llvm dev - Jul 2013 - [LLVMdev] Docs question: legality of inspecting other functions in a function pass

Hi,

I'm not planning on doing this, but I noticed that the documentation
in WritingAnLLVMPass.rst doesn't seem to specify whether or not it's
legal for a function pass to inspect (and thus depend upon the
contents of) other functions and I'm curious if this is an oversight
or by design:

    To be explicit, ``FunctionPass`` subclasses are not allowed to:

    #. Modify a ``Function`` other than the one currently being processed.
    ...

Whereas for basic block passes there is an explicit prohibition:

    ``BasicBlockPass``\ es are just like :ref:`FunctionPass's
    <writing-an-llvm-pass-FunctionPass>` , except that they must limit
their scope
    of inspection and modification to a single basic block at a time.  As such,
    they are **not** allowed to do any of the following:

    #. Modify or inspect any basic blocks outside of the current one.
    ...

Does anyone know if there's a defined policy about this, either way?
If so, I think it ought to be noted in the docs, for consistency.

Stephen

On Wed, Jul 3, 2013 at 3:05 PM, Stephen Lin <swlin at post.harvard.edu>
wrote:
>
> If so, I think it ought to be noted in the docs, for consistency.
>
Once you get your answer, definitely feel free to update the docs.

-- Sean Silva
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On 3 Jul 2013, at 23:05, Stephen Lin <swlin at post.harvard.edu> wrote:
> Does anyone know if there's a defined policy about this, either way?
> If so, I think it ought to be noted in the docs, for consistency.
The prohibition exists, at least in part, because in theory it would be nice to
be able to run passes in parallel.  It's not a real limitation at the moment
because updating instructions in a module is not thread safe (and making it so
with the current APIs would probably be somewhat problematic in terms of
performance) and so when we do eventually get the ability to run FunctionPasses
in parallel they will most likely need new APIs.  That said, it's a good
idea structurally to view the Function / Block as synchronisation boundaries so
that it will be easier to support concurrent execution in the future.

David

On Thu, Jul 4, 2013 at 1:45 AM, David Chisnall
<David.Chisnall at cl.cam.ac.uk> wrote:
> On 3 Jul 2013, at 23:05, Stephen Lin <swlin at post.harvard.edu>
wrote:
>
>> Does anyone know if there's a defined policy about this, either
way?
>> If so, I think it ought to be noted in the docs, for consistency.
>
> The prohibition exists, at least in part, because in theory it would be
nice to be able to run passes in parallel.  It's not a real limitation at
the moment because updating instructions in a module is not thread safe (and
making it so with the current APIs would probably be somewhat problematic in
terms of performance) and so when we do eventually get the ability to run
FunctionPasses in parallel they will most likely need new APIs.  That said,
it's a good idea structurally to view the Function / Block as
synchronisation boundaries so that it will be easier to support concurrent
execution in the future.
>
I understand the rationale but are you sure that the prohibition
against *inspecting* other functions during a function pass does exist
and is currently followed? If it does I think the docs ought to make
that clear so I want to make sure if the omission is not deliberate.

In theory you could still parallelize function pass execution if they
inspected other functions if they used some kind of read/write locking
and used transactional updates; I would think the main point is that
we want the results to be deterministic and not dependent on the order
in which functions are processed, which applies regardless of what
kind of parallelization and/or synchronization is used.

Stephen