[clang] [RISCV][clang] Use fcf-protection flag in Multilib Selection (PR #205202)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang

Author: Sam Elliott (lenary)

<details>
<summary>Changes</summary>

This ensures that we can separate out multilibs that are or are not built with control flow protection enabled.

The initial version of the patch claims all values of these flags are incompatible. It might be the case that we could make this logic more complex if some versions do become compatible.

---
Full diff: https://github.com/llvm/llvm-project/pull/205202.diff


2 Files Affected:

- (modified) clang/lib/Driver/ToolChain.cpp (+12) 
- (modified) clang/test/Driver/print-multi-selection-flags.c (+13) 


``````````diff
diff --git a/clang/lib/Driver/ToolChain.cpp b/clang/lib/Driver/ToolChain.cpp
index 7d93e7f65daf5..328f4f8c8f420 100644
--- a/clang/lib/Driver/ToolChain.cpp
+++ b/clang/lib/Driver/ToolChain.cpp
@@ -445,6 +445,18 @@ static void getRISCVMultilibFlags(const Driver &D, const llvm::Triple &Triple,
     Result.push_back("-fsanitize=shadow-call-stack");
   else
     Result.push_back("-fno-sanitize=shadow-call-stack");
+
+  const Arg *CFProtectionArg =
+      Args.getLastArgNoClaim(options::OPT_fcf_protection_EQ);
+  StringRef CFProtectionVal =
+      CFProtectionArg ? CFProtectionArg->getValue() : "none";
+  Result.push_back(("-fcf-protection=" + CFProtectionVal).str());
+
+  if (CFProtectionVal == "branch" || CFProtectionVal == "full") {
+    if (const Arg *SchemeArg =
+            Args.getLastArgNoClaim(options::OPT_mcf_branch_label_scheme_EQ))
+      Result.push_back(SchemeArg->getAsString(Args));
+  }
 }
 
 Multilib::flags_list
diff --git a/clang/test/Driver/print-multi-selection-flags.c b/clang/test/Driver/print-multi-selection-flags.c
index ba7b325892f9c..e5a116234c321 100644
--- a/clang/test/Driver/print-multi-selection-flags.c
+++ b/clang/test/Driver/print-multi-selection-flags.c
@@ -154,3 +154,16 @@
 // CHECK-OPT-OS: -Os
 // CHECK-OPT-NOT: -Oz
 // CHECK-OPT-NOT: -Os
+
+// RUN: %clang -multi-lib-config=%S/Inputs/multilib/empty.yaml -print-multi-flags-experimental --target=riscv64-none-elf                                                         | FileCheck --check-prefix=CHECK-CF-PROTECTION-NONE --implicit-check-not="mcf-branch-label-scheme" %s
+// RUN: %clang -multi-lib-config=%S/Inputs/multilib/empty.yaml -print-multi-flags-experimental --target=riscv64-none-elf -fcf-protection=none                                   | FileCheck --check-prefix=CHECK-CF-PROTECTION-NONE --implicit-check-not="mcf-branch-label-scheme" %s
+// RUN: %clang -multi-lib-config=%S/Inputs/multilib/empty.yaml -print-multi-flags-experimental --target=riscv64-none-elf -fcf-protection=return                                 | FileCheck --check-prefix=CHECK-CF-PROTECTION-RETURN --implicit-check-not="mcf-branch-label-scheme" %s
+// RUN: %clang -multi-lib-config=%S/Inputs/multilib/empty.yaml -print-multi-flags-experimental --target=riscv64-none-elf -fcf-protection=branch -mcf-branch-label-scheme=unlabeled | FileCheck --check-prefix=CHECK-CF-PROTECTION-BRANCH %s
+// RUN: %clang -multi-lib-config=%S/Inputs/multilib/empty.yaml -print-multi-flags-experimental --target=riscv64-none-elf -fcf-protection=full   -mcf-branch-label-scheme=unlabeled | FileCheck --check-prefix=CHECK-CF-PROTECTION-FULL   %s
+// RUN: %clang -multi-lib-config=%S/Inputs/multilib/empty.yaml -print-multi-flags-experimental --target=riscv64-none-elf -fcf-protection        -mcf-branch-label-scheme=unlabeled | FileCheck --check-prefix=CHECK-CF-PROTECTION-FULL   %s
+// CHECK-CF-PROTECTION-NONE: -fcf-protection=none
+// CHECK-CF-PROTECTION-RETURN: -fcf-protection=return
+// CHECK-CF-PROTECTION-BRANCH: -fcf-protection=branch
+// CHECK-CF-PROTECTION-BRANCH: -mcf-branch-label-scheme=unlabeled
+// CHECK-CF-PROTECTION-FULL: -fcf-protection=full
+// CHECK-CF-PROTECTION-FULL: -mcf-branch-label-scheme=unlabeled

``````````

</details>


https://github.com/llvm/llvm-project/pull/205202