[AArch64][SME] Disable NEON in streaming mode

In streaming mode most of the NEON instruction set is illegal, disable
NEON when compiling with `+streaming-sve`, unless NEON is explictly
requested.

Subsequent patches will add support for the small subset of NEON
instructions that are legal in streaming mode.

Reviewed By: paulwalker-arm, david-arm

Differential Revision: https://reviews.llvm.org/D107902

llvm/lib/Target/AArch64/MCTargetDesc/AArch64MCTargetDesc.cpp

@@ -57,7 +57,16 @@ createAArch64MCSubtargetInfo(const Triple &TT, StringRef CPU, StringRef FS) {
       CPU = "apple-a12";
   }
 
-  return createAArch64MCSubtargetInfoImpl(TT, CPU, /*TuneCPU*/ CPU, FS);
+  // Most of the NEON instruction set isn't supported in streaming mode on SME
+  // targets, disable NEON unless explicitly requested.
+  bool RequestedNEON = FS.contains("neon");
+  bool RequestedStreamingSVE = FS.contains("streaming-sve");
+  MCSubtargetInfo *STI =
+      createAArch64MCSubtargetInfoImpl(TT, CPU, /*TuneCPU*/ CPU, FS);
+  if (RequestedStreamingSVE && !RequestedNEON &&
+      STI->hasFeature(AArch64::FeatureNEON))
+    STI->ToggleFeature(AArch64::FeatureNEON);
+  return STI;
 }
 
 void AArch64_MC::initLLVMToCVRegMapping(MCRegisterInfo *MRI) {

llvm/test/MC/AArch64/SME/streaming-sve-feature.s

@@ -0,0 +1,8 @@
+// RUN: llvm-mc -triple=aarch64 -mattr=+streaming-sve,+neon < %s 2>&1 | FileCheck %s
+// RUN: not llvm-mc -triple=aarch64 -mattr=+streaming-sve < %s 2>&1 | FileCheck %s --check-prefix=CHECK-ERROR
+
+// Verify NEON is disabled when targeting streaming mode, if it's not
+// explicitly requested.
+add v0.8b, v1.8b, v2.8b
+// CHECK: add v0.8b, v1.8b, v2.8b
+// CHECK-ERROR: error: instruction requires: neon