Reduce the number of memory allocations when displaying a warning about clobbering reserved registers (NFC).

[chill]

Summary

Also address some minor inefficiencies and style issues.

[chill]

Ping?

[stuij]

Looks harmless :o) Thanks, LGTM.

llvm/lib/CodeGen/AsmPrinter/AsmPrinterInlineAsm.cpp

 //===-- AsmPrinterInlineAsm.cpp - AsmPrinter Inline Asm Handling ----------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements the inline assembler pieces of the AsmPrinter class.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/SmallString.h"
+#include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/CodeGen/AsmPrinter.h"
 #include "llvm/CodeGen/MachineBasicBlock.h"
 #include "llvm/CodeGen/MachineFunction.h"
 #include "llvm/CodeGen/MachineModuleInfo.h"
 #include "llvm/CodeGen/TargetInstrInfo.h"
 #include "llvm/CodeGen/TargetRegisterInfo.h"
 #include "llvm/IR/Constants.h"
void AsmPrinter::emitInlineAsm(const MachineInstr *MI) const {
   int AsmPrinterVariant = MAI->getAssemblerDialect();
   AsmPrinter *AP = const_cast<AsmPrinter*>(this);
   if (MI->getInlineAsmDialect() == InlineAsm::AD_ATT)
     EmitGCCInlineAsmStr(AsmStr, MI, MMI, AsmPrinterVariant, AP, LocCookie, OS);
   else
     EmitMSInlineAsmStr(AsmStr, MI, MMI, AP, LocCookie, OS);
 
   // Emit warnings if we use reserved registers on the clobber list, as
-  // that might give surprising results.
-  std::vector<std::string> RestrRegs;
+  // that might lead to undefined behaviour.
+  SmallVector<Register, 8> RestrRegs;
+  const TargetRegisterInfo *TRI = MF->getSubtarget().getRegisterInfo();
   // Start with the first operand descriptor, and iterate over them.
   for (unsigned I = InlineAsm::MIOp_FirstOperand, NumOps = MI->getNumOperands();
        I < NumOps; ++I) {
     const MachineOperand &MO = MI->getOperand(I);
-    if (MO.isImm()) {
-      unsigned Flags = MO.getImm();
-      const TargetRegisterInfo *TRI = MF->getSubtarget().getRegisterInfo();
-      if (InlineAsm::getKind(Flags) == InlineAsm::Kind_Clobber &&
-          !TRI->isAsmClobberable(*MF, MI->getOperand(I + 1).getReg())) {
-        RestrRegs.push_back(TRI->getName(MI->getOperand(I + 1).getReg()));
-      }
-      // Skip to one before the next operand descriptor, if it exists.
-      I += InlineAsm::getNumOperandRegisters(Flags);
-    }
-  }
+    if (!MO.isImm())
+      continue;
+    if (InlineAsm::getKind(Flags) == InlineAsm::Kind_Clobber) {
+      Register Reg = MI->getOperand(I + 1).getReg();
+      if (!TRI->isAsmClobberable(*MF, Reg))
+        RestrRegs.push_back(Reg);
 
   if (!RestrRegs.empty()) {
     unsigned BufNum = addInlineAsmDiagBuffer(OS.str(), LocMD);
     auto &SrcMgr = DiagInfo->SrcMgr;
     SMLoc Loc = SMLoc::getFromPointer(
         SrcMgr.getMemoryBuffer(BufNum)->getBuffer().begin());
 
     std::string Msg = "inline asm clobber list contains reserved registers: ";
-    for (auto I = RestrRegs.begin(), E = RestrRegs.end(); I != E; I++) {
+    for (auto I = RestrRegs.begin(), E = RestrRegs.end(); I != E; ++I) {
       if(I != RestrRegs.begin())
         Msg += ", ";
-      Msg += *I;
+      Msg += TRI->getName(*I);
     }
-    std::string Note = "Reserved registers on the clobber list may not be "
-                "preserved across the asm statement, and clobbering them may "
-                "lead to undefined behaviour.";
+    const char *Note =
+        "Reserved registers on the clobber list may not be "
     SrcMgr.PrintMessage(Loc, SourceMgr::DK_Warning, Msg);
     SrcMgr.PrintMessage(Loc, SourceMgr::DK_Note, Note);
   }
 
   emitInlineAsm(OS.str(), getSubtargetInfo(), TM.Options.MCOptions, LocMD,
                 MI->getInlineAsmDialect());
 
   // Emit the #NOAPP end marker.  This has to happen even if verbose-asm isn't