[LLVMdev] JIT question: Inner workings of getPointerToFunction()

[Evan Cheng]

On May 27, 2008, at 2:47 PM, Prakash Prabhu wrote:

> Hi,
>
> I was just reading through the Kaleidoscope tutorial (which is  
> greatly written and understandable, thanks! ) hoping to get some  
> glimpse about the workings of the JIT and the optimizations that are  
> done at run time. I am curious as to how LLVM's JIT dynamically  
> generates native code from bit code at run time and runs that code  
> (I think my question is also somewhat more general in the sense how  
> does any JIT system translate some form of low level IR (which  
> presumably is  JIT's data) into native code which is actually made  
> executable at runtime). Specifically, in the following code snippet  
> (from the tutorial), how does getPointerToFunction() actually  
> generate native code for function LF and the call to FP succeed as  
> if FPtr was a pointer to statically compiled code ?
>
> // JIT the function, returning a function pointer.
> void *FPtr = TheExecutionEngine->getPointerToFunction(LF);
>
> // Cast it to the right type (takes no arguments, returns a double)  
> so we
> // can call it as a native function.
>   double (*FP)() = (double (*)())FPtr;
>
> I took a look at getPointerToFunction() and it seems it calls  
> materializeFunction ( is this the run time code generator ? ) where  
> most of the work is done. It would be great if you could point out a  
> good starting place to understand the whole JIT'ing place in the  
> source  and relevant documentation (I read the paper about Jello).  
> Also are there any dynamic optimizations that are currently done  
> using the JIT ?

JIT::getPointerToFunction() calls runJITOnFunction(F) which starts the  
whole code generation process. LLVM jit doesn't do any dynamic  
optimizations at this point.

Evan


>
>
> Thanks for your time !
>
> - Prakash
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