[RISCV] Make it explicit that attributes use the MCSubtargetInfo from TargetMachine. NFC

[craig.topper]

Summary

The MCSTI variable is initialized to TM.getMCSubtargetInfo(), but is
re-assigned in every call to runOnMachineFunction. mitAttributes is
called before any call to runOnMachineFunction, but it's not
immediately obvious.

This patch removes the MCSTI variable, and instead queries
TM.getMCSubtargetInfo() at the time emitAttributes is called.

[craig.topper]

Remove the MCSTI variable which becomes dead after this.

[reames]

LGTM w/comment addressed.

llvm/lib/Target/RISCV/RISCVAsmPrinter.cpp

220–223

I think this needs a comment. The fact that we specifically *don't* want some arbitrary functions copy of the sub-target info is surprising, and worth a word or two here.

[craig.topper]

Add comment

[jrtc27]

It's called from emitStartOfAsmFile, so yes, it's going to be called before any of the functions are emitted and this really should be NFC. https://reviews.llvm.org/D73339 is responsible for (what is now) MCSTI and was done for RVC[1] but we don't need that any more.

[1] And that fix was more of a hack, because we do need the module-level features in order to emit the right attributes

[jrtc27]

Worth nothing that EmitHwasanMemaccessSymbols is surely doing bogus things with STI...

llvm/lib/Target/RISCV/RISCVAsmPrinter.cpp

 
 #define DEBUG_TYPE "asm-printer"
 
 STATISTIC(RISCVNumInstrsCompressed,
           "Number of RISC-V Compressed instructions emitted");
 
 namespace {
 class RISCVAsmPrinter : public AsmPrinter {
-  const MCSubtargetInfo *MCSTI;
   const RISCVSubtarget *STI;
 
 public:
   explicit RISCVAsmPrinter(TargetMachine &TM,
                            std::unique_ptr<MCStreamer> Streamer)
-      : AsmPrinter(TM, std::move(Streamer)), MCSTI(TM.getMCSubtargetInfo()) {}
+      : AsmPrinter(TM, std::move(Streamer)) {}
 
   StringRef getPassName() const override { return "RISCV Assembly Printer"; }
 
   bool runOnMachineFunction(MachineFunction &MF) override;
 
   void emitInstruction(const MachineInstr *MI) override;
 
   bool PrintAsmOperand(const MachineInstr *MI, unsigned OpNo,
if (!ExtraCode) {
     OS << "0(" << RISCVInstPrinter::getRegisterName(MO.getReg()) << ")";
     return false;
   }
 
   return AsmPrinter::PrintAsmMemoryOperand(MI, OpNo, ExtraCode, OS);
 }
 
 bool RISCVAsmPrinter::runOnMachineFunction(MachineFunction &MF) {
-  // Set the current MCSubtargetInfo to a copy which has the correct
-  // feature bits for the current MachineFunction
-  MCSubtargetInfo &NewSTI =
-    OutStreamer->getContext().getSubtargetCopy(*TM.getMCSubtargetInfo());
-  NewSTI.setFeatureBits(MF.getSubtarget().getFeatureBits());
-  MCSTI = &NewSTI;
   STI = &MF.getSubtarget<RISCVSubtarget>();
 
   SetupMachineFunction(MF);
   emitFunctionBody();
   return false;
 }
 
 void RISCVAsmPrinter::emitStartOfAsmFile(Module &M) {
void RISCVAsmPrinter::emitEndOfAsmFile(Module &M) {
   if (TM.getTargetTriple().isOSBinFormatELF())
     RTS.finishAttributeSection();
   EmitHwasanMemaccessSymbols(M);
 }
 
 void RISCVAsmPrinter::emitAttributes() {
   RISCVTargetStreamer &RTS =
       static_cast<RISCVTargetStreamer &>(*OutStreamer->getTargetStreamer());
-  RTS.emitTargetAttributes(*MCSTI);
+  // Use MCSubtargetInfo from TargetMachine. Individual functions may have
+  // attributes that differ from other functions in the module and we have no
+  // way to know which function is correct.
+  RTS.emitTargetAttributes(*TM.getMCSubtargetInfo());
 }
 
 void RISCVAsmPrinter::emitFunctionEntryLabel() {
   const auto *RMFI = MF->getInfo<RISCVMachineFunctionInfo>();
   if (RMFI->isVectorCall()) {
     auto &RTS =
         static_cast<RISCVTargetStreamer &>(*OutStreamer->getTargetStreamer());
     RTS.emitDirectiveVariantCC(*CurrentFnSym);