Add LLVM_ATTRIBUTE_USED to fix problems which could be exposed by aggressive global pointer variable removal

[MaskRay]

Summary

Note to BuryPointer.cpp:GraveYard. 'unused' cannot prevent (1) dead store
elimination and (2) removal of the global pointer variable (D69428) but 'used' can.

Discovered when comparing link maps between HEAD+D69428 and HEAD.

[lattner]

Looks great, thank you for catching this!

llvm/lib/Support/BuryPointer.cpp

 
 void BuryPointer(const void *Ptr) {
   // This function may be called only a small fixed amount of times per each
   // invocation, otherwise we do actually have a leak which we want to report.
   // If this function is called more than kGraveYardMaxSize times, the pointers
   // will not be properly buried and a leak detector will report a leak, which
   // is what we want in such case.
   static const size_t kGraveYardMaxSize = 16;
-  LLVM_ATTRIBUTE_UNUSED static const void *GraveYard[kGraveYardMaxSize];
+  LLVM_ATTRIBUTE_USED static const void *GraveYard[kGraveYardMaxSize];
   static std::atomic<unsigned> GraveYardSize;
   unsigned Idx = GraveYardSize++;
   if (Idx >= kGraveYardMaxSize)
     return;
   GraveYard[Idx] = Ptr;
 }
 
 } // namespace llvm

llvm/lib/Support/Unix/Signals.inc

 } RegisteredSignalInfo[NumSigs];
 
 #if defined(HAVE_SIGALTSTACK)
 // Hold onto both the old and new alternate signal stack so that it's not
 // reported as a leak. We don't make any attempt to remove our alt signal
 // stack if we remove our signal handlers; that can't be done reliably if
 // someone else is also trying to do the same thing.
 static stack_t OldAltStack;
-static void* NewAltStackPointer;
+LLVM_ATTRIBUTE_USED static void *NewAltStackPointer;
 
 static void CreateSigAltStack() {
   const size_t AltStackSize = MINSIGSTKSZ + 64 * 1024;
 
   // If we're executing on the alternate stack, or we already have an alternate
   // signal stack that we're happy with, there's nothing for us to do. Don't
   // reduce the size, some other part of the process might need a larger stack
   // than we do.