[mlir] use a thread-local alternative to llvm::nulls

[ftynse]

Summary

LLVM is not set up in a thread-safe way, which seems to be leading to
race conditions when sending stuff to llvm::nulls in opt builds. Try a
thread-local alternative.

[pzread]

LGTM, but might have someone else who is familiar with it to take another look

[mehdi_amini]

Why wouldn't this go to LLVM Support?

mlir/include/mlir/Dialect/Linalg/IR/LinalgInterfaces.h

 #include "mlir/IR/AffineMap.h"
 #include "mlir/IR/BuiltinTypes.h"
 #include "mlir/IR/IRMapping.h"
 #include "mlir/IR/ImplicitLocOpBuilder.h"
 #include "mlir/IR/OpDefinition.h"
 #include "mlir/Interfaces/DestinationStyleOpInterface.h"
 #include "mlir/Interfaces/InferTypeOpInterface.h"
 #include "mlir/Interfaces/ViewLikeInterface.h"
+#include "mlir/Support/RawOstreamExtras.h"
 
 namespace mlir {
 namespace linalg {
 class IteratorTypeAttr;
 class LinalgOp;
 
 namespace detail {
 /// Implementation of the method that that check if given operands

 /// are checked by `isaPair`. All operands of all operations may be supplied
 /// through a chain of side effect-free unary operations, such as casts, which
 /// is denoted as `cu` above.
 ///
 /// When the body does not contain a contraction, a more precise description of
 /// the failed precondition is send to the `errs` stream, if provided.
 bool isContractionBody(Block &block,
                        function_ref<bool(Operation *, Operation *)> isaPair,
-                       llvm::raw_ostream &errs = llvm::nulls());
+                       llvm::raw_ostream &errs = mlir::thread_safe_nulls());
 
 /// Result of matching a Linalg generic against the predicates of it being a
 /// contraction.
 enum class MatchContractionResult;
 
 /// Checks whether `op` conforms to ContractionOpInterface and populates
 /// `dimensions` with indexes of the different kinds of dimensions when
 /// present.

mlir/include/mlir/Support/RawOstreamExtras.h

+//===- RawOstreamExtras.h - Extensions to LLVM's raw_ostream.h --*- C++ -*-===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+namespace llvm {
+class raw_ostream;
+} // namespace llvm
+
+namespace mlir {
+/// Returns a raw output stream that simply discards the output, but in a
+/// thread-safe manner. Similar to llvm::nulls.
+llvm::raw_ostream &thread_safe_nulls();
+} // namespace mlir

mlir/lib/Support/CMakeLists.txt

 set(LLVM_OPTIONAL_SOURCES
   FileUtilities.cpp
   InterfaceSupport.cpp
   StorageUniquer.cpp
   Timing.cpp
   ToolUtilities.cpp
   TypeID.cpp
 )
 
 add_mlir_library(MLIRSupport
   FileUtilities.cpp
   InterfaceSupport.cpp
+  RawOstreamExtras.cpp
   StorageUniquer.cpp
   Timing.cpp
   ToolUtilities.cpp
   TypeID.cpp
 
   ADDITIONAL_HEADER_DIRS
   ${MLIR_MAIN_INCLUDE_DIR}/mlir/Support
 
   LINK_LIBS PUBLIC
   ${LLVM_PTHREAD_LIB})

mlir/lib/Support/RawOstreamExtras.cpp

+//===- RawOstreamExtras.cpp - Extensions to LLVM's raw_ostream ------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "mlir/Support/RawOstreamExtras.h"
+#include "llvm/Support/raw_ostream.h"
+
+llvm::raw_ostream &mlir::thread_safe_nulls() {
+  static thread_local llvm::raw_null_ostream stream;
+  return stream;
+}