[PATCH] musttail: Forward regparms of variadic functions on x86_64

[David Majnemer]

================
Comment at: lib/CodeGen/SelectionDAG/FunctionLoweringInfo.cpp:151
@@ +150,3 @@
+      // forward implicit register parameters.
+      if (auto *CI = dyn_cast<CallInst>(I)) {
+        if (CI->isMustTailCall() && Fn->isVarArg())
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`if (const auto *CI = dyn_cast<CallInst>(I)) {` would be a little nicer when considering that `I` is a `const_iterator`.

================
Comment at: lib/Target/X86/X86ISelLowering.cpp:2291-2296
@@ +2290,8 @@
+  assert(Subtarget->is64Bit());
+  if (Subtarget->isCallingConvWin64(CallConv)) {
+    // The XMM registers which might contain var arg parameters are shadowed
+    // in their paired GPR.  So we only need to save the GPR to their home
+    // slots.
+    return None;
+  }
+
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Do we want a TODO here for vectorcall?

http://reviews.llvm.org/D5060