[Clang][RISCV] Support -fcf-protection=return for RISC-V (#112477)

Enables the support of `-fcf-protection=return` on RISC-V, which
requires Zicfiss. It also adds a string attribute "hw-shadow-stack"
to every function if the option is set on RISC-V

Author: jaidTw

clang/lib/Basic/Targets/RISCV.h

@@ -141,6 +141,13 @@ class RISCVTargetInfo : public TargetInfo {
     return true;
   }
 
+  bool
+  checkCFProtectionReturnSupported(DiagnosticsEngine &Diags) const override {
+    if (ISAInfo->hasExtension("zicfiss"))
+      return true;
+    return TargetInfo::checkCFProtectionReturnSupported(Diags);
+  }
+
   CFBranchLabelSchemeKind getDefaultCFBranchLabelScheme() const override {
     return CFBranchLabelSchemeKind::FuncSig;
   }

clang/lib/CodeGen/Targets/RISCV.cpp

@@ -594,6 +594,11 @@ class RISCVTargetCodeGenInfo : public TargetCodeGenInfo {
     const auto *FD = dyn_cast_or_null<FunctionDecl>(D);
     if (!FD) return;
 
+    auto *Fn = cast<llvm::Function>(GV);
+
+    if (CGM.getCodeGenOpts().CFProtectionReturn)
+      Fn->addFnAttr("hw-shadow-stack");
+
     const auto *Attr = FD->getAttr<RISCVInterruptAttr>();
     if (!Attr)
       return;
@@ -604,8 +609,6 @@ class RISCVTargetCodeGenInfo : public TargetCodeGenInfo {
     case RISCVInterruptAttr::machine: Kind = "machine"; break;
     }
 
-    auto *Fn = cast<llvm::Function>(GV);
-
     Fn->addFnAttr("interrupt", Kind);
   }
 };

clang/test/CodeGen/RISCV/attr-hw-shadow-stack.c

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -triple riscv64 -target-feature +experimental-zicfiss -emit-llvm -o - %s -fcf-protection=return | FileCheck %s
+// RUN: %clang_cc1 -triple riscv64 -target-feature +experimental-zicfiss -emit-llvm -o - %s | FileCheck -check-prefix=NOSHADOWSTACK %s
+// RUN: %clang_cc1 -triple riscv32 -target-feature +experimental-zicfiss -emit-llvm -o - %s -fcf-protection=return | FileCheck %s
+// RUN: %clang_cc1 -triple riscv32 -target-feature +experimental-zicfiss -emit-llvm -o - %s | FileCheck -check-prefix=NOSHADOWSTACK %s
+
+int foo(int *a) { return *a; }
+
+// CHECK: attributes {{.*}}"hw-shadow-stack"{{.*}}
+// NOSHADOWSTACK-NOT: attributes {{.*}}"hw-shadow-stack"{{.*}}