[AArch64] Don't crash trying to resolve __stack_chk_guard.

[efriedma]

Summary

In certain cases, the compiler might try to merge __stack_chk_guard with another global variable. (Or someone could theoretically define __stack_chk_guard as an alias.) In that case, make sure we don't crash.

[davide]

LGTM, thanks

llvm/trunk/lib/CodeGen/TargetLoweringBase.cpp

if (getTargetMachine().getTargetTriple().isOSOpenBSD()) {
     return M.getOrInsertGlobal("__guard_local", PtrTy);
   }
   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()));
+  if (!M.getNamedValue("__stack_chk_guard"))
+    new GlobalVariable(M, Type::getInt8PtrTy(M.getContext()), false,
+                       GlobalVariable::ExternalLinkage,
+                       nullptr, "__stack_chk_guard");
 }
 
 // Currently only support "standard" __stack_chk_guard.
 // TODO: add LOAD_STACK_GUARD support.
 Value *TargetLoweringBase::getSDagStackGuard(const Module &M) const {
-  return M.getGlobalVariable("__stack_chk_guard", true);
+  return M.getNamedValue("__stack_chk_guard");
 }
 
 Value *TargetLoweringBase::getSSPStackGuardCheck(const Module &M) const {
   return nullptr;
 }
 
 unsigned TargetLoweringBase::getMinimumJumpTableEntries() const {
   return MinimumJumpTableEntries;

llvm/trunk/test/CodeGen/AArch64/stackguard-internal.ll

+; RUN: llc -O3 %s -o - | FileCheck %s
+target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+target triple = "aarch64-linux-gnu"
+
+; Make sure we correctly lower stack guards even if __stack_chk_guard
+; is an alias.  (The alias is created by GlobalMerge.)
+; CHECK: adrp {{.*}}, __stack_chk_guard
+; CHECK: ldr {{.*}}, [{{.*}}, :lo12:__stack_chk_guard]
+; CHECK: .set __stack_chk_guard, .L_MergedGlobals+4
+
+@__stack_chk_guard = internal global [8 x i32] zeroinitializer, align 4
+@x = internal global i32 0, align 4
+
+define i32 @b() nounwind sspstrong {
+entry:
+  %z = alloca [10 x i32], align 4
+  %arraydecay = getelementptr inbounds [10 x i32], [10 x i32]* %z, i64 0, i64 0
+  %call = call i32 @a(i32* getelementptr inbounds ([8 x i32], [8 x i32]* @__stack_chk_guard, i64 0, i64 0), i32* nonnull @x, i32* nonnull %arraydecay) #3
+  ret i32 %call
+}
+declare i32 @a(i32*, i32*, i32*)