llvm dev - Jun 2018 - How to force an unused function declaration in clang

It does, when the function has a body. When it doesn't, it ignores
<https://godbolt.org/g/2BCvht>. The body might be provided later on in the
toolchain via linking a library.

Regards,
Soham Sinha
PhD Student, Department of Computer Science
Boston University


On Tue, Jun 26, 2018 at 10:25 AM Hans Wennborg <hans at chromium.org>
wrote:
> It works for me:
>
> https://godbolt.org/g/3QzqAH
>
> Can you share an example?
>
> On Tue, Jun 26, 2018 at 5:21 PM, Soham Sinha via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > For the same reason GCC allowed the attribute. Even if I want to
> use/inline
> > a function later on in the pipeline.
> >
> > Regards,
> > Soham Sinha
> > PhD Student, Department of Computer Science
> > Boston University
> >
> >
> > On Tue, Jun 26, 2018 at 8:30 AM mayuyu.io <admin at mayuyu.io>
wrote:
> >>
> >> Out of curiosity, how does an unused declaration affect the
emitted
> object
> >> file
> >>
> >> Zhang
> >>
> >> > 在 2018年6月26日，08:48，Soham Sinha via llvm-dev <llvm-dev at
lists.llvm.org>
> >> > 写道：
> >> >
> >> > clang doesn't seem to respect __attribute__((used)) in C
functions.
> Even
> >> > if I declare a function like the following:  
__attribute__((used))
> void
> >> > function(), then also it doesn't declare the function in
its IR file
> if I
> >> > don't use the function. Is there any other way to force
the
> declaration of
> >> > "unused" function declarations with clang.
> >> >
> >> > I have hacked in clang 6.0.0 in
> >> > tools/clang/lib/CodeGen/CodeGenModule.cpp file. I changed in
> EmitGlobal
> >> > function to detect if the function has "used"
attribute and then if
> it has
> >> > then use "GetOrCreateLLVMFunction" this to create
the function
> declaration.
> >> > However, this hack fails in some other cases. Therefore I am
looking
> for a
> >> > proper way to do this.
> >> >
> >> > Regards,
> >> > Soham Sinha
> >> > PhD Student, Department of Computer Science
> >> > Boston University
> >> > _______________________________________________
> >> > LLVM Developers mailing list
> >> > llvm-dev at lists.llvm.org
> >> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >>
> >>
> >>
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
>
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I realize that gcc also doesn't provide <https://godbolt.org/g/ismnEj>
this
feature if the function doesn't have a body. I was wondering if there is
any elegant way to achieve this, instead of the "hack" I mentioned
earlier.

Regards,
Soham Sinha
PhD Student, Department of Computer Science
Boston University


On Tue, Jun 26, 2018 at 10:58 AM Soham Sinha <soham1 at bu.edu> wrote:
> It does, when the function has a body. When it doesn't, it ignores
> <https://godbolt.org/g/2BCvht>. The body might be provided later on
in
> the toolchain via linking a library.
>
> Regards,
> Soham Sinha
> PhD Student, Department of Computer Science
> Boston University
>
>
> On Tue, Jun 26, 2018 at 10:25 AM Hans Wennborg <hans at chromium.org>
wrote:
>
>> It works for me:
>>
>> https://godbolt.org/g/3QzqAH
>>
>> Can you share an example?
>>
>> On Tue, Jun 26, 2018 at 5:21 PM, Soham Sinha via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>> > For the same reason GCC allowed the attribute. Even if I want to
>> use/inline
>> > a function later on in the pipeline.
>> >
>> > Regards,
>> > Soham Sinha
>> > PhD Student, Department of Computer Science
>> > Boston University
>> >
>> >
>> > On Tue, Jun 26, 2018 at 8:30 AM mayuyu.io <admin at
mayuyu.io> wrote:
>> >>
>> >> Out of curiosity, how does an unused declaration affect the
emitted
>> object
>> >> file
>> >>
>> >> Zhang
>> >>
>> >> > 在 2018年6月26日，08:48，Soham Sinha via llvm-dev <llvm-dev
at lists.llvm.org
>> >
>> >> > 写道：
>> >> >
>> >> > clang doesn't seem to respect __attribute__((used))
in C functions.
>> Even
>> >> > if I declare a function like the following:  
__attribute__((used))
>> void
>> >> > function(), then also it doesn't declare the function
in its IR file
>> if I
>> >> > don't use the function. Is there any other way to
force the
>> declaration of
>> >> > "unused" function declarations with clang.
>> >> >
>> >> > I have hacked in clang 6.0.0 in
>> >> > tools/clang/lib/CodeGen/CodeGenModule.cpp file. I changed
in
>> EmitGlobal
>> >> > function to detect if the function has "used"
attribute and then if
>> it has
>> >> > then use "GetOrCreateLLVMFunction" this to
create the function
>> declaration.
>> >> > However, this hack fails in some other cases. Therefore I
am looking
>> for a
>> >> > proper way to do this.
>> >> >
>> >> > Regards,
>> >> > Soham Sinha
>> >> > PhD Student, Department of Computer Science
>> >> > Boston University
>> >> > _______________________________________________
>> >> > LLVM Developers mailing list
>> >> > llvm-dev at lists.llvm.org
>> >> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >>
>> >>
>> >>
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >
>>
>
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Define a variable that takes the address of the otherwise-unused function.
--paulr

