[CFGPrinter] Display branch weight on the edges

[anemet]

Summary

This is pretty useful especially in connection with
BFI's -view-block-freq-propagation-dags. It helped me to track down the
bug that is being fixed in D24118.

While -view-block-freq-propagation-dags displays the high-level
information with static heuristics included (and block frequencies), the
new thing only shows the raw weight as presented by PGO without any of
the static estimates. This helps to distinguished what has been
measured vs. estimated.

For the sample loop in D24118, -view-block-freq-propagation-dags=integer
looks like this:

https://reviews.llvm.org/F2381352

While with -view-cfg-only you can see the underlying branch weights:

https://reviews.llvm.org/F2381349

[davidxl]

The problem I see is that people may confuse 'weight' with actual profile count, while in-reality it may not be the case (due to scaling). Perhaps add '(W)' after the number?

include/llvm/Analysis/CFGPrinter.h

135 ↗

(On Diff #70026)

A better way to filter:

MDString *MDName = cast<MDString>(WeightsNode>getOperand(0));

if (MDName->getString() != "branch_weights")
  return "" ;

[anemet]

In D24144#531757, @davidxl wrote:

The problem I see is that people may confuse 'weight' with actual profile count, while in-reality it may not be the case (due to scaling). Perhaps add '(W)' after the number?

Makes sense. I went with 'W:', prepended, e.g.:

https://reviews.llvm.org/F2392296

The W after made me think of electric power ;)

[anemet]

Addressed David's comments. I still kept the check of the operand number
around to make sure we don't index out of the MD node.

[davidxl]

lgtm

llvm/trunk/include/llvm/Analysis/CFGPrinter.h

if (const SwitchInst *SI = dyn_cast<SwitchInst>(Node->getTerminator())) {
       raw_string_ostream OS(Str);
       SwitchInst::ConstCaseIt Case =
           SwitchInst::ConstCaseIt::fromSuccessorIndex(SI, SuccNo);
       OS << Case.getCaseValue()->getValue();
       return OS.str();
     }
     return "";
   }
+
+  /// Display the raw branch weights from PGO.
+  std::string getEdgeAttributes(const BasicBlock *Node, succ_const_iterator I,
+                                const Function *F) {
+    const TerminatorInst *TI = Node->getTerminator();
+    if (TI->getNumSuccessors() == 1)
+      return "";
+
+    MDNode *WeightsNode = TI->getMetadata(LLVMContext::MD_prof);
+    if (!WeightsNode)
+      return "";
+
+    MDString *MDName = cast<MDString>(WeightsNode->getOperand(0));
+    if (MDName->getString() != "branch_weights")
+      return "";
+
+    unsigned OpNo = I.getSuccessorIndex() + 1;
+    if (OpNo >= WeightsNode->getNumOperands())
+      return "";
+    ConstantInt *Weight =
+        mdconst::dyn_extract<ConstantInt>(WeightsNode->getOperand(OpNo));
+    if (!Weight)
+      return "";
+
+    // Append a 'W' to indicate that these are weights rather than actual
+    // profile
+    // count (due to scaling).
+    Twine Attrs = "label=\"W:" + Twine(Weight->getZExtValue()) + "\"";
+    return Attrs.str();
+  }
 };
 } // End llvm namespace
 
 namespace llvm {
   class FunctionPass;
   FunctionPass *createCFGPrinterPass ();
   FunctionPass *createCFGOnlyPrinterPass ();
 } // End llvm namespace
 
 #endif