llvm dev - Jan 2010 - [LLVMdev] Redefining function

Hi everybody.

I've just started learning about LLVM and didn't get too far studying
the
core.
I couldn't find the solution to my problem (if it has one) in the mailing
list or the source code. The problem is: how can I redefine a function
that's been called already by some other function?

Suppose I have 3 files, all compiled to bytecode through llvm-gcc (I think
it could be clang instead).

File1.c:
void do_print() { print(); }

File2.c:
void print() { printf("File2.c\n"); }

File3.c:
void print() { printf("File3.c\n"); }

Then, I have the main file compiled to executable like this:
int main() {
  // initialize and get context (not shown)
  Module *file1(LoadFile("file1.bc",Context));
  Module *file2(LoadFile("file2.bc",Context));
  Module *file3(LoadFile("file3.bc",Context));

  Linker::LinkModules(file1, file2, NULL);

  EngineBuilder builder(file1);
  ExecutionEngine *EE = builder.create();

  EE->runStaticConstructorsDestructors(false);

  func = EE->FindFunctionNamed("do_print");
  EE->runFunction(func, std::vector<GenericValue>());

  //swap the definition of the function "print" from the one in
File2.c to
File3.c
  swap (file1, file2, file3);

  EE->runFunction(func, std::vector<GenericValue>());

  EE->runStaticConstructorsDestructors(true);

  return 0;
}

I can get everything before the swap working (if I comment the rest, the
output is OK). I've tried to build the "swap" function many times
but I
can't get it to work.
The expected output is:
File2.c
File3.c

If someone know how to do this or knows it's impossible, I would be very
thankful. I don't even know if this is the best way to do it.

Miranda
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Hi Conrado,
> I couldn't find the solution to my problem (if it has one) in the 
> mailing list or the source code. The problem is: how can I redefine a 
> function that's been called already by some other function?
why do you want to do this?
> Suppose I have 3 files, all compiled to bytecode through llvm-gcc (I 
> think it could be clang instead).
> 
> File1.c:
> void do_print() { print(); }
> 
> File2.c:
> void print() { printf("File2.c\n"); }
> 
> File3.c:
> void print() { printf("File3.c\n"); }
The solution in C is to give the version in File2 a weak linkage type,
for example using gcc's "weak" attribute.  You then link all three
files
together, and the weak print in File2 will be magically replaced with the
non-weak print in File3.
>   //swap the definition of the function "print" from the one in
File2.c
> to File3.c
>   swap (file1, file2, file3);
If all the functions are in the same module, then you can use
FunctionA->replaceAllUsesWith(FunctionB) if they have the same
type.

Ciao,

Duncan.

Hi Duncan,
> I couldn't find the solution to my problem (if it has one) in the
mailing
>> list or the source code. The problem is: how can I redefine a function
>> that's been called already by some other function?
>>
>
> why do you want to do this?
>
To implement something that is common in Lisp. Suppose I have a program that
is running and can't be stopped or the cost being stoped is prohibitive. If
I find a better way to run an algorithm, I'd like to update the running
program non-stopping.


 Suppose I have 3 files, all compiled to bytecode through llvm-gcc (I
think
> it could be clang instead).
>
> File1.c:
> void do_print() { print(); }
>
> File2.c:
> void print() { printf("File2.c\n"); }
>
> File3.c:
> void print() { printf("File3.c\n"); }
>
The solution in C is to give the version in File2 a weak linkage
type,
> for example using gcc's "weak" attribute.  You then link all
three files
> together, and the weak print in File2 will be magically replaced with the
> non-weak print in File3.

Never heard of it before. After a quick reading, it sounds OK. Keeping the
rest of the file, changed the attribute and this section:

  func = EE->FindFunctionNamed ("do_print");
  EE->runFunction(func, std::vector<GenericValue> ());
  Linker::LinkModules(main_file, print2, &ErrorMessage);
  EE->runFunction(func, std::vector<GenericValue> ());

And now I get this error before the second runFunction:

While deleting: void ()* %print
An asserting value handle still pointed to this value!
UNREACHABLE executed at /home/miranda/llvm-2.6/lib/VMCore/Value.cpp:492!

I suppose that's because "do_print" was already called. By the
way, it seems
like weak attribute is only supported for ELF and a.out. Maybe not the
better solution.

  //swap the definition of the function "print" from the one in
File2.c to
> File3.c
>  swap (file1, file2, file3);
>
If all the functions are in the same module, then you can
use
> FunctionA->replaceAllUsesWith(FunctionB) if they have the same
> type.
>
Sorry I didn't see that function before. But, when I tried that (pastebin
code: http://pastebin.com/m2485ae4f), it still doesn't print as supposed. It
calls only the first function printing
File2.c
File2.c

Maybe that works when the functions haven't been called before. Am I using
the wrong way or had to do something before?

Thanks,

Miranda
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