llvm dev - Jan 2019 - Undefined symbols with inline functions using the ORC JIT on Linux

Hi,

I am developing an application that uses the ORC api to JIT compile C++
code using Clang. So far I have done most of the work on Windows, where it
now mostly works as expected. However, when I tried to run my application
on Linux I ran into some problems.

The problem I ran into is that symbols for jitted inline functions cannot
be resolved. Both LLVM and Clang are checked out with latest master branch.

It is probably me that is doing something wrong but I cannot figure out
what it is. Here is what I found so far. In the file RuntimeDyld.cpp in
function loadObjectImpl, a check is made whether a particular function is
weak or not. Since inline functions are weak (as I understood it) an
attempt is made to promote this symbol to a strong definition. But only if
it is present in the "ResponsibilitySet", that check is made on line
273.
However, the inline functions are not in that set so they are not promoted
to strong definitions. Shouldn't functions defined in the jitted code be
our responsibility?

Looking closer what happens with the ResponsibiltySet. When it is created
it tries to to find symbols for all the names it knows about. Eventually
the look up request ends up in my application where I use a
LegacyIRCompileLayer to search for the symbol in the jitted module. That
function call eventually ends up in getSymbol in RTDyldObjectLinkingLayer.h
where the symbol is found but it does not have and address (Address is zero
and Flags is 50). So an instance of JITSymbol is returned to the
LegacyRTDyldObjectLinkingLayer and the findSymbol function which checks if
an valid symbol was found (on line 406 in RTDyldObjectLinkingLayer.h).
Since the address is zero the JITSymbol is deemed to be not valid, via the
bool operator in JITSymbol.h and that is why that particular symbol name
does not end up in the ResponsibilitySet. That is as far as I got, I don't
know enough about LLVM to understand what the problem is (if any).

This issue can be replicated with the clang-interpreter application.

This program does not work, clang-interpreter crashes because it cannot
find the symbol for the Test constructor function.

class Test {
    public: Test() {}
};

int main()
{
    Test test;
    return 0;
}

This program works:

class Test {
    public: Test();
};

Test::Test() { }

int main()
{
    Test test;
    return 0;
}

The first version works on Windows since the inline constructor is not
marked as weak. Can anyone enlighten me on what is happening here? Is this
the expected behavior, and if it is, what am I doing wrong?

Btw, I also tried various compiler flags (fno-inline and
fno-inline-functions) but those do not help in this case.

//Martin
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20190104/1739ed0e/attachment.html>

Hi Martin
> However, the inline functions are not in that set so they are not
> promoted to strong definitions. Shouldn't functions defined in the
> jitted code be our responsibility?
> [...]
> This program does not work, clang-interpreter crashes because it
> cannot find the symbol for the Test constructor function.
>
> class Test {
>     public: Test() {}
> };
>
> int main()
> {
>     Test test;
>     return 0;
> }
You could compile your example to bitcode and run it with lli. This will
provide more information and the issue may be discussed easier on
bitcode level. (Not very familiar with clang-interpreter, but it looks
more like an example for illustration than a bulletproof tool.)
> Looking closer what happens with the ResponsibiltySet. When it is
> created it tries to to find symbols for all the names it knows about.
> Eventually the look up request ends up in my application [...]
Instead of using the legacy resolvers you might prefer a fallback symbol
generator here. The lli tool uses this approach to provide symbols from
the host process for the JITed code (see:
https://github.com/llvm-mirror/llvm/blob/8ffa038b3ef4448af8bf31f6c50281779939c774/tools/lli/lli.cpp#L807).

Hope it helps
Stefan

Am 04.01.19 um 18:49 schrieb Martin Andersson via
llvm-dev:
> Hi,
>
> I am developing an application that uses the ORC api to JIT compile
> C++ code using Clang. So far I have done most of the work on Windows,
> where it now mostly works as expected. However, when I tried to run my
> application on Linux I ran into some problems.
>
> The problem I ran into is that symbols for jitted inline functions
> cannot be resolved. Both LLVM and Clang are checked out with latest
> master branch.
>
> It is probably me that is doing something wrong but I cannot figure
> out what it is. Here is what I found so far. In the file
> RuntimeDyld.cpp in function loadObjectImpl, a check is made whether a
> particular function is weak or not. Since inline functions are weak
> (as I understood it) an attempt is made to promote this symbol to a
> strong definition. But only if it is present in the
> "ResponsibilitySet", that check is made on line 273. However, the
> inline functions are not in that set so they are not promoted to
> strong definitions. Shouldn't functions defined in the jitted code be
> our responsibility?
>
> Looking closer what happens with the ResponsibiltySet. When it is
> created it tries to to find symbols for all the names it knows about.
> Eventually the look up request ends up in my application where I use a
> LegacyIRCompileLayer to search for the symbol in the jitted module.
> That function call eventually ends up in getSymbol in
> RTDyldObjectLinkingLayer.h where the symbol is found but it does not
> have and address (Address is zero and Flags is 50). So an instance of
> JITSymbol is returned to the LegacyRTDyldObjectLinkingLayer and the
> findSymbol function which checks if an valid symbol was found (on line
> 406 in RTDyldObjectLinkingLayer.h). Since the address is zero the
> JITSymbol is deemed to be not valid, via the bool operator in
> JITSymbol.h and that is why that particular symbol name does not end
> up in the ResponsibilitySet. That is as far as I got, I don't know
> enough about LLVM to understand what the problem is (if any).
>
> This issue can be replicated with the clang-interpreter application.
>
> This program does not work, clang-interpreter crashes because it
> cannot find the symbol for the Test constructor function.
>
> class Test {
>     public: Test() {}
> };
>
> int main()
> {
>     Test test;
>     return 0;
> }
>
> This program works:
>
> class Test {
>     public: Test();
> };
>
> Test::Test() { }
>
> int main()
> {
>     Test test;
>     return 0;
> }
>
> The first version works on Windows since the inline constructor is not
> marked as weak. Can anyone enlighten me on what is happening here? Is
> this the expected behavior, and if it is, what am I doing wrong?
>
> Btw, I also tried various compiler flags (fno-inline and
> fno-inline-functions) but those do not help in this case.
>
> //Martin
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20190104/f2e53e73/attachment.html>

