[cfe-dev] Module build - tokenized form of intermediate source stream

[Richard Smith via cfe-dev]

On Tue, Oct 13, 2015 at 5:55 AM, Serge Pavlov <sepavloff at gmail.com> wrote:

> 2015-10-13 8:52 GMT+06:00 Sean Silva <chisophugis at gmail.com>:
>
>> On Mon, Oct 12, 2015 at 12:13 PM, Richard Smith via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>>> On Mon, Oct 12, 2015 at 11:33 AM, Serge Pavlov via cfe-dev <
>>> cfe-dev at lists.llvm.org> wrote:
>>>
>>>> Hi all,
>>>>
>>>> Now building a module involves creation of intermediate source streams
>>>> that includes/imports each header composing the  module. This source stream
>>>> is then parsed as if it were a source file. So to build a module several
>>>> transformations must be done:
>>>> - Module map is parsed to produce module objects(clang::Module),
>>>> - Module objects are used to build source stream (llvm::MemoryBuffer),
>>>> which contains include directives,
>>>> - The source stream is parsed to produce module content.
>>>>
>>>> The build process could be simpler, if instead of text source stream we
>>>> prepared a sequence of annotation tokens, annot_module_begin,
>>>> annot_module_end and some new token, say annot_module_header, which
>>>> represented a header of a module. It would be something like pretokenized
>>>> header but without a counterpart in file system.
>>>>
>>>> Such redesign would help in solving performance degradation reported in
>>>> PR24667 ([Regression] Quadratic module build time due to
>>>> Preprocessor::LeaveSubmodule). The reason of the problem is leaving module
>>>> after each header, even if the next header is of the same module.
>>>>
>>>
>>> We generally recommend that each header goes in its own submodule, so
>>> optimizing for this case doesn't address the problem for a lot of cases.
>>>
>>
> These are different use cases and there is nothing bad if the problem will
> be solved with different means. If a user follow this recommendation and
> puts each header into a separate module, he won't suffer from the tokenized
> form of the intermediate input stream. If the user chooses to put many
> headers into one module, this change can solve the problem. The cited PR
> refers to just the latter case.
>

I think you're missing my point. We seem to have a choice between a general
solution that addresses the problem in all cases, and a solution that only
helps for the "one big module with no submodules" case (which is not the
case that you get for, say, an umbrella directory module / umbrella header
/ libc++ / Darwin's libc / ...). If these solutions don't have drastically
different technical complexity, the former seems like the better choice.

I'm not opposed to providing a token sequence rather than text for the
synthesized module umbrella header, but we'd need a reasonably strong
argument to justify the added complexity, especially as we still need our
current mode to handle umbrella headers on the file system, #includes
within modular headers, and so on. If we want something like that, a
simpler approach might be to add a pragma for starting / ending a module,
and emit that into the header file we synthesize, and then teach
PPLexerChange not to do the extra work when switching modules if the source
and destination module are actually the same.


> The "one huge submodule" approach with no local visibility is actually
>> very useful to have because it (for better or for worse) is very close to
>> the semantics of PCH (which are very simple). This makes it a nice
>> incremental step away from PCH and very easy to understand.
>>
>> Also, I think "we generally recommend" is a bit strong considering that
>> this isn't documented anywhere to my knowledge. In fact, the documentation
>> I've written internally for my customers recommends the exact opposite for
>> the reason described above.
>>
>>
> This very convenient for users. Usually it is much simpler to write
> something like #include "clang.h" instead of listing dozen of includes.
> When API is distributed by many headers, a user must determine first where
> the necessary piece is declared. In pre-module era splitting API was
> unavoidable evil, as it reduced compile time. With modules we can enable
> more convenient solutions.
>

I agree, but that seems to me that this should be the choice of the user of
the API. If they want to import all of the Clang API, that should work (and
if you add an umbrella "clang.h" header, it will work), but if they just
#include some small part of that interface, should they really get the
whole thing?

-- Sean Silva
>>
>>
>>>
>>> Leaving module after the last header would be a solution but it is
>>>> problematic to reveal if the header just parsed is the last one, - there is
>>>> no such thing as look ahead of the next include directive. Using tokenized
>>>> input would mark module ends easily.
>>>>
>>>
>>> I have a different approach in mind for that case: namely, to produce a
>>> separate submodule state for distinct submodules even when not in local
>>> visibility mode, and lazily populate its Macros map when identifiers are
>>> queried. That way, the performance is linear in the number of macros the
>>> submodule actually defines or uses, not in the total number defined or used
>>> by the top-level module.
>>>
>>>
>>>> Is there any reason why textual form of the intermediate source stream
>>>> should be kept? Does implementing tokenized form of it make sense?
>>>>
>>>> Thanks,
>>>> --Serge
>>>>
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>>>>
>>>>
>>>
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>>
>
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