[llvm-mca] Emit a message when no bottlenecks are identified.

[mattd]

Summary

Since bottleneck hints are enabled via user request, it can be
confusing if no bottleneck information is presented. Such is the
case when no bottlenecks are identified. This patch emits a message
in that case.

[andreadb]

LGTM if you add flags -all-views=false -summary-view to that new test, and then you regenerate it with the update_mca script.

llvm/test/tools/llvm-mca/X86/BtVer2/bottleneck-hints-none.s

+# RUN: llvm-mca -mtriple=x86_64-unknown-unknown -mcpu=btver2 -bottleneck-analysis -iterations 1 < %s \
+# RUN:   | FileCheck %s -implicit-check-not 'Cycles with backend pressure increase'
+
+add %eax, %ebx
+
+# CHECK: No resource or data dependency bottlenecks discovered.

llvm/tools/llvm-mca/Views/SummaryView.h

class SummaryView : public View {
 
   // True if resource pressure events were notified during this cycle.
   bool PressureIncreasedBecauseOfResources;
   bool PressureIncreasedBecauseOfDataDependencies;
 
   // True if throughput was affected by dispatch stalls.
   bool SeenStallCycles;
 
+  // True if the bottleneck analysis should be displayed.
+  bool ShouldEmitBottleneckAnalysis;
+
   // Compute the reciprocal throughput for the analyzed code block.
   // The reciprocal block throughput is computed as the MAX between:
   //   - NumMicroOps / DispatchWidth
   //   - Total Resource Cycles / #Units   (for every resource consumed).
   double getBlockRThroughput() const;
 
   // Prints a bottleneck message to OS.
   void printBottleneckHints(llvm::raw_ostream &OS) const;
 
 public:
   SummaryView(const llvm::MCSchedModel &Model, llvm::ArrayRef<llvm::MCInst> S,
-              unsigned Width);
+              unsigned Width, bool EmitBottleneckAnalysis);
 
   void onCycleEnd() override {
     ++TotalCycles;
     if (PressureIncreasedBecauseOfResources ||
         PressureIncreasedBecauseOfDataDependencies) {
       ++BPI.PressureIncreaseCycles;
       if (PressureIncreasedBecauseOfDataDependencies)
         ++BPI.DataDependencyCycles;

llvm/tools/llvm-mca/Views/SummaryView.cpp

 #include "llvm/Support/Format.h"
 
 namespace llvm {
 namespace mca {
 
 #define DEBUG_TYPE "llvm-mca"
 
 SummaryView::SummaryView(const MCSchedModel &Model, ArrayRef<MCInst> S,
-                         unsigned Width)
+                         unsigned Width, bool EmitBottleneckAnalysis)
     : SM(Model), Source(S), DispatchWidth(Width), LastInstructionIdx(0),
       TotalCycles(0), NumMicroOps(0), BPI({0, 0, 0, 0, 0}),
       ResourcePressureDistribution(Model.getNumProcResourceKinds(), 0),
       ProcResourceUsage(Model.getNumProcResourceKinds(), 0),
       ProcResourceMasks(Model.getNumProcResourceKinds()),
       ResIdx2ProcResID(Model.getNumProcResourceKinds(), 0),
       PressureIncreasedBecauseOfResources(false),
       PressureIncreasedBecauseOfDataDependencies(false),
-      SeenStallCycles(false) {
+      SeenStallCycles(false),
+      ShouldEmitBottleneckAnalysis(EmitBottleneckAnalysis) {
   computeProcResourceMasks(SM, ProcResourceMasks);
   for (unsigned I = 1, E = SM.getNumProcResourceKinds(); I < E; ++I) {
     unsigned Index = getResourceStateIndex(ProcResourceMasks[I]);
     ResIdx2ProcResID[Index] = I;
   }
 }
 
 void SummaryView::onEvent(const HWInstructionEvent &Event) {
void SummaryView::onEvent(const HWPressureEvent &Event) {
   case HWPressureEvent::MEMORY_DEPS:
     PressureIncreasedBecauseOfDataDependencies = true;
     ++BPI.MemoryDependencyCycles;
     break;
   }
 }
 
 void SummaryView::printBottleneckHints(raw_ostream &OS) const {
-  if (!SeenStallCycles || !BPI.PressureIncreaseCycles)
+  if (!SeenStallCycles || !BPI.PressureIncreaseCycles) {
+    OS << "\nNo resource or data dependency bottlenecks discovered.\n";
     return;
+  }
 
   double PressurePerCycle =
       (double)BPI.PressureIncreaseCycles * 100 / TotalCycles;
   double ResourcePressurePerCycle =
       (double)BPI.ResourcePressureCycles * 100 / TotalCycles;
   double DDPerCycle = (double)BPI.DataDependencyCycles * 100 / TotalCycles;
   double RegDepPressurePerCycle =
       (double)BPI.RegisterDependencyCycles * 100 / TotalCycles;
void SummaryView::printView(raw_ostream &OS) const {
   TempStream << "\nDispatch Width:    " << DispatchWidth;
   TempStream << "\nuOps Per Cycle:    "
              << format("%.2f", floor((UOpsPerCycle * 100) + 0.5) / 100);
   TempStream << "\nIPC:               "
              << format("%.2f", floor((IPC * 100) + 0.5) / 100);
   TempStream << "\nBlock RThroughput: "
              << format("%.1f", floor((BlockRThroughput * 10) + 0.5) / 10)
              << '\n';
-
+  if (ShouldEmitBottleneckAnalysis)
     printBottleneckHints(TempStream);
   TempStream.flush();
   OS << Buffer;
 }
 } // namespace mca.
 } // namespace llvm

llvm/tools/llvm-mca/llvm-mca.cpp

if (PrintInstructionTables) {
       continue;
     }
 
     // Create a basic pipeline simulating an out-of-order backend.
     auto P = MCA.createDefaultPipeline(PO, IB, S);
     mca::PipelinePrinter Printer(*P);
 
     if (PrintSummaryView)
-      Printer.addView(llvm::make_unique<mca::SummaryView>(SM, Insts, Width));
+      Printer.addView(llvm::make_unique<mca::SummaryView>(
+          SM, Insts, Width, EnableBottleneckAnalysis));
 
     if (PrintInstructionInfoView)
       Printer.addView(
           llvm::make_unique<mca::InstructionInfoView>(*STI, *MCII, Insts, *IP));
 
     if (PrintDispatchStats)
       Printer.addView(llvm::make_unique<mca::DispatchStatistics>());