Update FEAT_PAuth_LR behaviour for AArch64 (#90614)

Currently, LLVM enables `-mbranch-protection=standard` as `bti+pac-ret`.
To align LLVM with the behaviour in GNU, this has been updated to
`bti+pac-ret+pc` when FEAT_PAuth_LR is enabled as an optional feature
via the `-mcpu=` options. If this is not enabled, then this will revert
to the existing behaviour.

Author: Stylie777

clang/lib/Basic/Targets/AArch64.cpp

@@ -225,7 +225,7 @@ bool AArch64TargetInfo::validateBranchProtection(StringRef Spec, StringRef,
                                                  BranchProtectionInfo &BPI,
                                                  StringRef &Err) const {
   llvm::ARM::ParsedBranchProtection PBP;
-  if (!llvm::ARM::parseBranchProtection(Spec, PBP, Err))
+  if (!llvm::ARM::parseBranchProtection(Spec, PBP, Err, HasPAuthLR))
     return false;
 
   BPI.SignReturnAddr =

clang/lib/Driver/ToolChains/Clang.cpp

@@ -55,6 +55,7 @@
 #include "llvm/Support/Path.h"
 #include "llvm/Support/Process.h"
 #include "llvm/Support/YAMLParser.h"
+#include "llvm/TargetParser/AArch64TargetParser.h"
 #include "llvm/TargetParser/ARMTargetParserCommon.h"
 #include "llvm/TargetParser/Host.h"
 #include "llvm/TargetParser/LoongArchTargetParser.h"
@@ -1511,7 +1512,24 @@ static void CollectARMPACBTIOptions(const ToolChain &TC, const ArgList &Args,
   } else {
     StringRef DiagMsg;
     llvm::ARM::ParsedBranchProtection PBP;
-    if (!llvm::ARM::parseBranchProtection(A->getValue(), PBP, DiagMsg))
+    bool EnablePAuthLR = false;
+
+    // To know if we need to enable PAuth-LR As part of the standard branch
+    // protection option, it needs to be determined if the feature has been
+    // activated in the `march` argument. This information is stored within the
+    // CmdArgs variable and can be found using a search.
+    if (isAArch64) {
+      auto isPAuthLR = [](const char *member) {
+        llvm::AArch64::ExtensionInfo pauthlr_extension =
+            llvm::AArch64::getExtensionByID(llvm::AArch64::AEK_PAUTHLR);
+        return (pauthlr_extension.Feature.compare(member) == 0);
+      };
+
+      if (std::any_of(CmdArgs.begin(), CmdArgs.end(), isPAuthLR))
+        EnablePAuthLR = true;
+    }
+    if (!llvm::ARM::parseBranchProtection(A->getValue(), PBP, DiagMsg,
+                                          EnablePAuthLR))
       D.Diag(diag::err_drv_unsupported_option_argument)
           << A->getSpelling() << DiagMsg;
     if (!isAArch64 && PBP.Key == "b_key")

