Remove DisableTailCalls from TargetOptions and the code in resetTargetOptions

that was resetting it.

Remove the uses of DisableTailCalls in subclasses of TargetLowering and use
the value of function attribute "disable-tail-calls" instead. Also,
unconditionally add pass TailCallElim to the pipeline and check the function
attribute at the start of runOnFunction to disable the pass on a per-function
basis. 
 
This is part of the work to remove TargetMachine::resetTargetOptions, and since
DisableTailCalls was the last non-fast-math option that was being reset in that
function, we should be able to remove the function entirely after the work to
propagate IR-level fast-math flags to DAG nodes is completed.

Out-of-tree users should remove the uses of DisableTailCalls and make changes
to attach attribute "disable-tail-calls"="true" or "false" to the functions in
the IR.

rdar://problem/13752163

Differential Revision: http://reviews.llvm.org/D10099


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@239427 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Scalar/TailRecursionElimination.cpp

@@ -158,6 +158,9 @@ bool TailCallElim::runOnFunction(Function &F) {
   if (skipOptnoneFunction(F))
     return false;
 
+  if (F.getFnAttribute("disable-tail-calls").getValueAsString() == "true")
+    return false;
+
   bool AllCallsAreTailCalls = false;
   bool Modified = markTails(F, AllCallsAreTailCalls);
   if (AllCallsAreTailCalls)