ChuanqiXu added inline comments.
================
Comment at: clang/test/CXX/modules-ts/basic/basic.def.odr/p4/module.cpp:5
// CHECK-DAG: @extern_var_exported = external {{(dso_local )?}}global
-// CHECK-DAG: @inline_var_exported = linkonce_odr {{(dso_local )?}}global
+// CHECK-DAG: @inline_var_exported = available_externally {{(dso_local
)?}}global
// CHECK-DAG: @const_var_exported = available_externally {{(dso_local
)?}}constant i32 3,
----------------
urnathan wrote:
> ChuanqiXu wrote:
> > dblaikie wrote:
> > > ChuanqiXu wrote:
> > > > ChuanqiXu wrote:
> > > > > urnathan wrote:
> > > > > > ChuanqiXu wrote:
> > > > > > > urnathan wrote:
> > > > > > > > ChuanqiXu wrote:
> > > > > > > > > urnathan wrote:
> > > > > > > > > > I don;t think this is correct. That should still be a
> > > > > > > > > > linkonce odr, otherwise you'll get conflicts with other
> > > > > > > > > > module implementation units.
> > > > > > > > > It is still linkonce_odr in the module it get defined. See
> > > > > > > > > the new added test case: inline-variable-in-module.cpp for
> > > > > > > > > example. The attribute `available_externally` is equivalent
> > > > > > > > > to external from the perspective of linker. See
> > > > > > > > > https://llvm.org/docs/LangRef.html#linkage-types. According
> > > > > > > > > to [dcl.inline]p7, inline variable attached to named module
> > > > > > > > > should be defined in that domain. Note that the variable
> > > > > > > > > attached to global module fragment and private module
> > > > > > > > > fragment shouldn't be accessed outside the module, so it
> > > > > > > > > implies that all the variable defined in the module could
> > > > > > > > > only be defined in the module unit itself.
> > > > > > > > There's a couple of issues with this. module.cppm is emitting
> > > > > > > > a (linkonce) definition of inlne_var_exported, but only because
> > > > > > > > it itself is ODR-using that variable. If you take out the
> > > > > > > > ODR-use in noninline_exported, there is no longer a symbol
> > > > > > > > emitted.
> > > > > > > >
> > > > > > > > But, even if you forced inline vars to be emitted in their
> > > > > > > > defining-module's interface unit, that would be an ABI change.
> > > > > > > > inline vars are emitted whereever ODR-used. They have no fixed
> > > > > > > > home TU. Now, we could alter the ABI and allow interface units
> > > > > > > > to define a home location for inline vars and similar entities
> > > > > > > > (eg, vtables for keyless classes). But we'd need buy-in from
> > > > > > > > other compilers to do that.
> > > > > > > >
> > > > > > > > FWIW such a discussion did come up early in implementing
> > > > > > > > modules-ts, but we decided there was enough going on just
> > > > > > > > getting the TS implemented. I'm fine with revisiting that, but
> > > > > > > > it is a more significant change.
> > > > > > > >
> > > > > > > > And it wouldn't apply to (eg) templated variables, which of
> > > > > > > > course could be instantiated anywhere.
> > > > > > > Oh, now the key point here is what the correct behavior is
> > > > > > > instead of the patch. Let's discuss it first.
> > > > > > >
> > > > > > > According to [[ http://eel.is/c++draft/dcl.inline#7 |
> > > > > > > [dcl.inline]p7 ]],
> > > > > > > > If an inline function or variable that is attached to a named
> > > > > > > > module is declared in a definition domain, it shall be defined
> > > > > > > > in that domain.
> > > > > > >
> > > > > > > I think the intention of the sentence is to define inline
> > > > > > > variable in the module interface. So if it is required by the
> > > > > > > standard, I think other compiler need to follow up. As I
> > > > > > > described in the summary, it might be a difference between C++20
> > > > > > > module and ModuleTS. Do you think it is necessary to send the
> > > > > > > question to WG21? (I get the behavior from reading the words.
> > > > > > > Maybe I misread or the word is not intentional).
> > > > > > >
> > > > > > > Maybe the ABI standard group need to discuss what the linkage
> > > > > > > should be. Now it may be weak_odr or linkonce_odr. It depends on
> > > > > > > how we compile the file. If we compile the .cppm file directly,
> > > > > > > it would be linkonce_odr. And if we compile it to *.pcm file
> > > > > > > first, it would be weak_odr. I have registered an issue for this:
> > > > > > > https://github.com/llvm/llvm-project/issues/53838.
> > > > > > > Oh, now the key point here is what the correct behavior is
> > > > > > > instead of the patch. Let's discuss it first.
> > > > > > >
> > > > > > > According to [[ http://eel.is/c++draft/dcl.inline#7 |
> > > > > > > [dcl.inline]p7 ]],
> > > > > > > > If an inline function or variable that is attached to a named
> > > > > > > > module is declared in a definition domain, it shall be defined
> > > > > > > > in that domain.
> > > > > > >
> > > > > > > I think the intention of the sentence is to define inline
> > > > > > > variable in the module interface. So if it is required by the
> > > > > > > standard, I think other compiler need to follow up. As I
> > > > > > > described in the summary, it might be a difference between C++20
> > > > > > > module and ModuleTS. Do you think it is necessary to send the
> > > > > > > question to WG21? (I get the behavior from reading the words.
> > > > > > > Maybe I misread or the word is not intentional).
> > > > > >
> > > > > > You are reading more into the std than it says. The std specifies
> > > > > > what /source code/ is meaningful. It says nothing about how a
> > > > > > computation system might represent the program in another form.
