[clang] [llvm] [clang][llvm][AArch64] Set hardening fn attrs on synthetic functions (PR #211013)

[Anatoly Trosinenko via cfe-commits]

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@@ -1543,6 +1543,27 @@ void CodeGenModule::Release() {
                                 "sign-return-address-with-bkey", 2);
   }
   if (T.isAArch64()) {
+    // Emit the following 4 module flags so LLVM can derive corresponding
+    // function attributes for synthetically generated functions (e.g.
+    // __llvm_gcov_writeout). It is safe to only emit the flags conditionally
+    // and set the Max behavior because of two reasons:
+    // 1) all 4 hardening features gated behind the attributes do not break ABI
+    //    compatibility, so we do not need to error on flag mismatch (thus,
+    //    conditional emission);
+    // 2) promoting an absent flag to a present flag enables the corresponding
+    //    hardening feature for newly emitted functions which does not affect
+    //    correctness and is guaranteed to have sufficient target features for
+    //    it, since the module we are merging with already has the flag set.
+    if (LangOpts.PointerAuthReturns)
+      getModule().addModuleFlag(llvm::Module::Max, "ptrauth-returns", 1);
+    if (LangOpts.PointerAuthAuthTraps)
+      getModule().addModuleFlag(llvm::Module::Max, "ptrauth-auth-traps", 1);
+    if (LangOpts.PointerAuthIndirectGotos)
+      getModule().addModuleFlag(llvm::Module::Max, "ptrauth-indirect-gotos", 1);
+    if (LangOpts.AArch64JumpTableHardening)
+      getModule().addModuleFlag(llvm::Module::Max,
+                                "aarch64-jump-table-hardening", 1);
+
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atrosinenko wrote:

[nit] I would add a tiny comment here mentioning why the below flags are treated differently.

https://github.com/llvm/llvm-project/pull/211013