The frontend option -fno-optimize-sibling-calls resolves to -cc1's
-mdisable-tail-calls, which is passed to the TargetMachine in the
backend. PassManagerBuilder was adding the -tailcallelim pass anyway.
Use a new DisableTailCalls option in PassManagerBuilder to disable tail
calls harder.
Requires the matching commit in LLVM that adds DisableTailCalls.
<rdar://problem/16050591>
llvm-svn: 206543
Duncan P. N. Exon Smith