Hi Stefan,

Thanks for your reply. I tried running my simple example on Linux using lli
and it does work fine. So I think the best long-term solution is to migrate
my code to the new lazy orc jit. Unfortunately, even the simplest example
does not work on Windows:
int main() { return 0; }

This is the output:
JITDylib "<main>" (ES: 0x000001b6e4ad3670):
Search order: [ ("<main>", all) ]
Symbol table:
    "__cxa_atexit": <not resolved> ( Lazy (MU=0x1b6e4ae3110),
[Data] )
    "main": <not resolved> ( Lazy (MU=0x1b6e4ae7f10), [Callable]
)
    "__dso_handle": <not resolved> ( Lazy (MU=0x1b6e4ae3110),
[Data] )

I run with these arguments, and I checked and -fno-use-cxa-atexit is set by
clang:
clang -S -emit-llvm main.cpp
lli -jit-kind=orc-lazy main.ll

Using mcjit and orc-mcjit as the jit-kind in lli works fine on Windows.

I will keep investigating how to best proceed, thanks for pointing me in
the right direction.

//Martin

On Fri, Jan 4, 2019 at 10:49 PM Stefan Gränitz <stefan.graenitz at
gmail.com>
wrote:
> Hi Martin
>
> However, the inline functions are not in that set so they are not promoted
> to strong definitions. Shouldn't functions defined in the jitted code
be
> our responsibility?
> [...]
> This program does not work, clang-interpreter crashes because it cannot
> find the symbol for the Test constructor function.
>
> class Test {
>     public: Test() {}
> };
>
> int main()
> {
>     Test test;
>     return 0;
> }
>
> You could compile your example to bitcode and run it with lli. This will
> provide more information and the issue may be discussed easier on bitcode
> level. (Not very familiar with clang-interpreter, but it looks more like an
> example for illustration than a bulletproof tool.)
>
> Looking closer what happens with the ResponsibiltySet. When it is created
> it tries to to find symbols for all the names it knows about. Eventually
> the look up request ends up in my application [...]
>
> Instead of using the legacy resolvers you might prefer a fallback symbol
> generator here. The lli tool uses this approach to provide symbols from the
> host process for the JITed code (see:
>
https://github.com/llvm-mirror/llvm/blob/8ffa038b3ef4448af8bf31f6c50281779939c774/tools/lli/lli.cpp#L807
> ).
>
> Hope it helps
> Stefan
>
> Am 04.01.19 um 18:49 schrieb Martin Andersson via llvm-dev:
>
> Hi,
>
> I am developing an application that uses the ORC api to JIT compile C++
> code using Clang. So far I have done most of the work on Windows, where it
> now mostly works as expected. However, when I tried to run my application
> on Linux I ran into some problems.
>
> The problem I ran into is that symbols for jitted inline functions cannot
> be resolved. Both LLVM and Clang are checked out with latest master branch.
>
> It is probably me that is doing something wrong but I cannot figure out
> what it is. Here is what I found so far. In the file RuntimeDyld.cpp in
> function loadObjectImpl, a check is made whether a particular function is
> weak or not. Since inline functions are weak (as I understood it) an
> attempt is made to promote this symbol to a strong definition. But only if
> it is present in the "ResponsibilitySet", that check is made on
line 273.
> However, the inline functions are not in that set so they are not promoted
> to strong definitions. Shouldn't functions defined in the jitted code
be
> our responsibility?
>
> Looking closer what happens with the ResponsibiltySet. When it is created
> it tries to to find symbols for all the names it knows about. Eventually
> the look up request ends up in my application where I use a
> LegacyIRCompileLayer to search for the symbol in the jitted module. That
> function call eventually ends up in getSymbol in RTDyldObjectLinkingLayer.h
> where the symbol is found but it does not have and address (Address is zero
> and Flags is 50). So an instance of JITSymbol is returned to the
> LegacyRTDyldObjectLinkingLayer and the findSymbol function which checks if
> an valid symbol was found (on line 406 in RTDyldObjectLinkingLayer.h).
> Since the address is zero the JITSymbol is deemed to be not valid, via the
> bool operator in JITSymbol.h and that is why that particular symbol name
> does not end up in the ResponsibilitySet. That is as far as I got, I
don't
> know enough about LLVM to understand what the problem is (if any).
>
> This issue can be replicated with the clang-interpreter application.
>
> This program does not work, clang-interpreter crashes because it cannot
> find the symbol for the Test constructor function.
>
> class Test {
>     public: Test() {}
> };
>
> int main()
> {
>     Test test;
>     return 0;
> }
>
> This program works:
>
> class Test {
>     public: Test();
> };
>
> Test::Test() { }
>
> int main()
> {
>     Test test;
>     return 0;
> }
>
> The first version works on Windows since the inline constructor is not
> marked as weak. Can anyone enlighten me on what is happening here? Is this
> the expected behavior, and if it is, what am I doing wrong?
>
> Btw, I also tried various compiler flags (fno-inline and
> fno-inline-functions) but those do not help in this case.
>
> //Martin
>
>
> _______________________________________________
> LLVM Developers mailing listllvm-dev at
lists.llvm.orghttp://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20190105/5ed65281/attachment.html>