Add an option to hide "cold" blocks from CFG graph

[apilipenko]

Summary

Introduce a new cl::opt to hide "cold" blocks from CFG DOT graphs. Use BFI to get block relative frequency. Hide the block if the frequency is below the threshold set by the command line option value.

This is a generalization of the existing cfg-hide-unreachable-paths and cfg-hide-deoptimize-paths options. Just like with the existing options the main use case is performance analysis. Hiding cold blocks reduces the size of the graph by highlighting the common path.

[hoy]

LGTM, thanks for working on this.

[davidxl]

lgtm

llvm/lib/Analysis/CFGPrinter.cpp

static cl::opt<std::string> CFGDotFilenamePrefix(
     cl::desc("The prefix used for the CFG dot file names."));
 
 static cl::opt<bool> HideUnreachablePaths("cfg-hide-unreachable-paths",
                                           cl::init(false));
 
 static cl::opt<bool> HideDeoptimizePaths("cfg-hide-deoptimize-paths",
                                          cl::init(false));
 
+static cl::opt<double> HideColdPaths(
+    "cfg-hide-cold-paths", cl::init(0.0),
+    cl::desc("Hide blocks with relative frequency below the given value"));
+
 static cl::opt<bool> ShowHeatColors("cfg-heat-colors", cl::init(true),
                                     cl::Hidden,
                                     cl::desc("Show heat colors in CFG"));
 
 static cl::opt<bool> UseRawEdgeWeight("cfg-raw-weights", cl::init(false),
                                       cl::Hidden,
                                       cl::desc("Use raw weights for labels. "
                                                "Use percentages as default."));
void DOTGraphTraits<DOTFuncInfo *>::computeDeoptOrUnreachablePaths(
   };
   /// The post order traversal iteration is done to know the status of
   /// isOnDeoptOrUnreachablePath for all the successors on the current BB.
   llvm::for_each(post_order(&F->getEntryBlock()), evaluateBB);
 }
 
 bool DOTGraphTraits<DOTFuncInfo *>::isNodeHidden(const BasicBlock *Node,
                                                  const DOTFuncInfo *CFGInfo) {
+  if (HideColdPaths.getNumOccurrences() > 0)
+    if (auto *BFI = CFGInfo->getBFI()) {
+      uint64_t NodeFreq = BFI->getBlockFreq(Node).getFrequency();
+      uint64_t EntryFreq = BFI->getEntryFreq();
+      // Hide blocks with relative frequency below HideColdPaths threshold.
+      if ((double)NodeFreq / EntryFreq < HideColdPaths)
+        return true;
+    }
   if (HideUnreachablePaths || HideDeoptimizePaths) {
     if (isOnDeoptOrUnreachablePath.find(Node) == 
         isOnDeoptOrUnreachablePath.end())
       computeDeoptOrUnreachablePaths(Node->getParent());
     return isOnDeoptOrUnreachablePath[Node];
   }
   return false;
 }