clang/test/Preprocessor/aarch64-target-features.c

@@ -616,6 +616,9 @@
 // ================== Check Armv9.5-A Pointer Authentication Enhancements(PAuth_LR).
 // RUN: %clang -target arm64-none-linux-gnu -march=armv8-a -x c -E -dM %s -o - | FileCheck -check-prefix=CHECK-PAUTH-LR-OFF %s
 // RUN: %clang -target arm64-none-linux-gnu -march=armv9.5-a -x c -E -dM %s -o - | FileCheck -check-prefix=CHECK-PAUTH-LR-OFF %s
+// RUN: %clang -target arm64-none-linux-gnu -march=armv9.5-a -mbranch-protection=standard -x c -E -dM %s -o - | FileCheck -check-prefixes=CHECK-PAUTH-LR-OFF,CHECK-BRANCH-PROTECTION-NO-PC %s
+// RUN: %clang -target arm64-none-linux-gnu -march=armv9.5-a+pauth-lr -mbranch-protection=standard -x c -E -dM %s -o - | FileCheck -check-prefixes=CHECK-PAUTH-LR,CHECK-BRANCH-PROTECTION-PC %s
+// RUN: %clang -target arm64-none-linux-gnu -march=armv9.5-a+nopauth-lr -mbranch-protection=standard -x c -E -dM %s -o - | FileCheck -check-prefixes=CHECK-PAUTH-LR-OFF,CHECK-BRANCH-PROTECTION-NO-PC %s
 // RUN: %clang -target arm64-none-linux-gnu -march=armv8-a+pauth -mbranch-protection=none -x c -E -dM %s -o - | FileCheck -check-prefix=CHECK-PAUTH-LR-OFF %s
 // RUN: %clang -target arm64-none-linux-gnu -march=armv8-a+pauth-lr -mbranch-protection=none -x c -E -dM %s -o - | FileCheck -check-prefix=CHECK-PAUTH-LR %s
 // RUN: %clang -target arm64-none-linux-gnu -march=armv8-a+pauth-lr -mbranch-protection=bti -x c -E -dM %s -o - | FileCheck -check-prefix=CHECK-PAUTH-LR %s
@@ -636,6 +639,7 @@
 // RUN: %clang -target arm64-none-linux-gnu -march=armv8-a+pauth-lr -mbranch-protection=pac-ret+pc+b-key -x c -E -dM %s -o - | FileCheck -check-prefixes=CHECK-PAUTH-LR,CHECK-BRANCH-PROTECTION-PC-BKEY %s
 // RUN: %clang -target arm64-none-linux-gnu -march=armv8-a+pauth-lr -mbranch-protection=pac-ret+pc+leaf -x c -E -dM %s -o - | FileCheck -check-prefixes=CHECK-PAUTH-LR,CHECK-BRANCH-PROTECTION-PC-LEAF %s
 // RUN: %clang -target arm64-none-linux-gnu -march=armv8-a+pauth-lr -mbranch-protection=pac-ret+pc+leaf+b-key -x c -E -dM %s -o - | FileCheck -check-prefixes=CHECK-PAUTH-LR,CHECK-BRANCH-PROTECTION-PC-LEAF-BKEY %s
+// CHECK-BRANCH-PROTECTION-NO-PC:        #define __ARM_FEATURE_PAC_DEFAULT 1
 // CHECK-BRANCH-PROTECTION-PC:           #define __ARM_FEATURE_PAC_DEFAULT 9
 // CHECK-BRANCH-PROTECTION-PC-BKEY:      #define __ARM_FEATURE_PAC_DEFAULT 10
 // CHECK-BRANCH-PROTECTION-PC-LEAF:      #define __ARM_FEATURE_PAC_DEFAULT 13

llvm/docs/ReleaseNotes.rst

@@ -76,6 +76,11 @@ Changes to the AArch64 Backend
 * Added support for Cortex-A78AE, Cortex-A520AE, Cortex-A720AE,
   Cortex-R82AE, Neoverse-N3, Neoverse-V3 and Neoverse-V3AE CPUs.
 
+* ``-mbranch-protection=standard`` now enables FEAT_PAuth_LR by
+  default when the feature is enabled. The new behaviour results 
+  in ``standard`` being equal to ``bti+pac-ret+pc`` when ``+pauth-lr``
+  is passed as part of ``-mcpu=`` options.
+
 Changes to the AMDGPU Backend
 -----------------------------
 

llvm/include/llvm/TargetParser/AArch64TargetParser.h

@@ -676,6 +676,8 @@ inline constexpr Alias CpuAliases[] = {{"cobalt-100", "neoverse-n2"},
 
 inline constexpr Alias ExtAliases[] = {{"rdma", "rdm"}};
 
+const ExtensionInfo &getExtensionByID(ArchExtKind(ExtID));
+
 bool getExtensionFeatures(
     const AArch64::ExtensionBitset &Extensions,
     std::vector<StringRef> &Features);

llvm/include/llvm/TargetParser/ARMTargetParserCommon.h

@@ -46,7 +46,7 @@ struct ParsedBranchProtection {
 };
 
 bool parseBranchProtection(StringRef Spec, ParsedBranchProtection &PBP,
-                           StringRef &Err);
+                           StringRef &Err, bool EnablePAuthLR = false);
 
 } // namespace ARM
 } // namespace llvm

llvm/lib/TargetParser/AArch64TargetParser.cpp

@@ -280,3 +280,8 @@ bool AArch64::ExtensionSet::parseModifier(StringRef Modifier) {
   }
   return false;
 }
+
+const AArch64::ExtensionInfo &
+AArch64::getExtensionByID(AArch64::ArchExtKind ExtID) {
+  return lookupExtensionByID(ExtID);
+}

llvm/lib/TargetParser/ARMTargetParserCommon.cpp

@@ -139,7 +139,7 @@ ARM::EndianKind ARM::parseArchEndian(StringRef Arch) {
 // returned in `PBP`. Returns false in error, with `Err` containing
 // an erroneous part of the spec.
 bool ARM::parseBranchProtection(StringRef Spec, ParsedBranchProtection &PBP,
-                                StringRef &Err) {
+                                StringRef &Err, bool EnablePAuthLR) {
   PBP = {"none", "a_key", false, false, false};
   if (Spec == "none")
     return true; // defaults are ok
@@ -148,6 +148,7 @@ bool ARM::parseBranchProtection(StringRef Spec, ParsedBranchProtection &PBP,
     PBP.Scope = "non-leaf";
     PBP.BranchTargetEnforcement = true;
     PBP.GuardedControlStack = true;
+    PBP.BranchProtectionPAuthLR = EnablePAuthLR;
     return true;
   }