[LLVMdev] Redefining function

Conrado Miranda miranda.conrado at gmail.com
Sat Jan 30 14:03:57 PST 2010

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

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


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