[llvm-commits] MachineInstr serializer and parser

[Bob Wilson]

On May 31, 2012, at 9:27 PM, Bin Zeng <bzeng at apple.com> wrote:

> Hi all,
> 
> Thanks for the reviews. I am working on a serializer and parser for MachineInstr's in the backend.
> The opt driver has a very nice feature that you can serialize IR after a specific pass and, if you
> want, deserialize it and get an identical copy of the original code before serialization and resume
> compilation there.  The project I am working on tries to bring that nice feature to the backend so that
> the MachineInstr's can be serialized after a specific pass and deserialized later and continue codegen
> there if you want. Since its goal is to help debug passes,  the format will be in human readable yaml 
> format instead of a binary format.
> 
> This patch contains the skeleton of the serializer pass and the parser pass: MachineInstrSerializerPass
> and MachineInstrParserPass and their command line options: -serialize-mi=pass-arg and
> -parse-mi=pass-arg where pass-arg is the argument of the pass after which the MachineInstr's will
> be serialized and parsed respectively. If you have better ideas for their names, please do not hold them
> to yourself.

I had earlier suggested naming the command-line options:

  --stop-after=<pass>
  --start-after=<pass>

Those may not be the greatest names but I really dislike the proposed --serialize-mi and --parse-mi names.  Two reasons: (1) From the point of view of someone running llc, these options are controlling which passes get run.  The fact that the machine instructions get (de)serialized is just a prerequisite to support that.  The option names should reflect that.  (2) llc also runs some IR-level passes and I would eventually like these same options to be used to allow stopping/starting after IR-level passes.  Please reconsider the names.

> Also, these two passes are MachineFunctionPass's. They can only serialize a MachineFunction at a
> time so far. To get a snapshot of the whole module after a specific pass, I need to visit the other MachineFunction's.
> The backend generates code for each MachineFunction at a time. I would appreciate it if you can point
> me to the right direction.

I'm not sure offhand what is the right direction, but this isn't it.  The output file header should not be written by a function pass.

Consider the case of the --start-after option (to use my name).  You don't just want to insert a function pass to parse the machine instructions -- you want it to skip all the prior passes.

> 
> Here is the patch. Thanks for reading.
> -Bin
> <serialize-parse-mi-0.3.diff>

One more comment below:

> Index: include/llvm/InitializePasses.h
> ===================================================================
> --- include/llvm/InitializePasses.h	(revision 157693)
> +++ include/llvm/InitializePasses.h	(working copy)
> @@ -253,6 +253,8 @@
>  void initializeFinalizeMachineBundlesPass(PassRegistry&);
>  void initializeBBVectorizePass(PassRegistry&);
>  void initializeMachineFunctionPrinterPassPass(PassRegistry&);
> +void initializeMachineInstrSerializerPassPass(PassRegistry&);
> +void initializeMachineInstrParserPassPass(PassRegistry&);
>  }

What's with the "PassPass" names?  I see a bunch of other initialization functions have that naming style, but I don't understand.  At least for your new functions, can't we drop the redundant "Pass" in the name?

Also, I thought you were going to call these "Reader" and "Writer" instead of "Serializer" and "Parser"?