[CHERI-RISC-V] Add a temporary -mxcheri-v9-semantics clang flag

This will be removed once we default to the new semantics. This also
adds a mxcheri-v8-compat flag to opt-in to the old semantics (although
that is currently the default).

clang/include/clang/Driver/Options.td

@@ -3397,6 +3397,14 @@ def mno_xcheri_rvc : Flag<["-"], "mno-xcheri-rvc">, Alias<mxcheri_norvc>;
 def mno_xcheri_norvc : Flag<["-"], "mno-xcheri-norvc">, Group<m_riscv_Features_Group>,
   HelpText<"Enable using compressed CHERI instructions">;
 def mxcheri_rvc : Flag<["-"], "mxcheri-rvc">, Alias<mno_xcheri_norvc>;
+// Temporary flags to enable/disable CHERI ISAv8 compatibility.
+// Flag name is a bit odd but this is required by handleTargetFeaturesGroup().
+def mxcheri_v9_semantics : Flag<["-"], "mxcheri-v9-semantics">, Group<m_riscv_Features_Group>,
+  HelpText<"Generate code that is no longer compatible with CHERI ISAv8">;
+def mno_xcheri_v9_semantics : Flag<["-"], "mno-xcheri-v9-semantics">, Group<m_riscv_Features_Group>,
+  HelpText<"Generate code that is compatible with CHERI ISAv8">;
+// Add an alias with a more sensible name for when the default is flipped.
+def mxcheri_v8_compat : Flag<["-"], "mxcheri-v8-compat">, Alias<mno_xcheri_v9_semantics>;
 
 def munaligned_access : Flag<["-"], "munaligned-access">, Group<m_arm_Features_Group>,
   HelpText<"Allow memory accesses to be unaligned (AArch32/AArch64 only)">;

clang/test/Driver/riscv-default-features.c

@@ -13,6 +13,9 @@
 // RUN: %clang --target=riscv32-unknown-elf -march=rv32ixcheri -S -mno-xcheri-norvc -emit-llvm %s -o - | FileCheck %s -check-prefix=RV32-XCHERI-EXPLICIT-RVC
 // RUN: %clang --target=riscv64-unknown-elf -march=rv64ixcheri -S -mno-xcheri-norvc -emit-llvm %s -o - | FileCheck %s -check-prefix=RV64-XCHERI-EXPLICIT-RVC
 
+// RUN: %clang --target=riscv64-unknown-elf -march=rv64ixcheri -mxcheri-v9-semantics -S -emit-llvm %s -o - | FileCheck %s -check-prefix=RV64-XCHERI-V9
+// RUN: %clang --target=riscv64-unknown-elf -march=rv64ixcheri -mxcheri-v8-compat -S -emit-llvm %s -o - | FileCheck %s -check-prefix=RV64-XCHERI-V8
+
 // RV32: "target-features"="+a,+c,+m,+relax,-save-restore"
 // RV64: "target-features"="+64bit,+a,+c,+m,+relax,-save-restore"
 
@@ -26,6 +29,9 @@
 // RV32-XCHERI-EXPLICIT-RVC: "target-features"="+relax,+xcheri,-save-restore,-xcheri-norvc"
 // RV64-XCHERI-EXPLICIT-RVC: "target-features"="+64bit,+relax,+xcheri,-save-restore,-xcheri-norvc"
 
+// RV64-XCHERI-V8: "target-features"="+64bit,+relax,+xcheri,-save-restore,-xcheri-v9-semantics"
+// RV64-XCHERI-V9: "target-features"="+64bit,+relax,+xcheri,+xcheri-v9-semantics,-save-restore"
+
 // Dummy function
 int foo(){
   return  3;