llvm dev - Mar 2017 - Sharing MemoryBuffers between front ends and LLVM

Hi all,

I'm implementing interleaved source in assembly output. Early reviews raised
the concern that the current implementation will be opening files (using a
llvm::MemoryBuffer) that are likely to be in the memory of the front end
(commonly clang but I think we want this to be front end agnostic).

I'm now exploring ideas to avoid reopening files and let LLVM reuse the
files the FE had to open.

I am assuming that the front end will use llvm::MemoryBuffer (e.g.: clang does
indirectly through clang::SourceManager).

So for buffers related to named files (including stdin, which does not have name
and is handled in a special way) we could have in the LLVM context a
MemoryBufferRegistry. The idea is to add new creators of MemoryBuffer (the ones
that work on named files and stdin) that can be passed a reference to that
llvm::MemoryBufferRegistry. MemoryBuffer objects would register/deregister
themselves at creation/destruction. This registry can then be used as a cache of
already opened files from which retrieve a reference to the MemoryBuffer itself
using the file path. These new interfaces would be opt-in for all users of
MemoryBuffer.

Back to my case,  the new AsmPrinterHandler could now use the
MemoryBufferRegistry of the LLVM context. If there is none or the memory buffer
associated to a file path has been already deregistered (or was never registered
e.g. because we are using a .ll file directly), it would open the file as usual,
otherwise it would reuse the registered MemoryBuffer.

I see a few downsides of this approach, though.

It overlaps a bit with the existing SourceManager in clang which already does
some caching work through the clang::ContentCache class. At first the cache
seems hard to abstract away as it uses clang::FileEntry and looks pretty
tailored for clang needs.

Also, assuming that the front end is using a MemoryBuffer may be a too strong
requirement, in particular for FE's that are mostly unaware of LLVM except
for a final LLVM codegen pass. This would mean that the files would be reopened
even if they are already in the memory of the FE.

Finally the file path may not even be a good identifier to reuse MemoryBuffer
objects.

Thoughts?

Thank you,
Roger

On 03/16/2017 12:29 PM, Roger Ferrer Ibanez via llvm-dev
wrote:
> Hi all,
>
> I'm implementing interleaved source in assembly output. Early reviews
raised the concern that the current implementation will be opening files (using
a llvm::MemoryBuffer) that are likely to be in the memory of the front end
(commonly clang but I think we want this to be front end agnostic).
>
> I'm now exploring ideas to avoid reopening files and let LLVM reuse the
files the FE had to open.
I think this is a good idea.
>
> I am assuming that the front end will use llvm::MemoryBuffer (e.g.: clang
does indirectly through clang::SourceManager).
>
> So for buffers related to named files (including stdin, which does not have
name and is handled in a special way) we could have in the LLVM context a
MemoryBufferRegistry. The idea is to add new creators of MemoryBuffer (the ones
that work on named files and stdin) that can be passed a reference to that
llvm::MemoryBufferRegistry. MemoryBuffer objects would register/deregister
themselves at creation/destruction. This registry can then be used as a cache of
already opened files from which retrieve a reference to the MemoryBuffer itself
using the file path. These new interfaces would be opt-in for all users of
MemoryBuffer.
>
> Back to my case,  the new AsmPrinterHandler could now use the
MemoryBufferRegistry of the LLVM context. If there is none or the memory buffer
associated to a file path has been already deregistered (or was never registered
e.g. because we are using a .ll file directly), it would open the file as usual,
otherwise it would reuse the registered MemoryBuffer.
>
> I see a few downsides of this approach, though.
>
> It overlaps a bit with the existing SourceManager in clang which already
does some caching work through the clang::ContentCache class. At first the cache
seems hard to abstract away as it uses clang::FileEntry and looks pretty
tailored for clang needs.
I don't see why this is hard to abstract? As you describe it, it sounds 
like we just need to define some callback that can be implemented by the 
FE. For example, we could add the following to LLVMContext:

typedef ErrorOr<MemoryBufferRef> (*FileBufferCallbackTy)(StringRef 
FileName, void *Context);

void setFileBufferCallback(FileBufferCallbackTy Callback, void *Context 
= nullptr);
FileBufferCallbackTy getFileBufferCallback() const;

