[cfe-dev] Controlling instantiation of templates from PCH

[David Blaikie via cfe-dev]

Thanks Richard - yeah sounds pretty similar though I'm a bit confused about
what's happening in this case, in part because I know next to nothing about
how Clang's PCH works (& especially how it differs from PCM/modules).

Richard: why would any module or PCH cause a subsequent compilation to
perform more pending instantiations? (I would've thought/my understanding
was that nothing in the module would be used if it wasn't referenced from
the source file, so why would a pch cause more pending instantiations?)

Lubos: Could you provide a small standalone example of this increase in
pending instantiations so it's a bit easier for me to understand the kind
of code & what's happening?
You mentioned in the blog post that the use of a PCH causes more functions
to be emitted into the final object file (than if a PCH had not been used,
and the source remained the same). Especially the possibility of functions
being emitted into the object file that are totally unused by the object
file. (again, I'm especially interested in comparing the non-PCH with the
PCH case here, rather than the Clang PCH with the VS PCH situation) - those
are situations that would be very surprising to me.

On Sat, May 25, 2019 at 6:38 PM Richard Smith <richard at metafoo.co.uk> wrote:

> This seems like a nice idea, and has a lot in common with our existing
> "modular codegen" mode, which does largely the same thing but for PCMs
> rather than PCHs. I'd hope we could share a lot of the implementation
> between the two features.
>
> +David Blaikie, who implemented modular codegen and might be able to
> advise as to the best way to integrate similar functionality into our PCH
> support.
>
> On Sat, 25 May 2019 at 12:32, Lubos Lunak via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>>
>>  Hello,
>>
>>  I'm working on a Clang patch that can make C++ builds noticeably faster
>> in
>> some setups by allowing control over how templates are instantiated, but
>> I
>> have some problems finishing it and need advice.
>>
>>  Background: I am a LibreOffice developer. When enabling precompiled
>> headers,
>> e.g. for LO Calc precompiled headers save ~2/3 of build time when MSVC is
>> used, but with Clang they save only ~10%. Moreover the larger the PCH the
>> more time is saved with MSVC, but this is not so with Clang, in fact
>> larger
>> PCHs often make things slower.
>>
>>  The recent -ftime-trace feature allowed me to investigate this and it
>> turns
>> out that the time saved by having to parse less is outweighted by having
>> to
>> instantiate (many) more templates. You can see -ftime-trace graphs at
>>
>> http://llunak.blogspot.com/2019/05/why-precompiled-headers-do-not-improve.html
>> (1nd row - no PCH, 2nd row - small PCH, 3rd row - large PCH), the .json
>> files
>> are at http://ge.tt/7RHeLHw2 if somebody wants to see them.
>>
>>  Specifically, the time is spent in Sema::PerformPendingInstantiations()
>> and
>> Sema::InstantiateFunctionDefinition(). The vast majority of the
>> instantiations comes from the PCH itself. This means that this is
>> performed
>> for every TU using the PCH, and it also means that it's useless work, as
>> the
>> linker will discard all but one copy of that.
>>
>>  My WIP patch implements a new option to avoid that. The idea is that all
>> sources using the PCH will be built with
>> -fpch-template-instantiation=skip,
>> which will prevent Sema::InstantiateFunctionDefinition() from actually
>> instantiating templates coming from the PCH if they would be uneeded
>> duplicates (note that means almost all PCH template instantiations in the
>> case of a developer build with -O0 -g, which is my primary use case).
>> Then
>> one extra source file is built with -fpch-template-instantiation=force,
>> which
>> will provide one copy of instantiations. I assume that this is similar to
>> how
>> MSVC manages to have much better gains with PCH, the .obj created during
>> PCH
>> creation presumably contains single instantiations.
>>
>>  In the -ftime-trace graphs linked above, the 4th row is large PCH with
>> my
>> patch. The compilation time saved by this is 50% and 60% for the two
>> examples
>> (and I think moving some templates into the PCH might get it to 70-75%
>> for
>> the second file).
>>
>>  As I said, I have some problems that prevent the patch from being fully
>> usable, so in order to finish it, could somebody help me with the
>> following:
>>
>> - I don't understand how it is controlled which kind of ctor/dtor is
>> emitted
>> (complete ctor vs base ctor, i.e. C1 vs C2 type in the Itanium ABI). I
>> get
>> many undefined references because the TU built with instances does not
>> have
>> both types, yet other TUs refer to them. How can I force both of them be
>> emitted?
>>
>> - I have an undefined reference to one template function that should be
>> included in the TU with instances, but it isn't. The Sema part
>> instantiates
>> it and I could track it as far as getting generated in Codegen, but then
>> I'm
>> lost. I assume that it gets discarded because something in codegen or
>> llvm
>> considers it unused. Is there a place like that and where is it? Are
>> there
>> other places in codegen/llvm where I could check to see why this function
>> doesn't get generated in the object file?
>>
>> - In Sema::InstantiateFunctionDefinition() the code for extern templates
>> still
>> instantiates a function if it has getContainedAutoType(), so my code
>> should
>> probably also check that. But I'm not sure what that is (is that 'auto
>> foo()
>> { return 1; }' ?) or why that would need an instance in every TU.
>>
>> - I used BENIGN_ENUM_LANGOPT because Clang otherwise complains that the
>> PCH is
>> used with a different option than what it was generated with, which is
>> necessary in this case, but I'm not sure if this is the correct handling
>> of
>> the option.
>>
>> - Is there a simple rule for what decides that a template needs to be
>> instantiated? As far as I can tell, even using a template as a class
>> member
>> or having an inline member function manipulating it doesn't. When I
>> mentioned
>> moving some templates into the PCH in order to get possible 70% savings,
>> I
>> actually don't know how to cause an instantiation from the PCH, the
>> templates
>> and their uses are included there.
>>
>>  Thank you.
>>
>> --
>>  Lubos Lunak
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>
>
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