if (InlinedMustTailCalls && RI->getPrecedingMustTailCallIfPresent())
           continue;
         IRBuilder<>(RI).CreateLifetimeEnd(AI, AllocaSize);
       }
@@ -789,7 +762,7 @@
     for (ReturnInst *RI : Returns) {
       // Don't insert llvm.stackrestore calls between a musttail call and a
       // return.  The return will restore the stack pointer.
-      if (InlinedMustTailCalls && getPrecedingMustTailCall(RI))
+      if (InlinedMustTailCalls && RI->getPrecedingMustTailCallIfPresent())
         continue;
       IRBuilder<>(RI).CreateCall(StackRestore, SavedPtr);
     }
@@ -812,7 +785,7 @@
     // Handle the returns preceded by musttail calls separately.
     SmallVector<ReturnInst *, 8> NormalReturns;
     for (ReturnInst *RI : Returns) {
-      CallInst *ReturnedMustTail = getPrecedingMustTailCall(RI);
+      CallInst *ReturnedMustTail = RI->getPrecedingMustTailCallIfPresent();
       if (!ReturnedMustTail) {
         NormalReturns.push_back(RI);
         continue;
Index: test/Instrumentation/MemorySanitizer/msan_basic.ll
===================================================================
--- test/Instrumentation/MemorySanitizer/msan_basic.ll
+++ test/Instrumentation/MemorySanitizer/msan_basic.ll
@@ -860,3 +860,16 @@
 ; CHECK-LABEL: define void @MismatchedReturnTypeTailCall
 ; CHECK: tail call i32 @InnerTailCall
 ; CHECK: ret void
+
+declare i32 @InnerMustTailCall(i32 %a)
+
+define i32 @MustTailCall(i32 %a) {
+  %b = musttail call i32 @InnerMustTailCall(i32 %a)
+  ret i32 %b
+}
+
+; Test that 'musttail' is preserved.  The ABI should make this work.
+
+; CHECK-LABEL: define i32 @MustTailCall
+; CHECK: musttail call i32 @InnerMustTailCall
+; CHECK-NEXT: ret i32