> > > > > > Most of the latter, for ahead-of-time translation, is at the
> > > > > > discretion of compiler implementors. Part of that is the domain of
> > > > > > the ABI, which specifies an interface to which different compilers
> > > > > > may target, and then have compatibility at the object-file
> > > > > > boundary.
> > > > > >
> > > > > > > Maybe the ABI standard group need to discuss what the linkage
> > > > > > > should be.
> > > > > >
> > > > > > Correct. And right now there is no consensus to do anything
> > > > > > different with such entities.
> > > > > > The ABI (http://itanium-cxx-abi.github.io/cxx-abi/abi.html) 5.2
> > > > > > documents such vague-linkage entities. That section would need
> > > > > > changes to bless what you are trying to do.
> > > > > >
> > > > > >
> > > > > > You are reading more into the std than it says. The std specifies
> > > > > > what /source code/ is meaningful. It says nothing about how a
> > > > > > computation system might represent the program in another form.
> > > > > > Most of the latter, for ahead-of-time translation, is at the
> > > > > > discretion of compiler implementors. Part of that is the domain of
> > > > > > the ABI, which specifies an interface to which different compilers
> > > > > > may target, and then have compatibility at the object-file boundary.
> > > > >
> > > > > OK, your words make sense. In fact, I don't care much about whether
> > > > > or not could we define `inline variable` in the module unit. The
> > > > > problem I tried to solve is about `the definition static variable in
> > > > > module`. We couldn't run a simple hello world example if we don't
> > > > > solve it.
> > > > >
> > > > > What I care about is where should we define inline function. I want
> > > > > to define inline function in the module unit it get declared. And my
> > > > > theory comes from [dcl.inline]p7. And our experiments show that it is
> > > > > the key reason why module could speed up compilation. Our data shows
> > > > > that the compilation could speed up about 40% for the feature. Since
> > > > > most of the time consumed in compilation spent on the middle end, it
> > > > > is really not significant to save the time in frontend. So it matters
> > > > > a lot if we could avoid compiling same functions in middle end.
> > > > >
> > > > > Originally, I thought I am doing right. But from your words, we
> > > > > couldn't do this until the ABI standard group get in consensus, right?
> > > > >
> > > > > Finally, I feel it is odd about [dcl.inline]p7. Since if it is not
> > > > > for implementors, I feel it is meaningless for users.
> > > > Or given that the CXXABI doesn't discuss the case about inline function
> > > > in named module. Could we think it is a free space? Another question
> > > > maybe where could we ask for opinion? WG21 or Itanium CXXABI group?
> > > > Finally, I feel it is odd about [dcl.inline]p7. Since if it is not for
> > > > implementors, I feel it is meaningless for users.
> > >
> > > Presumably that means that a user can't declare an inline function in a
> > > module, and define it somewhere else (like in a private implementation
> > > unit) - they must define it in the same definition domain it is declared.
> > > That's a concrete requirement for the user, irrespective of what
> > > object-file-level implementation strategy (where the definition gets
> > > emitted, what linkage is used, etc) is used.
> > > Presumably that means that a user can't declare an inline function in a
> > > module, and define it somewhere else (like in a private implementation
> > > unit) - they must define it in the same definition domain it is declared.
> > > That's a concrete requirement for the user, irrespective of what
> > > object-file-level implementation strategy (where the definition gets
> > > emitted, what linkage is used, etc) is used.
> >
> > Oh, I get it. Thanks.
> yes, I understand the problem you are trying to solve (had the same in GCC).
> The issue is with internal-linkage entities in global module fragments.
> Let's consider 3 separate cases.
> a) the GMF is a header-unit.
>
> 1) We could either emit it in a header-unit-specific object file (if
> ODR-used when building that header unit). This would surprise users as now
> we have a thing that is morally a header-file, but comes with an object file.
> That is likely to break build flow and is not what GCC does. There is of
> course a trade off here, in that it's either emitted exactly once, or emitted
> into every TU that ODR uses it. But is that a significant extra burden?
> The only variable case I came across was _ioinit. (There are many static
> inline functions, due to C compatibility, but for those you want to inline
> them anyway.)
>
> 2) Or we could emit it in every TU that directly or indirectly imports the
> header unit and ODR uses the entity. This is what GCC does. in the case of
> _ioinit that means making sure to call its dynamic constructor from the TU's
> initialization function.
>
> 3) Note we do not clone the internal entity within a single TU, once for each
> header or module that we import that itself ODR uses the entity.
>
> b) textual inclusion in the GMF of a module. If the module ODR-uses the
> entity, it needs to be emitted into the object file.
>
> c) Both textual inclusion AND importing of a header-unit. We could emit 2
> copies, or we could merge these two definitions. Merging is better (and what
> GCC does).
>
> The std allows all the alternatives considered above. Does that help?
Thanks for the explanation. It really helps. Since I didn't touch header-unit
before (I mainly focus on named module), I don't have an opinion for (a) and
(c). For the (b), it might not be the case. I think we need to emit it all the
time. Here is the pattern of <iostream> in libstdc++:
```
extern istream cin;
// ...
static ios_base::Init __ioinit;
```
And after including, `__ioinit` is not used in the module unit, but we couldn't
erase it. Otherwise the problem might met segfault. You could find the example
in the issue I linked.
---
In fact, I care more about whether or not should we emit `inline` function body
in module purview to the module unit. But given that this is not the intention
of the patch, let's talk it in other places.
CHANGES SINCE LAST ACTION
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https://reviews.llvm.org/D119409
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