[cfe-dev] [RFC] A C++ pseudo parser for tooling

[David Blaikie via cfe-dev]

On Tue, Nov 9, 2021 at 8:38 PM Andrew Tomazos via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On Tue, Nov 9, 2021 at 7:28 PM Sam McCall via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> On Tue, Nov 9, 2021 at 7:30 PM Demi Marie Obenour <demiobenour at gmail.com>
>> wrote:
>>
>>> On 11/9/21 1:03 PM, Sam McCall wrote:
>>> >> Could an index be persisted to disk, reloaded at startup
>>> >
>>> > (Clangd does have this too, FWIW)
>>> >
>>> >
>>> >> and incrementally changed as the user edits?
>>> >
>>> > User edits mean we need to parse new code.
>>> > Absent a pseudoparser, the only thing that parses code is clang.
>>> > Clang needs a full AST, so this step is slow.
>>>
>>> What I meant is that clang could reparse only the code the user has
>>> edited, rather than reparsing the entire source file from scratch.
>>> I believe rust-analyzer does exactly this for Rust source, and my
>>> understanding is that it is considered fast enough.
>>
>>
>
>> Yes, you could do this, but not with clang as the parser.
>>
>
> If it were modified to efficiently persist the salient parser state
> (including partial AST) at checkpoints during translation after every few
> lines (see https://en.wikipedia.org/wiki/Persistent_data_structure), and
> then able to serialize that history and reload from a checkpoint just above
> a modification, then it wouldn't have to start from scratch every time.
> Again, I would spend some time thinking about how pre-compiled headers and
> modules work.
>

To be clear, the folks making this proposal are pretty closely connected to
how PCH and modules work - I think it's fair to assume they've spent quite
a while thinking about how these things work and could be applied.
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