[cfe-dev] [llvm-dev] Sharing MemoryBuffers between front ends and LLVM

[Hal Finkel via cfe-dev]

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
>>
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-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory