diff --git a/mlir/lib/Dialect/Linalg/IR/LinalgOps.cpp b/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;
 }
 
 //===----------------------------------------------------------------------===//

diff --git a/mlir/lib/Dialect/Linalg/TransformOps/LinalgMatchOps.cpp b/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()) {

diff --git a/mlir/lib/Dialect/OpenMP/IR/OpenMPDialect.cpp b/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,

diff --git a/mlir/lib/Dialect/SparseTensor/IR/Detail/Var.cpp b/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()); }

diff --git a/mlir/lib/Dialect/Traits.cpp b/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) {

diff --git a/mlir/lib/Dialect/Transform/DebugExtension/DebugExtensionOps.cpp b/mlir/lib/Dialect/Transform/DebugExtension/DebugExtensionOps.cpp
@@ -60,10 +60,10 @@ DiagnosedSilenceableFailure transform::DebugEmitParamAsRemarkOp::apply(
     os << *getMessage() << " ";
   llvm::interleaveComma(state.getParams(getParam()), os);
   if (!getAnchor()) {
-    emitRemark() << os.str();
+    emitRemark() << str;
     return DiagnosedSilenceableFailure::success();
   }
   for (Operation *payload : state.getPayloadOps(getAnchor()))
-    ::mlir::emitRemark(payload->getLoc()) << os.str();
+    ::mlir::emitRemark(payload->getLoc()) << str;
   return DiagnosedSilenceableFailure::success();
 }
diff --git a/mlir/lib/Dialect/Transform/IR/TransformOps.cpp b/mlir/lib/Dialect/Transform/IR/TransformOps.cpp
@@ -1967,7 +1967,7 @@ transform::MatchParamCmpIOp::apply(transform::TransformRewriter &rewriter,
     std::string str;
     llvm::raw_string_ostream os(str);
     value.print(os, /*isSigned=*/true);
-    return os.str();
+    return str;
   };
 
   ArrayRef<Attribute> params = state.getParams(getParam());

diff --git a/mlir/lib/ExecutionEngine/ExecutionEngine.cpp b/mlir/lib/ExecutionEngine/ExecutionEngine.cpp
@@ -432,7 +432,7 @@ Expected<void *> ExecutionEngine::lookup(StringRef name) const {
     llvm::raw_string_ostream os(errorMessage);
     llvm::handleAllErrors(expectedSymbol.takeError(),
                           [&os](llvm::ErrorInfoBase &ei) { ei.log(os); });
-    return makeStringError(os.str());
+    return makeStringError(errorMessage);
   }
 
   if (void *fptr = expectedSymbol->toPtr<void *>())

diff --git a/mlir/lib/IR/Diagnostics.cpp b/mlir/lib/IR/Diagnostics.cpp
@@ -146,15 +146,15 @@ Diagnostic &Diagnostic::appendOp(Operation &op, const OpPrintingFlags &flags) {
   // multiple lines.
   if (str.find('\n') != std::string::npos)
     *this << '\n';
-  return *this << os.str();
+  return *this << str;
 }
 
 /// Stream in a Value.
 Diagnostic &Diagnostic::operator<<(Value val) {
   std::string str;
   llvm::raw_string_ostream os(str);
   val.print(os, adjustPrintingFlags(OpPrintingFlags(), severity));
-  return *this << os.str();
+  return *this << str;
 }
 
 /// Outputs this diagnostic to a stream.
@@ -168,7 +168,7 @@ std::string Diagnostic::str() const {
   std::string str;
   llvm::raw_string_ostream os(str);
   print(os);
-  return os.str();
+  return str;
 }
 
 /// Attaches a note to this diagnostic. A new location may be optionally
@@ -451,7 +451,7 @@ void SourceMgrDiagnosticHandler::emitDiagnostic(Location loc, Twine message,
     if (!llvm::isa<UnknownLoc>(loc))
       strOS << loc << ": ";
     strOS << message;
-    return mgr.PrintMessage(os, SMLoc(), getDiagKind(kind), strOS.str());
+    return mgr.PrintMessage(os, SMLoc(), getDiagKind(kind), str);
   }
 
   // Otherwise if we are displaying the source line, try to convert the file
@@ -469,7 +469,7 @@ void SourceMgrDiagnosticHandler::emitDiagnostic(Location loc, Twine message,
   llvm::raw_string_ostream locOS(locStr);
   locOS << fileLoc.getFilename().getValue() << ":" << fileLoc.getLine() << ":"
         << fileLoc.getColumn();
-  llvm::SMDiagnostic diag(locOS.str(), getDiagKind(kind), message.str());
+  llvm::SMDiagnostic diag(locStr, getDiagKind(kind), message.str());
   diag.print(nullptr, os);
 }
 
