Disable MLIR multithreading while parsing input, this speeds up the

parser (which doesn't a lot of IR generation) by about 18%!

lib/FIRParser/FIRParser.cpp

@@ -7,10 +7,10 @@
 #include "cirt/FIRParser.h"
 #include "FIRLexer.h"
 #include "cirt/Dialect/FIRRTL/Ops.h"
-#include "mlir/IR/Verifier.h"
 #include "mlir/IR/Diagnostics.h"
 #include "mlir/IR/Module.h"
 #include "mlir/IR/StandardTypes.h"
+#include "mlir/IR/Verifier.h"
 #include "mlir/Translation.h"
 #include "llvm/ADT/ScopedHashTable.h"
 #include "llvm/Support/SourceMgr.h"

tools/cirt-translate/cirt-translate.cpp

@@ -79,6 +79,10 @@ int main(int argc, char **argv) {
   auto processBuffer = [&](std::unique_ptr<llvm::MemoryBuffer> ownedBuffer,
                            raw_ostream &os) {
     MLIRContext context;
+
+    // Nothing here is threaded.  Disable synchronization overhead.
+    context.disableMultithreading();
+
     llvm::SourceMgr sourceMgr;
     sourceMgr.AddNewSourceBuffer(std::move(ownedBuffer), llvm::SMLoc());
 

tools/firtool/firtool.cpp

@@ -70,6 +70,9 @@ processBuffer(std::unique_ptr<llvm::MemoryBuffer> ownedBuffer,
   sourceMgr.AddNewSourceBuffer(std::move(ownedBuffer), llvm::SMLoc());
   SourceMgrDiagnosticHandler sourceMgrHandler(sourceMgr, &context);
 
+  // Nothing in the parser is threaded.  Disable synchronization overhead.
+  context.disableMultithreading();
+
   OwningModuleRef module;
   if (inputFormat == InputFIRFile) {
     FIRParserOptions options;
@@ -82,6 +85,9 @@ processBuffer(std::unique_ptr<llvm::MemoryBuffer> ownedBuffer,
   if (!module)
     return failure();
 
+  // Allow optimizations to run multithreaded.
+  context.disableMultithreading(false);
+
   // If enabled, run the optimizer.
   if (!disableOptimization) {
     // Apply any pass manager command line options.