Restore visibility attribute for OpenBSD's stack guard

[sisnkemp]

Summary

This diff is from Philip Guenther ([e-mail])
and got committed to the OpenBSD tree, see http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/gnu/llvm/lib/CodeGen/TargetLoweringBase.cpp?rev=1.3&content-type=text/x-cvsweb-markup
and http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/gnu/llvm/lib/CodeGen/TargetLoweringBase.cpp.diff?r1=1.2&r2=1.3&f=h

This repairs my previous patch (https://reviews.llvm.org/rL279449) that
missed the fact that removing visibility diff results accesses to
the stack protector look like this:

two loads

movq __guard_local@GOTPCREL(%rip), %rbx
movq (%rbx), %rax

But better is one load only, which is what Philip's diff achieves
movq __guard_local(%rip), %rax

[joerg]

Missing test case.

[timshen]

Though I don't know a way off my hand, it's indeed good to have a testcase for this.

[brad]

Any suggestions for a test case?

[brad]

ping.

[joerg]

Create a function that does check the guard variable and make sure that it has the correct visibility declaration and/or access.

lib/CodeGen/TargetLoweringBase.cpp

 //===----------------------------------------------------------------------===//
 
 // For OpenBSD return its special guard variable. Otherwise return nullptr,
 // so that SelectionDAG handle SSP.
 Value *TargetLoweringBase::getIRStackGuard(IRBuilder<> &IRB) const {
   if (getTargetMachine().getTargetTriple().isOSOpenBSD()) {
     Module &M = *IRB.GetInsertBlock()->getParent()->getParent();
     PointerType *PtrTy = Type::getInt8PtrTy(M.getContext());
-    return M.getOrInsertGlobal("__guard_local", PtrTy);
+    Constant *C = M.getOrInsertGlobal("__guard_local", PtrTy);
+    if (GlobalVariable *G = dyn_cast_or_null<GlobalVariable>(C))
+      G->setVisibility(GlobalValue::HiddenVisibility);
+    return C;
   }
   return nullptr;
 }
 
 // Currently only support "standard" __stack_chk_guard.
 // TODO: add LOAD_STACK_GUARD support.
 void TargetLoweringBase::insertSSPDeclarations(Module &M) const {
   M.getOrInsertGlobal("__stack_chk_guard", Type::getInt8PtrTy(M.getContext()));