@@ -637,7 +637,7 @@ struct ExpectedDiag {
       regexOS << '(' << regexStr << ')';
       strToProcess = strToProcess.drop_front(regexEndIt + 2);
     }
-    substringRegex = llvm::Regex(regexOS.str());
+    substringRegex = llvm::Regex(regexStr);
     return success();
   }
 

diff --git a/mlir/lib/Interfaces/DataLayoutInterfaces.cpp b/mlir/lib/Interfaces/DataLayoutInterfaces.cpp
@@ -29,7 +29,7 @@ using namespace mlir;
   os << "neither the scoping op nor the type class provide data layout "
         "information for "
      << type;
-  llvm::report_fatal_error(Twine(os.str()));
+  llvm::report_fatal_error(Twine(message));
 }
 
 /// Returns the bitwidth of the index type if specified in the param list.

diff --git a/mlir/lib/Interfaces/RuntimeVerifiableOpInterface.cpp b/mlir/lib/Interfaces/RuntimeVerifiableOpInterface.cpp
@@ -30,7 +30,7 @@ RuntimeVerifiableOpInterface::generateErrorMessage(Operation *op,
   stream << "\n^ " << msg;
   stream << "\nLocation: ";
   op->getLoc().print(stream);
-  return stream.str();
+  return buffer;
 }
 } // namespace mlir
 

diff --git a/mlir/lib/Pass/Pass.cpp b/mlir/lib/Pass/Pass.cpp
@@ -66,7 +66,6 @@ LogicalResult Pass::initializeOptions(
   std::string errStr;
   llvm::raw_string_ostream os(errStr);
   if (failed(passOptions.parseFromString(options, os))) {
-    os.flush();
     return errorHandler(errStr);
   }
   return success();
@@ -700,7 +699,7 @@ std::string OpToOpPassAdaptor::getAdaptorName() {
     os << '\'' << pm.getOpAnchorName() << '\'';
   });
   os << ']';
-  return os.str();
+  return name;
 }
 
 void OpToOpPassAdaptor::runOnOperation() {

diff --git a/mlir/lib/Pass/PassCrashRecovery.cpp b/mlir/lib/Pass/PassCrashRecovery.cpp
@@ -321,7 +321,7 @@ void PassCrashReproducerGenerator::prepareReproducerFor(Pass *pass,
     passOS << ")";
 
   impl->activeContexts.push_back(std::make_unique<RecoveryReproducerContext>(
-      passOS.str(), op, impl->streamFactory, impl->pmFlagVerifyPasses));
+      passStr, op, impl->streamFactory, impl->pmFlagVerifyPasses));
 }
 void PassCrashReproducerGenerator::prepareReproducerFor(
     iterator_range<PassManager::pass_iterator> passes, Operation *op) {
@@ -331,7 +331,7 @@ void PassCrashReproducerGenerator::prepareReproducerFor(
       passes, passOS, [&](Pass &pass) { pass.printAsTextualPipeline(passOS); });
 
   impl->activeContexts.push_back(std::make_unique<RecoveryReproducerContext>(
-      passOS.str(), op, impl->streamFactory, impl->pmFlagVerifyPasses));
+      passStr, op, impl->streamFactory, impl->pmFlagVerifyPasses));
 }
 
 void PassCrashReproducerGenerator::removeLastReproducerFor(Pass *pass,

diff --git a/mlir/lib/Query/Matcher/RegistryManager.cpp b/mlir/lib/Query/Matcher/RegistryManager.cpp
@@ -124,7 +124,7 @@ RegistryManager::getMatcherCompletions(llvm::ArrayRef<ArgKind> acceptedTypes,
     else if (argKinds[0][0] == ArgKind::String)
       typedText += "\"";
 
-    completions.emplace_back(typedText, os.str());
+    completions.emplace_back(typedText, decl);
   }
 
   return completions;

diff --git a/mlir/lib/Query/QueryParser.cpp b/mlir/lib/Query/QueryParser.cpp
@@ -123,7 +123,7 @@ makeInvalidQueryFromDiagnostics(const matcher::internal::Diagnostics &diag) {
   std::string errStr;
   llvm::raw_string_ostream os(errStr);
   diag.print(os);
-  return new InvalidQuery(os.str());
+  return new InvalidQuery(errStr);
 }
 } // namespace