[IR] Teach getConstraintString to return StringRef (NFC) (#139401)

With this change, some callers get to use StringRef::starts_with.

I'm planning to teach getAsmString to return StringRef also, but
I'ld like to keep that separate from this patch.

Author: kazutakahirata

llvm/include/llvm/IR/InlineAsm.h

@@ -84,7 +84,7 @@ class InlineAsm final : public Value {
   FunctionType *getFunctionType() const;
 
   const std::string &getAsmString() const { return AsmString; }
-  const std::string &getConstraintString() const { return Constraints; }
+  StringRef getConstraintString() const { return Constraints; }
   void collectAsmStrs(SmallVectorImpl<StringRef> &AsmStrs) const;
 
   /// This static method can be used by the parser to check to see if the

llvm/lib/Bitcode/Reader/BitcodeReader.cpp

@@ -6022,7 +6022,7 @@ Error BitcodeReader::parseFunctionBody(Function *F) {
               FunctionType::get(FTy->getReturnType(), ArgTys, FTy->isVarArg());
 
           // Update constraint string to use label constraints.
-          std::string Constraints = IA->getConstraintString();
+          std::string Constraints = IA->getConstraintString().str();
           unsigned ArgNo = 0;
           size_t Pos = 0;
           for (const auto &CI : ConstraintInfo) {

llvm/lib/Bitcode/Writer/BitcodeWriter.cpp

@@ -2805,7 +2805,7 @@ void ModuleBitcodeWriter::writeConstants(unsigned FirstVal, unsigned LastVal,
       Record.append(AsmStr.begin(), AsmStr.end());
 
       // Add the constraint string.
-      const std::string &ConstraintStr = IA->getConstraintString();
+      StringRef ConstraintStr = IA->getConstraintString();
       Record.push_back(ConstraintStr.size());
       Record.append(ConstraintStr.begin(), ConstraintStr.end());
       Stream.EmitRecord(bitc::CST_CODE_INLINEASM, Record);

llvm/lib/IR/Core.cpp

@@ -529,11 +529,10 @@ const char *LLVMGetInlineAsmAsmString(LLVMValueRef InlineAsmVal, size_t *Len) {
 const char *LLVMGetInlineAsmConstraintString(LLVMValueRef InlineAsmVal,
                                              size_t *Len) {
   Value *Val = unwrap<Value>(InlineAsmVal);
-  const std::string &ConstraintString =
-      cast<InlineAsm>(Val)->getConstraintString();
+  StringRef ConstraintString = cast<InlineAsm>(Val)->getConstraintString();
 
-  *Len = ConstraintString.length();
-  return ConstraintString.c_str();
+  *Len = ConstraintString.size();
+  return ConstraintString.data();
 }
 
 LLVMInlineAsmDialect LLVMGetInlineAsmDialect(LLVMValueRef InlineAsmVal) {

llvm/lib/Target/ARM/ARMISelLowering.cpp

@@ -20276,9 +20276,9 @@ bool ARMTargetLowering::ExpandInlineAsm(CallInst *CI) const {
     SplitString(AsmStr, AsmPieces, " \t,");
 
     // rev $0, $1
-    if (AsmPieces.size() == 3 &&
-        AsmPieces[0] == "rev" && AsmPieces[1] == "$0" && AsmPieces[2] == "$1" &&
-        IA->getConstraintString().compare(0, 4, "=l,l") == 0) {
+    if (AsmPieces.size() == 3 && AsmPieces[0] == "rev" &&
+        AsmPieces[1] == "$0" && AsmPieces[2] == "$1" &&
+        IA->getConstraintString().starts_with("=l,l")) {
       IntegerType *Ty = dyn_cast<IntegerType>(CI->getType());
       if (Ty && Ty->getBitWidth() == 32)
         return IntrinsicLowering::LowerToByteSwap(CI);

llvm/lib/Target/DirectX/DXILWriter/DXILBitcodeWriter.cpp

@@ -1979,7 +1979,7 @@ void DXILBitcodeWriter::writeConstants(unsigned FirstVal, unsigned LastVal,
       Record.append(AsmStr.begin(), AsmStr.end());
 
       // Add the constraint string.
-      const std::string &ConstraintStr = IA->getConstraintString();
+      StringRef ConstraintStr = IA->getConstraintString();
       Record.push_back(ConstraintStr.size());
       Record.append(ConstraintStr.begin(), ConstraintStr.end());
       Stream.EmitRecord(bitc::CST_CODE_INLINEASM, Record);

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -60860,7 +60860,7 @@ bool X86TargetLowering::ExpandInlineAsm(CallInst *CI) const {
 
     // rorw $$8, ${0:w}  -->  llvm.bswap.i16
     if (CI->getType()->isIntegerTy(16) &&
-        IA->getConstraintString().compare(0, 5, "=r,0,") == 0 &&
+        IA->getConstraintString().starts_with("=r,0,") &&
         (matchAsm(AsmPieces[0], {"rorw", "$$8,", "${0:w}"}) ||
          matchAsm(AsmPieces[0], {"rolw", "$$8,", "${0:w}"}))) {
       AsmPieces.clear();
@@ -60873,7 +60873,7 @@ bool X86TargetLowering::ExpandInlineAsm(CallInst *CI) const {
     break;
   case 3:
     if (CI->getType()->isIntegerTy(32) &&
-        IA->getConstraintString().compare(0, 5, "=r,0,") == 0 &&
+        IA->getConstraintString().starts_with("=r,0,") &&
         matchAsm(AsmPieces[0], {"rorw", "$$8,", "${0:w}"}) &&
         matchAsm(AsmPieces[1], {"rorl", "$$16,", "$0"}) &&
         matchAsm(AsmPieces[2], {"rorw", "$$8,", "${0:w}"})) {