Using this, we can create an additional abstraction in LLVM that will 
open a given file and return a memory buffer, asking the FE first if 
there's a callback, or otherwise just opening the file directly.
>
> Also, assuming that the front end is using a MemoryBuffer may be a too
strong requirement, in particular for FE's that are mostly unaware of LLVM
except for a final LLVM codegen pass. This would mean that the files would be
reopened even if they are already in the memory of the FE.
Given that you can construct a MemoryBuffer from a StringRef, and a 
StringRef can be constructed around an arbitrary buffer, if we wanted to 
create a wrapper around this interface usable by a less-LLVM-aware FE, 
that seems possible. I don't know how important this is right now.
>
> Finally the file path may not even be a good identifier to reuse
MemoryBuffer objects.
It might not be, but I think that we can make clear that the keys used 
are intended to correspond to the file names from the debug info (or 
module name).

Thanks again,
Hal
>
> Thoughts?
>
> Thank you,
> Roger
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

> On Mar 16, 2017, at 10:29 AM, Roger Ferrer Ibanez via llvm-dev <llvm-dev
at lists.llvm.org> wrote:
> 
> Hi all,
> 
> I'm implementing interleaved source in assembly output. Early reviews
raised the concern
Is there a patch up for review?
I’m wondering how is the frontend enabling this interleaved output mode?

— 
Mehdi

> that the current implementation will be opening files (using a
llvm::MemoryBuffer) that are likely to be in the memory of the front end
(commonly clang but I think we want this to be front end agnostic).
> 
> I'm now exploring ideas to avoid reopening files and let LLVM reuse the
files the FE had to open.
> 
> I am assuming that the front end will use llvm::MemoryBuffer (e.g.: clang
does indirectly through clang::SourceManager).
> 
> So for buffers related to named files (including stdin, which does not have
name and is handled in a special way) we could have in the LLVM context a
MemoryBufferRegistry. The idea is to add new creators of MemoryBuffer (the ones
that work on named files and stdin) that can be passed a reference to that
llvm::MemoryBufferRegistry. MemoryBuffer objects would register/deregister
themselves at creation/destruction. This registry can then be used as a cache of
already opened files from which retrieve a reference to the MemoryBuffer itself
using the file path. These new interfaces would be opt-in for all users of
MemoryBuffer.
> 
> Back to my case,  the new AsmPrinterHandler could now use the
MemoryBufferRegistry of the LLVM context. If there is none or the memory buffer
associated to a file path has been already deregistered (or was never registered
e.g. because we are using a .ll file directly), it would open the file as usual,
otherwise it would reuse the registered MemoryBuffer.
> 
> I see a few downsides of this approach, though.
> 
> It overlaps a bit with the existing SourceManager in clang which already
does some caching work through the clang::ContentCache class. At first the cache
seems hard to abstract away as it uses clang::FileEntry and looks pretty
tailored for clang needs.
> 
> Also, assuming that the front end is using a MemoryBuffer may be a too
strong requirement, in particular for FE's that are mostly unaware of LLVM
except for a final LLVM codegen pass. This would mean that the files would be
reopened even if they are already in the memory of the FE.
> 
> Finally the file path may not even be a good identifier to reuse
MemoryBuffer objects.
> 
> Thoughts?
> 
> Thank you,
> Roger
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

> On Mar 16, 2017, at 4:22 PM, Mehdi Amini via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
>> 
>> On Mar 16, 2017, at 10:29 AM, Roger Ferrer Ibanez via llvm-dev
<llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
wrote:
>> 
>> Hi all,
>> 
>> I'm implementing interleaved source in assembly output. Early
reviews raised the concern
> 
> Is there a patch up for review?
> I’m wondering how is the frontend enabling this interleaved output mode?
> 
>> that the current implementation will be opening files (using a
llvm::MemoryBuffer) that are likely to be in the memory of the front end
(commonly clang but I think we want this to be front end agnostic).
If we mmap files, is it a problem if a process mmap two times the same file? I
was assuming this to be “free”. (I’d like to make sure there is a real issue to
solve here).

