[IR] Make stack protector symbol dso_local according to -f[no-]direct-access-external-data

There are two motivations.

`-fno-pic -fstack-protector -mstack-protector-guard=global` created
`__stack_chk_guard` is referenced directly on all ELF OSes except FreeBSD.
This patch allows referencing the symbol indirectly with
-fno-direct-access-external-data.

Some Linux kernel folks want
`-fno-pic -fstack-protector -mstack-protector-guard-reg=gs -mstack-protector-guard-symbol=__stack_chk_guard`
created `__stack_chk_guard` to be referenced directly, avoiding
R_X86_64_REX_GOTPCRELX (even if the relocation may be optimized out by the linker).
https://github.com/llvm/llvm-project/issues/60116
Why they need this isn't so clear to me.

---

Add module flag "direct-access-external-data" and set the dso_local property of
the stack protector symbol. The module flag can benefit other LLVMCodeGen
synthesized symbols that are not represented in LLVM IR.

Nowadays, with `-fno-pic` being uncommon, ideally we should set
"direct-access-external-data" when it is true. However, doing so would require
~90 clang/test tests to be updated, which are too much.

As a compromise, we set "direct-access-external-data" only when it's different
from the implied default value.

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D150841

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/TargetLoweringBase.cpp b/llvm/lib/CodeGen/TargetLoweringBase.cpp
index 22472d4..081c9e5 100644
--- a/llvm/lib/CodeGen/TargetLoweringBase.cpp
+++ b/llvm/lib/CodeGen/TargetLoweringBase.cpp
@@ -1977,7 +1977,7 @@ void TargetLoweringBase::insertSSPDeclarations(Module &M) const {
                                   "__stack_chk_guard");
 
     // FreeBSD has "__stack_chk_guard" defined externally on libc.so
-    if (TM.getRelocationModel() == Reloc::Static &&
+    if (M.getDirectAccessExternalData() &&
         !TM.getTargetTriple().isWindowsGNUEnvironment() &&
         !TM.getTargetTriple().isOSFreeBSD())
       GV->setDSOLocal(true);