[llvm-mca] Return the total number of cycles from method Pipeline::run().

[andreadb]

Summary

If a user only cares about the overall latency, then the best/quickest way is to change method Pipeline::run() so that it returns the total number of cycles to the caller.

When the simulation pipeline is run, the number of cycles (or an error) is returned from method Pipeline::run().
The advantage is that no hardware event listener is needed for computing that latency. So, the whole process should be faster (and simpler - for that particular use case).

[mattd]

LGTM!

[andreadb]

Committed at revision 347767.

tools/llvm-mca/include/Pipeline.h

class Pipeline {
   Error runCycle();
   bool hasWorkToProcess();
   void notifyCycleBegin();
   void notifyCycleEnd();
 
 public:
   Pipeline() : Cycles(0) {}
   void appendStage(std::unique_ptr<Stage> S);
-  Error run();
+
+  // Returns the total number of simulated cycles.
+  Expected<unsigned> run();
+
   void addEventListener(HWEventListener *Listener);
 };
 } // namespace mca
 } // namespace llvm
 
 #endif // LLVM_TOOLS_LLVM_MCA_PIPELINE_H

tools/llvm-mca/lib/Pipeline.cpp

 }
 
 bool Pipeline::hasWorkToProcess() {
   return any_of(Stages, [](const std::unique_ptr<Stage> &S) {
     return S->hasWorkToComplete();
   });
 }
 
-Error Pipeline::run() {
+Expected<unsigned> Pipeline::run() {
   assert(!Stages.empty() && "Unexpected empty pipeline found!");
 
   do {
     notifyCycleBegin();
     if (Error Err = runCycle())
       return Err;
     notifyCycleEnd();
     ++Cycles;
   } while (hasWorkToProcess());
 
-  return ErrorSuccess();
+  return Cycles;
 }
 
 Error Pipeline::runCycle() {
   Error Err = ErrorSuccess();
   // Update stages before we start processing new instructions.
   for (auto I = Stages.rbegin(), E = Stages.rend(); I != E && !Err; ++I) {
     const std::unique_ptr<Stage> &S = *I;
     Err = S->cycleStart();

tools/llvm-mca/llvm-mca.cpp

static void processViewOptions() {
   processOptionImpl(PrintDispatchStats, Default);
   processOptionImpl(PrintSchedulerStats, Default);
   processOptionImpl(PrintRetireStats, Default);
 }
 
 // Returns true on success.
 static bool runPipeline(mca::Pipeline &P) {
   // Handle pipeline errors here.
-  if (auto Err = P.run()) {
-    WithColor::error() << toString(std::move(Err));
+  Expected<unsigned> Cycles = P.run();
+  if (!Cycles) {
+    WithColor::error() << toString(Cycles.takeError());
     return false;
   }
   return true;
 }
 
 int main(int argc, char **argv) {
   InitLLVM X(argc, argv);