llvm dev - Nov 2006 - [LLVMdev] question about the LLVM JIT

<cc'ing llvmdev>

On Tue, 28 Nov 2006, Eric van Riet Paap wrote:
> I'm working on using the LLVM JIT in PyPy and I hop you can give me a
few
> hint.
ok
> I have some things working at and try to write C++ code for what I need 
> from Python. The unittest I am working on at the moment is looks like 
> this
>
> --- Python code...
> llglobalmul4 = '''%my_global_int = external global int
>
> implementation
>
> int %globalmul4(int %a) {
>     % v0 = load int* %my_global_data
>     % v1 = mul int %v0, 4
>     % v2 = add int %v1, %a
>      store int %v2, int* %my_global_int
>      ret int %v2
> }'''
>
> def test_modify_global_data():
>      llvmjit.restart()
>      llvmjit.set_global_data(10)
>      assert llvmjit.get_global_data() == 10
>      gp_data = llvmjit.get_pointer_to_global_data()
>      llvmjit.add_global_mapping('my_global_data', gp_data) #note: 
> should be prior to compile()
>      llvmjit.compile(llglobalmul4)
>      globalmul4 = llvmjit.find_function('globalmul4')
>      assert llvmjit.execute(globalmul4, 5) == 10 * 4 + 5
>      assert llvmjit.get_global_data() == 10 * 4 + 5
> ---
>
> This gives a segfault in llvm::Type::getForwardedTypeInternal as far as I
can
> tell. Probably I don't understand how global data mapping works in
llvm.
> The C++ code for add_global_mapping(...) looks like this
>
> --- C++ code...
> void    add_global_mapping(const char* name, void* address) {
>    GlobalVariable  var(Type::IntTy, false, GlobalVariable::ExternalLinkage,
> 0, name, gp_module);
>    gp_execution_engine->addGlobalMapping(&var, address);
> }
This is creating a new global variable on the stack, instead of finding 
the existing global variable in the module.  Try something like this:

   gp_execution_engine->addGlobalMapping(YourModule->getNamedGlobal(name),
              address);

> The 2nd question I have has to do with recompiling code.
>
> The 'library' I'm creating, that sits between llvm's (c++)
libs and Python,
> will remain minimal so I decided to use ParseAssemblyString to get .ll 
> sourcecode into the JIT.
Ok.
> I do not see how I can tell the jit when this .ll code contains a function 
> that should overwrite a previous version of it.
> For instance, when there is a function "int %func() { ret int 5
}" and later
> I want this replaced by "int %func() { ret int 10 }".
> If I remember correctly this gives a 'function already defined'
error
> message. I tried to use a seperate module for each call to 
> ParseAssemblyString + addModuleProvider.
> In that way there was a problem with functions calling into other modules. 
> (which perhaps I could try to solve if would understand addGlobalMapping 
> better)
This is somewhat tricky.  To replace a function in-place like this, you 
should actually modify the function in the original module, then call 
EE->recompileAndRelinkFunction(fn).  This will require some C++ code to 
delete the original function body, then splice the body of the new 
function into the old function.

-Chris

-- 
http://nondot.org/sabre/
http://llvm.org/

>> <snip>.
>> void    add_global_mapping(const char* name, void* address) {
>>    GlobalVariable  var(Type::IntTy, false,  
>> GlobalVariable::ExternalLinkage, 0, name, gp_module);
>>    gp_execution_engine->addGlobalMapping(&var, address);
>> }
>
> This is creating a new global variable on the stack, instead of  
> finding the existing global variable in the module.  Try something  
> like this:
>
>   gp_execution_engine->addGlobalMapping(YourModule->getNamedGlobal 
> (name),
>              address);
>
Hi Chris,

Thanks, that was simpler than I thought!
I actually thought I had to create a new global so it would be  
available for the JIT when it gets to process the "%var = external  
global int" line.
> <snip>
>
>> I do not see how I can tell the jit when this .ll code contains a  
>> function that should overwrite a previous version of it.
>> For instance, when there is a function "int %func() { ret int 5
}"
>> and later I want this replaced by "int %func() { ret int 10
}".
>> If I remember correctly this gives a 'function already defined'
>> error message. I tried to use a seperate module for each call to  
>> ParseAssemblyString + addModuleProvider.
>> In that way there was a problem with functions calling into other  
>> modules. (which perhaps I could try to solve if would understand  
>> addGlobalMapping better)
>
> This is somewhat tricky.  To replace a function in-place like this,  
> you should actually modify the function in the original module,  
> then call EE->recompileAndRelinkFunction(fn).  This will require  
> some C++ code to delete the original function body, then splice the  
> body of the new function into the old function.
For my purpose it would be ideal if I could give ParseAssemblyString  
an additional option (or maybe set a global flag somehow) to indicate  
what should happen when an already defined function is parsed. Would  
something like this be possible and fit into llvm's design?

 >>> Eric

On Thu, 30 Nov 2006, Eric van Riet Paap wrote:
>> This is somewhat tricky.  To replace a function in-place like this,
>> you should actually modify the function in the original module,
>> then call EE->recompileAndRelinkFunction(fn).  This will require
>> some C++ code to delete the original function body, then splice the
>> body of the new function into the old function.
>
> For my purpose it would be ideal if I could give ParseAssemblyString
> an additional option (or maybe set a global flag somehow) to indicate
> what should happen when an already defined function is parsed. Would
> something like this be possible and fit into llvm's design?
Sure, it would be possible to add something like this.  I'd prefer a new 
function, not a bool to ParseAssemblyString though.

-Chris

-- 
http://nondot.org/sabre/
http://llvm.org/