llvm dev - Jun 2007 - [LLVMdev] How to call native functions from bytecode run in JIT?

On Mon, 11 Jun 2007, Jonas Maebe wrote:
> On 11 Jun 2007, at 22:35, Jan Rehders wrote:
>> It's inside PPCJITInfo::relocate but unfortunately I could not
figure
>> out anything from the source. It looks like it's calculating new
>> addresses for functions which does not make much sense for a native
>> function, at all
>
> On the PPC, unconditional branches are limited to 24 bit signed
> displacements. When you call a function in a program, the compiler
> therefore will create a stub which will indirectly load the address
> of the function (the first time it's called, usually with the help of
> the dynamic linker) and then perform an indirect branch using this
> calculated address. This stub will be placed close to the call site,
> so the displacement to the stub will be in range.
Right.
> LLVM presumably needs to generate such stubs for native functions
> introduced at run time as well, which it may not yet do. I have not
> looked into the code though, so I can't really say for sure what goes
> wrong here.
The LLVM PPC JIT certainly does produce these stubs.  The 
PPCJITInfo::emitFunctionStub method is the one that actually creates them 
:)

Jan, how are you doing this?  Are you creating an external LLVM Function 
object named "get5", then using EE::addGlobalMapping?  If
'get5' exists in
the address space, why not just let the JIT resolve it (which will then 
create the stub)?

-Chris

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

Hi,
> Jan, how are you doing this?  Are you creating an external LLVM  
> Function
> object named "get5", then using EE::addGlobalMapping?  If
'get5'
> exists in
> the address space, why not just let the JIT resolve it (which will  
> then
> create the stub)?
Yes. I create a Function with matching signature, calling conventions  
and external linkage which I pass together with the function's  
address to addGlobalMapping.

I have an application which creates an execution engine and a module  
and creates some bytecode functions at runtime. I need to call back  
to some of the functions in the app (for a language runtime). What do  
you mean with "address space"? The function is part of the  
application but I don't see how LLVM would find it? If I remove the  
addGlobalMapping call, the function is not found at all

- Jan

Hi,

I ran into the same issue some time ago, so you might find the
following message from the mailing list useful. You probably want to
look at some of the previous emails in the thread to get the
background.

http://lists.cs.uiuc.edu/pipermail/llvmdev/2006-September/006817.html

/ Andreas

On Tue, 12 Jun 2007, Jan Rehders wrote:
>> Jan, how are you doing this?  Are you creating an external LLVM 
>> Function object named "get5", then using
EE::addGlobalMapping?  If
>> 'get5' exists in the address space, why not just let the JIT
resolve it
>> (which will then create the stub)?
>
> Yes. I create a Function with matching signature, calling conventions
> and external linkage which I pass together with the function's
> address to addGlobalMapping.
Okay.  If the function exists in your application's address space already, 
just name the LLVM function the same name as the native function and the 
JIT should find it an do the right thing.  This is how it finds printf and 
a variety of other things.  You don't need to call addGlobalMapping at 
all.

Does this work?

-Chris

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