[llvm] r280442 - [CFGPrinter] Display branch weight on the edges

[Justin Bogner via llvm-commits]

Adam Nemet via llvm-commits <llvm-commits at lists.llvm.org> writes:
> Author: anemet
> Date: Thu Sep  1 19:28:26 2016
> New Revision: 280442
>
> URL: http://llvm.org/viewvc/llvm-project?rev=280442&view=rev
> Log:
> [CFGPrinter] Display branch weight on the edges
>
> 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

These links aren't visible without logging into phab, which seems a bit
unfortunate. I filed llvm.org/PR30252.

> While with -view-cfg-only you can see the underlying branch weights:
>
> https://reviews.llvm.org/F2392296
>
> Reviewers: dexonsmith, bogner, davidxl

When committing, please edit "Reviewers" to accurately reflect people
who reviewed the patch, or just rip it out completely. The change does
LGTM, but I find the way that phab/arc/whatever blindly lists people who
happened to be CC'd as reviewers quite misleading.

> Subscribers: llvm-commits
>
> Differential Revision: https://reviews.llvm.org/D24144
>
> Modified:
>     llvm/trunk/include/llvm/Analysis/CFGPrinter.h
>
> Modified: llvm/trunk/include/llvm/Analysis/CFGPrinter.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/CFGPrinter.h?rev=280442&r1=280441&r2=280442&view=diff
> ==============================================================================
> --- llvm/trunk/include/llvm/Analysis/CFGPrinter.h (original)
> +++ llvm/trunk/include/llvm/Analysis/CFGPrinter.h Thu Sep  1 19:28:26 2016
> @@ -118,6 +118,36 @@ struct DOTGraphTraits<const Function*> :
>      }
>      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).

typo - this should say "Prepend a 'W'". The comment is also wrapped a
little funny, probably from clang-format getting confused.

> +    Twine Attrs = "label=\"W:" + Twine(Weight->getZExtValue()) + "\"";
> +    return Attrs.str();
> +  }
>  };
>  } // End llvm namespace
>  
>
>
> _______________________________________________
> llvm-commits mailing list
> llvm-commits at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits