Implement the local-dynamic TLS model for x86 (PR3985)

This implements codegen support for accesses to thread-local variables
using the local-dynamic model, and adds a clean-up pass so that the base
address for the TLS block can be re-used between local-dynamic access on
an execution path.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@157818 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/X86/X86.h

@@ -36,6 +36,11 @@ FunctionPass *createX86ISelDag(X86TargetMachine &TM,
 /// register for PIC on x86-32.
 FunctionPass* createGlobalBaseRegPass();
 
+/// createCleanupLocalDynamicTLSPass() - This pass combines multiple accesses
+/// to local-dynamic TLS variables so that the TLS base address for the module
+/// is only fetched once per execution path through the function.
+FunctionPass *createCleanupLocalDynamicTLSPass();
+
 /// createX86FloatingPointStackifierPass - This function returns a pass which
 /// converts floating point register references and pseudo instructions into
 /// floating point stack references and physical instructions.