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

[Andrea Di Biagio via llvm-dev]

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.

-Andrea


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