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

Hi,
> I know nothing about this, but the failed assertion suggests the PPC
> code generator can't cope with a constant that's bigger than  
> expected at
> that point.  Have you taken a look at PPCJITInfo.cpp:382?  It may shed
> some light.
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

Greetings,
Jan

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.

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.


Jonas

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

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