[clang][ARM] PACBTI-M frontend support

Handle branch protection option on the commandline as well as a function
attribute. One patch for both mechanisms, as they use the same underlying
parsing mechanism.

These are recorded in a set of LLVM IR module-level attributes like we do for
AArch64 PAC/BTI (see https://reviews.llvm.org/D85649):

- command-line options are "translated" to module-level LLVM IR
  attributes (metadata).

- functions have PAC/BTI specific attributes iff the
  __attribute__((target("branch-protection=...))) was used in the function
  declaration.

- command-line option -mbranch-protection to armclang targeting Arm,
following this grammar:

branch-protection ::= "-mbranch-protection=" <protection>
protection ::=  "none" | "standard" | "bti" [ "+" <pac-ret-clause> ]
                | <pac-ret-clause> [ "+" "bti"]
pac-ret-clause ::= "pac-ret" [ "+" <pac-ret-option> ]
pac-ret-option ::= "leaf" ["+" "b-key"] | "b-key" ["+" "leaf"]

b-key is simply a placeholder to make it consistent with AArch64's
version. In Arm, however, it triggers a warning informing that b-key is
unsupported and a-key will be selected instead.

- Handle _attribute_((target(("branch-protection=..."))) for AArch32 with the
same grammer as the commandline options.

This patch is part of a series that adds support for the PACBTI-M extension of
the Armv8.1-M architecture, as detailed here:

https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/armv8-1-m-pointer-authentication-and-branch-target-identification-extension

The PACBTI-M specification can be found in the Armv8-M Architecture Reference
Manual:

https://developer.arm.com/documentation/ddi0553/latest

The following people contributed to this patch:

- Momchil Velikov
- Victor Campos
- Ties Stuij

Reviewed By: vhscampos

Differential Revision: https://reviews.llvm.org/D112421

1 files changed, 23 insertions, 6 deletions

diff --git a/clang/lib/CodeGen/CodeGenModule.cpp b/clang/lib/CodeGen/CodeGenModule.cpp
index 7a7ed22e4381..9ba1a5c25e81 100644
--- a/clang/lib/CodeGen/CodeGenModule.cpp
+++ b/clang/lib/CodeGen/CodeGenModule.cpp
@@ -710,10 +710,25 @@ void CodeGenModule::Release() {
                               1);
   }
 
-  if (Arch == llvm::Triple::aarch64 || Arch == llvm::Triple::aarch64_32 ||
+  // Add module metadata for return address signing (ignoring
+  // non-leaf/all) and stack tagging. These are actually turned on by function
+  // attributes, but we use module metadata to emit build attributes. This is
+  // needed for LTO, where the function attributes are inside bitcode
+  // serialised into a global variable by the time build attributes are
+  // emitted, so we can't access them.
+  if (Context.getTargetInfo().hasFeature("ptrauth") &&
+      LangOpts.getSignReturnAddressScope() !=
+          LangOptions::SignReturnAddressScopeKind::None)
+    getModule().addModuleFlag(llvm::Module::Override,
+                              "sign-return-address-buildattr", 1);
+  if (LangOpts.Sanitize.has(SanitizerKind::MemTag))
+    getModule().addModuleFlag(llvm::Module::Override,
+                              "tag-stack-memory-buildattr", 1);
+
+  if (Arch == llvm::Triple::thumb || Arch == llvm::Triple::thumbeb ||
+      Arch == llvm::Triple::aarch64 || Arch == llvm::Triple::aarch64_32 ||
       Arch == llvm::Triple::aarch64_be) {
-    getModule().addModuleFlag(llvm::Module::Error,
-                              "branch-target-enforcement",
+    getModule().addModuleFlag(llvm::Module::Error, "branch-target-enforcement",
                               LangOpts.BranchTargetEnforcement);
 
     getModule().addModuleFlag(llvm::Module::Error, "sign-return-address",
@@ -722,9 +737,11 @@ void CodeGenModule::Release() {
     getModule().addModuleFlag(llvm::Module::Error, "sign-return-address-all",
                               LangOpts.isSignReturnAddressScopeAll());
 
-    getModule().addModuleFlag(llvm::Module::Error,
-                              "sign-return-address-with-bkey",
-                              !LangOpts.isSignReturnAddressWithAKey());
+    if (Arch != llvm::Triple::thumb && Arch != llvm::Triple::thumbeb) {
+      getModule().addModuleFlag(llvm::Module::Error,
+                                "sign-return-address-with-bkey",
+                                !LangOpts.isSignReturnAddressWithAKey());
+    }
   }
 
   if (!CodeGenOpts.MemoryProfileOutput.empty()) {