[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, 12 insertions, 0 deletions

diff --git a/llvm/lib/IR/Module.cpp b/llvm/lib/IR/Module.cpp
index a1f8cd0..73354a8 100644
--- a/llvm/lib/IR/Module.cpp
+++ b/llvm/lib/IR/Module.cpp
@@ -672,6 +672,18 @@ void Module::setRtLibUseGOT() {
   addModuleFlag(ModFlagBehavior::Max, "RtLibUseGOT", 1);
 }
 
+bool Module::getDirectAccessExternalData() const {
+  auto *Val = cast_or_null<ConstantAsMetadata>(
+      getModuleFlag("direct-access-external-data"));
+  if (Val)
+    return cast<ConstantInt>(Val->getValue())->getZExtValue() > 0;
+  return getPICLevel() == PICLevel::NotPIC;
+}
+
+void Module::setDirectAccessExternalData(bool Value) {
+  addModuleFlag(ModFlagBehavior::Max, "direct-access-external-data", Value);
+}
+
 UWTableKind Module::getUwtable() const {
   if (auto *Val = cast_or_null<ConstantAsMetadata>(getModuleFlag("uwtable")))
     return UWTableKind(cast<ConstantInt>(Val->getValue())->getZExtValue());