[llvm-dev] [RFC] Moving tools/llvm-mca/lib into lib/MCA

Wed Dec 12 07:58:56 PST 2018

On 12/12/18 6:51 AM, Andrea Di Biagio via llvm-dev wrote:
Thanks Clement for working on this.

I like the idea of moving the core abstractions of llvm-mca in llvm/lib, as it potentially enables a number of interesting use cases and future developments.

As you wrote, the patch is very simple (a pretty mechanical change).
From my point of view, this all looks good. Let see if other people have the same opinion on this.

+1

 -Hal

-Andrea

On Wed, Dec 12, 2018 at 11:58 AM Clement Courbet <courbet at google.com<mailto:courbet at google.com>> wrote:

(on the correct mailing list)

Hi all,

tl;dr: We'd like to propose moving tools/llvm-mca/lib into lib/MCA and create a new MCA library in LLVM.

llvm-mca has recently been split<https://bugs.llvm.org/show_bug.cgi?id=37696> into its core part and the tool part.

  *   The core part simulates the execution of a basic block of machine instructions as modeled by the llvm SchedModel.

  *   The tool part deals with the plumbing and interacting with the user.

The core part can be used by parts of LLVM that deal with cost modeling (e.g. scheduling and vectorization). MCA provides a more realistic target for optimization than the heuristics typically used to drive these passes. We think that using approaches based on fine-grained cost modeling can greatly improve the performance of critically hot code, by trading compile time for performance.

As an example of how MCA can be used to improve scheduling, we have built a prototype machine scheduler that optimizes the simulated latency of a block of code under the constraints of the scheduling DAG. We've had interesting wins of 2-3% on assembly kernels for webp<https://github.com/webmproject/libwebp/commit/67748b41dbb21a43e88f2b6ddf6117f4338873a3> and gemmlowp<https://github.com/google/gemmlowp/pull/91>*.

The patch is completely mechanical and can be seen here<https://reviews.llvm.org/D55557>. It only moves the files and updates all references.

We'd like to hear if there are any things comments/objections to this change.

Thanks !

*note: We did this before llvm-mca was released and were using a different simulator, but the idea remains the same.

On Wed, Dec 12, 2018 at 11:44 AM Clement Courbet <courbet at google.com<mailto:courbet at google.com>> wrote: