Add 'musttail' marker to call instructions

This is similar to the 'tail' marker, except that it guarantees that
tail call optimization will occur.  It also comes with convervative IR
verification rules that ensure that tail call optimization is possible.

Reviewers: nicholas

Differential Revision: http://llvm-reviews.chandlerc.com/D3240

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207143 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/PowerPC/PPCISelLowering.cpp

@@ -3720,6 +3720,10 @@ PPCTargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
     isTailCall = IsEligibleForTailCallOptimization(Callee, CallConv, isVarArg,
                                                    Ins, DAG);
 
+  if (!isTailCall && CLI.CS && CLI.CS->isMustTailCall())
+    report_fatal_error("failed to perform tail call elimination on a call "
+                       "site marked musttail");
+
   if (PPCSubTarget.isSVR4ABI()) {
     if (PPCSubTarget.isPPC64())
       return LowerCall_64SVR4(Chain, Callee, CallConv, isVarArg,