llvm dev - Sep 2015 - JIT: Mapping global variable in JIT'ted code to variable in running program

Hi,

I think this is probably easiest to explain with code (I only provided the
essentials for clarity):

// begin file jit.cpp

int myglobal;

void printMyGlobal() {
  printf("myglobal: %d\n", myglobal);
}

int main(int argc, char *argv[]) {

  // This file, jit.cpp has been compiled to bitcode (clang -S -emit-llvm
jit.cpp)
  // and is read into Module M here
  Module *M = ...
  ExecutionEngine *ee = ...

  myglobal = 42;

  ee->addGlobalMapping(M->getGlobalVariable("myglobal"),
(void*)&myglobal);
  ee->finalizeObject();

  void(*fptr)() =
(void(*)())ee->getFunctionAddress("printMyGlobal");
  fptr();
  printMyGlobal();
}

// end file jit.cpp

I compile this file (jit.cpp) into bitcode which I then load int M. M  also
contains a global variable called myglobal. I want to "map" the
myglobal
variable in M to the global variable in the running program. The idea is
that the value of myglobal inside the JIT'ted module will also be 42.
However, as it is right now, when I run the code, I get:

myglobal: 0
myglobal: 42

The first is the JIT'ted printMyGlobal() and the second is the AOT'ed
printMyGlobal(). But when I lookup the address of the global variable
myglobal (ee->getGlobalValueAddress()), it is mapped to the correct
address. Similarly, if I lookup ee->getGlobalValueAtAddress(), I get the
expected result.

Am I missing an important step? Is what I am trying to do even possible?

Thanks for the help.

Tarun
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Hi Tarun,

My first thought is that you might be adding a definition (e.g. @x = global
i32 7) to your module, rather than an external declaration (e.g. @x external
global i32). Make sure you have the latter: If you provide a
definition in your module it will shadow any definitions in your mapping,
which would explain the behavior you're seeing.

Cheers,
Lang.


On Mon, Sep 14, 2015 at 2:55 PM, Tarun Prabhu via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi,
>
> I think this is probably easiest to explain with code (I only provided the
> essentials for clarity):
>
> // begin file jit.cpp
>
> int myglobal;
>
> void printMyGlobal() {
>   printf("myglobal: %d\n", myglobal);
> }
>
> int main(int argc, char *argv[]) {
>
>   // This file, jit.cpp has been compiled to bitcode (clang -S -emit-llvm
> jit.cpp)
>   // and is read into Module M here
>   Module *M = ...
>   ExecutionEngine *ee = ...
>
>   myglobal = 42;
>
>   ee->addGlobalMapping(M->getGlobalVariable("myglobal"),
(void*)&myglobal);
>   ee->finalizeObject();
>
>   void(*fptr)() =
(void(*)())ee->getFunctionAddress("printMyGlobal");
>   fptr();
>   printMyGlobal();
> }
>
> // end file jit.cpp
>
> I compile this file (jit.cpp) into bitcode which I then load int M. M
>  also contains a global variable called myglobal. I want to "map"
the
> myglobal variable in M to the global variable in the running program. The
> idea is that the value of myglobal inside the JIT'ted module will also
be
> 42. However, as it is right now, when I run the code, I get:
>
> myglobal: 0
> myglobal: 42
>
> The first is the JIT'ted printMyGlobal() and the second is the
AOT'ed
> printMyGlobal(). But when I lookup the address of the global variable
> myglobal (ee->getGlobalValueAddress()), it is mapped to the correct
> address. Similarly, if I lookup ee->getGlobalValueAtAddress(), I get the
> expected result.
>
> Am I missing an important step? Is what I am trying to do even possible?
>
> Thanks for the help.
>
> Tarun
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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Hi Lang,

That's exactly what it was.

Thanks a heap.


On Mon, Sep 14, 2015 at 5:44 PM, Lang Hames <lhames at gmail.com> wrote:
> Hi Tarun,
>
> My first thought is that you might be adding a definition (e.g. @x >
global i32 7) to your module, rather than an external declaration (e.g. @x
> = external global i32). Make sure you have the latter: If you provide a
> definition in your module it will shadow any definitions in your mapping,
> which would explain the behavior you're seeing.
>
> Cheers,
> Lang.
>
>
> On Mon, Sep 14, 2015 at 2:55 PM, Tarun Prabhu via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi,
>>
>> I think this is probably easiest to explain with code (I only provided
>> the essentials for clarity):
>>
>> // begin file jit.cpp
>>
>> int myglobal;
>>
>> void printMyGlobal() {
>>   printf("myglobal: %d\n", myglobal);
>> }
>>
>> int main(int argc, char *argv[]) {
>>
>>   // This file, jit.cpp has been compiled to bitcode (clang -S
-emit-llvm
>> jit.cpp)
>>   // and is read into Module M here
>>   Module *M = ...
>>   ExecutionEngine *ee = ...
>>
>>   myglobal = 42;
>>
>>  
ee->addGlobalMapping(M->getGlobalVariable("myglobal"),
>> (void*)&myglobal);
>>   ee->finalizeObject();
>>
>>   void(*fptr)() =
(void(*)())ee->getFunctionAddress("printMyGlobal");
>>   fptr();
>>   printMyGlobal();
>> }
>>
>> // end file jit.cpp
>>
>> I compile this file (jit.cpp) into bitcode which I then load int M. M
>>  also contains a global variable called myglobal. I want to
"map" the
>> myglobal variable in M to the global variable in the running program.
The
>> idea is that the value of myglobal inside the JIT'ted module will
also be
>> 42. However, as it is right now, when I run the code, I get:
>>
>> myglobal: 0
>> myglobal: 42
>>
>> The first is the JIT'ted printMyGlobal() and the second is the
AOT'ed
>> printMyGlobal(). But when I lookup the address of the global variable
>> myglobal (ee->getGlobalValueAddress()), it is mapped to the correct
>> address. Similarly, if I lookup ee->getGlobalValueAtAddress(), I get
the
>> expected result.
>>
>> Am I missing an important step? Is what I am trying to do even
possible?
>>
>> Thanks for the help.
>>
>> Tarun
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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