[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, 6 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 66c2abdb903a..d809e7063cf0 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -908,6 +908,12 @@ void CodeGenModule::Release() {
 
   if (CodeGenOpts.NoPLT)
     getModule().setRtLibUseGOT();
+  if (getTriple().isOSBinFormatELF() &&
+      CodeGenOpts.DirectAccessExternalData !=
+          getModule().getDirectAccessExternalData()) {
+    getModule().setDirectAccessExternalData(
+        CodeGenOpts.DirectAccessExternalData);
+  }
   if (CodeGenOpts.UnwindTables)
     getModule().setUwtable(llvm::UWTableKind(CodeGenOpts.UnwindTables));