— 
Mehdi


>> 
>> I'm now exploring ideas to avoid reopening files and let LLVM reuse
the files the FE had to open.
>> 
>> I am assuming that the front end will use llvm::MemoryBuffer (e.g.:
clang does indirectly through clang::SourceManager).
>> 
>> So for buffers related to named files (including stdin, which does not
have name and is handled in a special way) we could have in the LLVM context a
MemoryBufferRegistry. The idea is to add new creators of MemoryBuffer (the ones
that work on named files and stdin) that can be passed a reference to that
llvm::MemoryBufferRegistry. MemoryBuffer objects would register/deregister
themselves at creation/destruction. This registry can then be used as a cache of
already opened files from which retrieve a reference to the MemoryBuffer itself
using the file path. These new interfaces would be opt-in for all users of
MemoryBuffer.
>> 
>> Back to my case,  the new AsmPrinterHandler could now use the
MemoryBufferRegistry of the LLVM context. If there is none or the memory buffer
associated to a file path has been already deregistered (or was never registered
e.g. because we are using a .ll file directly), it would open the file as usual,
otherwise it would reuse the registered MemoryBuffer.
>> 
>> I see a few downsides of this approach, though.
>> 
>> It overlaps a bit with the existing SourceManager in clang which
already does some caching work through the clang::ContentCache class. At first
the cache seems hard to abstract away as it uses clang::FileEntry and looks
pretty tailored for clang needs.
>> 
>> Also, assuming that the front end is using a MemoryBuffer may be a too
strong requirement, in particular for FE's that are mostly unaware of LLVM
except for a final LLVM codegen pass. This would mean that the files would be
reopened even if they are already in the memory of the FE.
>> 
>> Finally the file path may not even be a good identifier to reuse
MemoryBuffer objects.
>> 
>> Thoughts?
>> 
>> Thank you,
>> Roger
>> 
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
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On 03/16/2017 06:22 PM, Mehdi Amini via llvm-dev wrote:
>> On Mar 16, 2017, at 10:29 AM, Roger Ferrer Ibanez via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>>
>> Hi all,
>>
>> I'm implementing interleaved source in assembly output. Early
reviews raised the concern
> Is there a patch up for review?
https://reviews.llvm.org/D30898
https://reviews.llvm.org/D30897
> I’m wondering how is the frontend enabling this interleaved output mode?
I'm not sure this has been really discussed in the review yet (although 
there certainly is a proposed mechanism in the patches).

  -Hal
>
> —
> Mehdi
>
>
>> that the current implementation will be opening files (using a
llvm::MemoryBuffer) that are likely to be in the memory of the front end
(commonly clang but I think we want this to be front end agnostic).
>>
>> I'm now exploring ideas to avoid reopening files and let LLVM reuse
the files the FE had to open.
>>
>> I am assuming that the front end will use llvm::MemoryBuffer (e.g.:
clang does indirectly through clang::SourceManager).
>>
>> So for buffers related to named files (including stdin, which does not
have name and is handled in a special way) we could have in the LLVM context a
MemoryBufferRegistry. The idea is to add new creators of MemoryBuffer (the ones
that work on named files and stdin) that can be passed a reference to that
llvm::MemoryBufferRegistry. MemoryBuffer objects would register/deregister
themselves at creation/destruction. This registry can then be used as a cache of
already opened files from which retrieve a reference to the MemoryBuffer itself
using the file path. These new interfaces would be opt-in for all users of
MemoryBuffer.
>>
>> Back to my case,  the new AsmPrinterHandler could now use the
MemoryBufferRegistry of the LLVM context. If there is none or the memory buffer
associated to a file path has been already deregistered (or was never registered
e.g. because we are using a .ll file directly), it would open the file as usual,
otherwise it would reuse the registered MemoryBuffer.
>>
>> I see a few downsides of this approach, though.
>>
>> It overlaps a bit with the existing SourceManager in clang which
already does some caching work through the clang::ContentCache class. At first
the cache seems hard to abstract away as it uses clang::FileEntry and looks
pretty tailored for clang needs.
>>
>> Also, assuming that the front end is using a MemoryBuffer may be a too
strong requirement, in particular for FE's that are mostly unaware of LLVM
except for a final LLVM codegen pass. This would mean that the files would be
reopened even if they are already in the memory of the FE.
>>
>> Finally the file path may not even be a good identifier to reuse
MemoryBuffer objects.
>>
>> Thoughts?
>>
>> Thank you,
>> Roger
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory