[Clang][AArch64] Emit 'unimplemented' diagnostic for SME.

When a function F has ZA and ZT0 state, calls another function G that only
shares ZT0 state with its caller, F will have to save ZA before the call to G,
and restore it afterwards (rather than setting up a lazy-sve).

This is not yet implemented in LLVM and does not result in a compile-time
error either. So instead of silently generating incorrect code, it's better to
emit an error saying this is not yet implemented.

Author: sdesmalen-arm

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -3711,6 +3711,12 @@ def err_sme_za_call_no_za_state : Error<
   "call to a shared ZA function requires the caller to have ZA state">;
 def err_sme_zt0_call_no_zt0_state : Error<
   "call to a shared ZT0 function requires the caller to have ZT0 state">;
+def err_sme_unimplemented_za_save_restore : Error<
+  "call to a function that shares state other than 'za' from a "
+  "function that has live 'za' state requires a spill/fill of ZA, which is not yet "
+  "implemented">;
+def note_sme_use_preserves_za : Note<
+  "add '__arm_preserves(\"za\")' to the callee if it preserves ZA">;
 def err_sme_definition_using_sm_in_non_sme_target : Error<
   "function executed in streaming-SVE mode requires 'sme'">;
 def err_sme_definition_using_za_in_non_sme_target : Error<

clang/lib/Sema/SemaChecking.cpp

@@ -7544,6 +7544,12 @@ void Sema::checkCall(NamedDecl *FDecl, const FunctionProtoType *Proto,
 
       if (CalleeArmZT0State != FunctionType::ARM_None && !CallerHasZT0State)
         Diag(Loc, diag::err_sme_zt0_call_no_zt0_state);
+
+      if (CallerHasZAState && CalleeArmZAState == FunctionType::ARM_None &&
+          CalleeArmZT0State != FunctionType::ARM_None) {
+        Diag(Loc, diag::err_sme_unimplemented_za_save_restore);
+        Diag(Loc, diag::note_sme_use_preserves_za);
+      }
     }
   }
 

clang/test/Sema/aarch64-sme-func-attrs.c

@@ -1,5 +1,5 @@
-// RUN: %clang_cc1 -triple aarch64-none-linux-gnu -target-feature +sme -fsyntax-only -verify %s
-// RUN: %clang_cc1 -triple aarch64-none-linux-gnu -target-feature +sme -fsyntax-only -verify=expected-cpp -x c++ %s
+// RUN: %clang_cc1 -triple aarch64-none-linux-gnu -target-feature +sme2 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -triple aarch64-none-linux-gnu -target-feature +sme2 -fsyntax-only -verify=expected-cpp -x c++ %s
 
 // Valid attributes
 
@@ -445,3 +445,12 @@ void conflicting_state_attrs_preserves_out_zt0(void) __arm_preserves("zt0") __ar
 // expected-cpp-error@+2 {{conflicting attributes for state 'zt0'}}
 // expected-error@+1 {{conflicting attributes for state 'zt0'}}
 void conflicting_state_attrs_preserves_inout_zt0(void) __arm_preserves("zt0") __arm_inout("zt0");
+
+// Test that we get a diagnostic for unimplemented case.
+void unimplemented_spill_fill_za(void (*share_zt0_only)(void) __arm_inout("zt0")) __arm_inout("za", "zt0") {
+  // expected-cpp-error@+4 {{call to a function that shares state other than 'za' from a function that has live 'za' state requires a spill/fill of ZA, which is not yet implemented}}
+  // expected-cpp-note@+3 {{add '__arm_preserves("za")' to the callee if it preserves ZA}}
+  // expected-error@+2 {{call to a function that shares state other than 'za' from a function that has live 'za' state requires a spill/fill of ZA, which is not yet implemented}}
+  // expected-note@+1 {{add '__arm_preserves("za")' to the callee if it preserves ZA}}
+  share_zt0_only();
+}