[mlir] Tidy uses of llvm::raw_stream_ostream (NFC)

As specified in the docs,
1) raw_string_ostream is always unbuffered and
2) the underlying buffer may be used directly

( 65b1361 for further reference )

* Don't call raw_string_ostream::flush(), which is essentially a no-op.
* Avoid unneeded calls to raw_string_ostream::str(), to avoid excess indirection.

Author: JOE1994

mlir/lib/Debug/Observers/ActionProfiler.cpp

@@ -58,6 +58,6 @@ void ActionProfiler::print(const ActionActiveStack *action,
   if (printComma)
     os << ",\n";
   printComma = true;
-  os << event.str();
+  os << str;
   os.flush();
 }

mlir/lib/Dialect/Bufferization/Transforms/OneShotAnalysis.cpp

@@ -1299,7 +1299,7 @@ static void annotateOpsWithAliasSets(Operation *op,
       std::string buffer;
       llvm::raw_string_ostream stream(buffer);
       alias.printAsOperand(stream, asmState);
-      aliases.push_back(b.getStringAttr(stream.str()));
+      aliases.push_back(b.getStringAttr(buffer));
     });
     return b.getArrayAttr(aliases);
   };

mlir/lib/Dialect/LLVMIR/IR/LLVMAttrs.cpp

@@ -326,7 +326,7 @@ std::string TargetFeaturesAttr::getFeaturesString() const {
   llvm::raw_string_ostream ss(featuresString);
   llvm::interleave(
       getFeatures(), ss, [&](auto &feature) { ss << feature.strref(); }, ",");
-  return ss.str();
+  return featuresString;
 }
 
 TargetFeaturesAttr TargetFeaturesAttr::featuresAt(Operation *op) {

mlir/lib/Dialect/LLVMIR/IR/LLVMDialect.cpp

@@ -3247,7 +3247,7 @@ LogicalResult LLVMDialect::verifyDataLayoutString(
   std::string message;
   llvm::raw_string_ostream messageStream(message);
   llvm::logAllUnhandledErrors(maybeDataLayout.takeError(), messageStream);
-  reportError("invalid data layout descriptor: " + messageStream.str());
+  reportError("invalid data layout descriptor: " + message);
   return failure();
 }
 

mlir/lib/Dialect/LLVMIR/IR/NVVMDialect.cpp

@@ -521,7 +521,7 @@ LogicalResult MmaOp::verify() {
       }
       errorStream << "but got ";
       llvm::interleaveComma(operandTySeg, errorStream);
-      return emitOpError(errorStream.str());
+      return emitOpError(errorMessage);
     }
   }
 
@@ -533,7 +533,7 @@ LogicalResult MmaOp::verify() {
         << "Could not match allowed types for the result; expected one of ";
     llvm::interleaveComma(expectedResult, errorStream);
     errorStream << " but got " << getResult().getType();
-    return emitOpError(errorStream.str());
+    return emitOpError(errorMessage);
   }
 
   // Ensure that binary MMA variants have a b1 MMA operation defined.
@@ -967,7 +967,6 @@ std::string NVVM::WgmmaMmaAsyncOp::getPtx() {
   }
   ss << ";\n"
      << "}\n";
-  ss.flush();
   return ptx;
 }
 

mlir/lib/Dialect/Linalg/IR/LinalgOps.cpp

@@ -2334,9 +2334,8 @@ std::string mlir::linalg::generateLibraryCallName(Operation *op) {
       return std::string();
     ss << "_";
   }
-  std::string res = ss.str();
-  res.pop_back();
-  return res;
+  name.pop_back();
+  return name;
 }
 
 //===----------------------------------------------------------------------===//

mlir/lib/Dialect/Linalg/TransformOps/LinalgMatchOps.cpp

@@ -209,7 +209,7 @@ DiagnosedSilenceableFailure transform::MatchStructuredBodyOp::matchOperation(
         os);
     if (result)
       return DiagnosedSilenceableFailure::success();
-    return emitSilenceableError() << "contraction: " << os.str();
+    return emitSilenceableError() << "contraction: " << message;
   }
   return emitDefiniteFailure() << "unknown body condition";
 }
@@ -226,7 +226,7 @@ LogicalResult transform::MatchStructuredBodyOp::verify() {
                                                getElementwiseAttrName(),
                                                getContractionAttrName()},
                           os);
-    return emitOpError() << "only one of {" << os.str() << "} is allowed";
+    return emitOpError() << "only one of {" << attributeNames << "} is allowed";
   }
 
   if (std::optional<ArrayAttr> contractionAttr = getContraction()) {

mlir/lib/Dialect/OpenMP/IR/OpenMPDialect.cpp

@@ -1258,7 +1258,7 @@ static void printCaptureType(OpAsmPrinter &p, Operation *op,
     typeCap << "VLAType";
   if (mapCaptureType.getValue() == mlir::omp::VariableCaptureKind::This)
     typeCap << "This";
-  p << typeCap.str();
+  p << typeCapStr;
 }
 
 static ParseResult parseCaptureType(OpAsmParser &parser,

mlir/lib/Dialect/SparseTensor/IR/Detail/Var.cpp

@@ -29,7 +29,7 @@ std::string Var::str() const {
   std::string str;
   llvm::raw_string_ostream os(str);
   print(os);
-  return os.str();
+  return str;
 }
 
 void Var::print(AsmPrinter &printer) const { print(printer.getStream()); }

mlir/lib/Dialect/Traits.cpp

@@ -223,7 +223,7 @@ static std::string getShapeString(ArrayRef<int64_t> shape) {
       },
       "x");
   ss << '\'';
-  return ss.str();
+  return ret;
 }
 
 LogicalResult OpTrait::impl::verifyCompatibleOperandBroadcast(Operation *op) {