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Soham
Sinha via llvm-dev
Sent: Tuesday, June 26, 2018 12:20 PM
To: hans at chromium.org
Cc: via llvm-dev
Subject: Re: [llvm-dev] How to force an unused function declaration in clang

I realize that gcc also doesn't provide<https://godbolt.org/g/ismnEj>
this feature if the function doesn't have a body. I was wondering if there
is any elegant way to achieve this, instead of the "hack" I mentioned
earlier.

Regards,
Soham Sinha
PhD Student, Department of Computer Science
Boston University


On Tue, Jun 26, 2018 at 10:58 AM Soham Sinha <soham1 at
bu.edu<mailto:soham1 at bu.edu>> wrote:
It does, when the function has a body. When it doesn't, it
ignores<https://godbolt.org/g/2BCvht>. The body might be provided later on
in the toolchain via linking a library.

Regards,
Soham Sinha
PhD Student, Department of Computer Science
Boston University


On Tue, Jun 26, 2018 at 10:25 AM Hans Wennborg <hans at
chromium.org<mailto:hans at chromium.org>> wrote:
It works for me:

https://godbolt.org/g/3QzqAH

Can you share an example?

On Tue, Jun 26, 2018 at 5:21 PM, Soham Sinha via llvm-dev
<llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
wrote:
> For the same reason GCC allowed the attribute. Even if I want to use/inline
> a function later on in the pipeline.
>
> Regards,
> Soham Sinha
> PhD Student, Department of Computer Science
> Boston University
>
>
> On Tue, Jun 26, 2018 at 8:30 AM mayuyu.io<http://mayuyu.io> <admin
at mayuyu.io<mailto:admin at mayuyu.io>> wrote:
>>
>> Out of curiosity, how does an unused declaration affect the emitted
object
>> file
>>
>> Zhang
>>
>> > 在 2018年6月26日，08:48，Soham Sinha via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
>> > 写道：
>> >
>> > clang doesn't seem to respect __attribute__((used)) in C
functions. Even
>> > if I declare a function like the following:  
__attribute__((used)) void
>> > function(), then also it doesn't declare the function in its
IR file if I
>> > don't use the function. Is there any other way to force the
declaration of
>> > "unused" function declarations with clang.
>> >
>> > I have hacked in clang 6.0.0 in
>> > tools/clang/lib/CodeGen/CodeGenModule.cpp file. I changed in
EmitGlobal
>> > function to detect if the function has "used" attribute
and then if it has
>> > then use "GetOrCreateLLVMFunction" this to create the
function declaration.
>> > However, this hack fails in some other cases. Therefore I am
looking for a
>> > proper way to do this.
>> >
>> > Regards,
>> > Soham Sinha
>> > PhD Student, Department of Computer Science
>> > Boston University
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org<mailto:llvm-dev at
lists.llvm.org>
>> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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