x86_64: Fix calls to __morestack under the large code model.

Under the large code model, we cannot assume that __morestack lives within
2^31 bytes of the call site, so we cannot use pc-relative addressing. We
cannot perform the call via a temporary register, as the rax register may
be used to store the static chain, and all other suitable registers may be
either callee-save or used for parameter passing. We cannot use the stack
at this point either because __morestack manipulates the stack directly.

To avoid these issues, perform an indirect call via a read-only memory
location containing the address.

This solution is not perfect, as it assumes that the .rodata section
is laid out within 2^31 bytes of each function body, but this seems to
be sufficient for JIT.

Differential Revision: http://reviews.llvm.org/D6787

llvm-svn: 225003

llvm/lib/CodeGen/MachineModuleInfo.cpp

@@ -273,7 +273,7 @@ bool MachineModuleInfo::doInitialization(Module &M) {
   CurCallSite = 0;
   CallsEHReturn = 0;
   CallsUnwindInit = 0;
-  DbgInfoAvailable = UsesVAFloatArgument = false; 
+  DbgInfoAvailable = UsesVAFloatArgument = UsesMorestackAddr = false;
   // Always emit some info, by default "no personality" info.
   Personalities.push_back(nullptr);
   AddrLabelSymbols = nullptr;