[llvm] Use StringRef::starts_with (NFC)

Author: kazutakahirata

llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp

@@ -5547,7 +5547,7 @@ std::pair<unsigned, const TargetRegisterClass *>
 TargetLowering::getRegForInlineAsmConstraint(const TargetRegisterInfo *RI,
                                              StringRef Constraint,
                                              MVT VT) const {
-  if (Constraint.empty() || Constraint[0] != '{')
+  if (!Constraint.starts_with("{"))
     return std::make_pair(0u, static_cast<TargetRegisterClass *>(nullptr));
   assert(*(Constraint.end() - 1) == '}' && "Not a brace enclosed constraint?");
 

llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp

@@ -479,7 +479,7 @@ static SectionKind getELFKindForNamedSection(StringRef Name, SectionKind K) {
       Name == ".llvmbc" || Name == ".llvmcmd")
     return SectionKind::getMetadata();
 
-  if (Name.empty() || Name[0] != '.') return K;
+  if (!Name.starts_with(".")) return K;
 
   // Default implementation based on some magic section names.
   if (Name == ".bss" || Name.starts_with(".bss.") ||

llvm/lib/MC/MCContext.cpp

@@ -397,7 +397,7 @@ MCContext::createXCOFFSymbolImpl(const StringMapEntry<bool> *Name,
   // If it's an entry point symbol, we will keep the '.'
   // in front for the convention purpose. Otherwise, add "_Renamed.."
   // as prefix to signal this is an renamed symbol.
-  const bool IsEntryPoint = !InvalidName.empty() && InvalidName[0] == '.';
+  const bool IsEntryPoint = InvalidName.starts_with(".");
   SmallString<128> ValidName =
       StringRef(IsEntryPoint ? "._Renamed.." : "_Renamed..");
 

llvm/lib/Support/CommandLine.cpp

@@ -422,7 +422,7 @@ void Option::removeArgument() { GlobalParser->removeOption(this); }
 void Option::setArgStr(StringRef S) {
   if (FullyInitialized)
     GlobalParser->updateArgStr(this, S);
-  assert((S.empty() || S[0] != '-') && "Option can't start with '-");
+  assert(!S.starts_with("-") && "Option can't start with '-");
   ArgStr = S;
   if (ArgStr.size() == 1)
     setMiscFlag(Grouping);

llvm/lib/Support/StringRef.cpp

@@ -458,7 +458,7 @@ bool llvm::consumeSignedInteger(StringRef &Str, unsigned Radix,
   unsigned long long ULLVal;
 
   // Handle positive strings first.
-  if (Str.empty() || Str.front() != '-') {
+  if (!Str.starts_with("-")) {
     if (consumeUnsignedInteger(Str, Radix, ULLVal) ||
         // Check for value so large it overflows a signed value.
         (long long)ULLVal < 0)

llvm/lib/Target/SystemZ/SystemZISelLowering.cpp

@@ -1302,7 +1302,7 @@ SystemZTargetLowering::getRegForInlineAsmConstraint(
       break;
     }
   }
-  if (Constraint.size() > 0 && Constraint[0] == '{') {
+  if (Constraint.starts_with("{")) {
 
     // A clobber constraint (e.g. ~{f0}) will have MVT::Other which is illegal
     // to check the size on.

llvm/utils/TableGen/CodeGenInstruction.cpp

@@ -232,7 +232,7 @@ bool CGIOperandList::hasSubOperandAlias(
 
 std::pair<unsigned,unsigned>
 CGIOperandList::ParseOperandName(StringRef Op, bool AllowWholeOp) {
-  if (Op.empty() || Op[0] != '$')
+  if (!Op.starts_with("$"))
     PrintFatalError(TheDef->getLoc(),
                     TheDef->getName() + ": Illegal operand name: '" + Op + "'");