Index: llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -3996,11 +3996,22 @@
   if (!Callee)
     return false;
 
-  // The prototype of a thunk is a lie. Don't directly call such a function.
+  // If this is a call to a thunk function, don't remove the cast. Thunks are
+  // used to transparently forward all incoming parameters and outgoing return
+  // values, so it's important to leave the cast in place.
   if (Callee->hasFnAttribute("thunk"))
     return false;
 
   Instruction *Caller = CS.getInstruction();
+
+  // If this is a musttail call, the callee's prototype must match the caller's
+  // prototype with the exception of pointee types. The code below doesn't
+  // implement that, so we can't do this transform.
+  // TODO: Do the transform if it only requires adding pointer casts.
+  CallInst *CI = dyn_cast<CallInst>(Caller);
+  if (CI && CI->isMustTailCall())
+    return false;
+
   const AttributeList &CallerPAL = CS.getAttributes();
 
   // Okay, this is a cast from a function to a different type.  Unless doing so
Index: llvm/test/Transforms/InstCombine/musttail-thunk.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/InstCombine/musttail-thunk.ll
@@ -0,0 +1,32 @@
+; RUN: opt -instcombine -S < %s | FileCheck %s
+
+; These are both direct calls, but make sure instcombine leaves the casts
+; alone.
+
+define i32 @call_thunk(i32 %x, i32 %y) {
+  %r = call i32 bitcast (void (i32, ...)* @inc_first_arg_thunk to i32 (i32, i32)*)(i32 %x, i32 %y)
+  ret i32 %r
+}
+
+; CHECK-LABEL: define i32 @call_thunk(i32 %x, i32 %y)
+; CHECK:   %r = call i32 bitcast (void (i32, ...)* @inc_first_arg_thunk to i32 (i32, i32)*)(i32 %x, i32 %y)
+; CHECK:   ret i32 %r
+
+define internal void @inc_first_arg_thunk(i32 %arg1, ...) #0 {
+entry:
+  %inc = add i32 %arg1, 1
+  musttail call void (i32, ...) bitcast (i32 (i32, i32)* @plus to void (i32, ...)*)(i32 %inc, ...)
+  ret void
+}
+
+; CHECK-LABEL: define internal void @inc_first_arg_thunk(i32 %arg1, ...) #0
+; CHECK:   %inc = add i32 %arg1, 1
+; CHECK:   musttail call void (i32, ...) bitcast (i32 (i32, i32)* @plus to void (i32, ...)*)(i32 %inc, ...)
+; CHECK:   ret void
+
+define internal i32 @plus(i32 %x, i32 %y) {
+  %r = add i32 %x, %y
+  ret i32 %r
+}
+
+attributes #0 = { "thunk" }