llvm dev - Jul 2011 - [LLVMdev] two questions about JIT compilation

hi all,

I've written a small language implementation that JIT-compiles code prior to
execution.  At the moment, i execute the function through
ExecutionEngine::runFunction.  However, this requires me to keep all of the
code generation artifacts (instructions, constants, etc) around during
execution.  Given that execution compiles the program down to machine code,
is there a reasonably straight-forward way to discard the contents of my
generated Module, and simply call the JITted function through a pointer
obtained through ExecutionEngine::getPointerToFunction?  I tried creating an
independent JITMemoryManager and keeping that around, but the execution
engine takes ownership of it (maybe to prevent this behaviour?).

My second question is about how to embed a reference to a c/c++ function
into LLVM-generated code.  I'm only interested in the JIT case, so this code
doesn't have to outlive the generating executable's lifetime.  The way i
think this can be achieved is to cast a pointer to this function to an int,
insert it into the generated code via ConstantInt, then use inttoptr to cast
it back to the correct function type and call it.  Is there a more direct
way of doing this?

thanks

Nick
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On Jul 26, 2011, at 6:22 PM, Nicholas Lester wrote:
> hi all,
> 
> I've written a small language implementation that JIT-compiles code
prior to execution.  At the moment, i execute the function through
ExecutionEngine::runFunction.  However, this requires me to keep all of the code
generation artifacts (instructions, constants, etc) around during execution. 
Given that execution compiles the program down to machine code, is there a
reasonably straight-forward way to discard the contents of my generated Module,
and simply call the JITted function through a pointer obtained through
ExecutionEngine::getPointerToFunction?  I tried creating an independent
JITMemoryManager and keeping that around, but the execution engine takes
ownership of it (maybe to prevent this behaviour?).
> 
> My second question is about how to embed a reference to a c/c++ function
into LLVM-generated code.  I'm only interested in the JIT case, so this code
doesn't have to outlive the generating executable's lifetime.  The way i
think this can be achieved is to cast a pointer to this function to an int,
insert it into the generated code via ConstantInt, then use inttoptr to cast it
back to the correct function type and call it.  Is there a more direct way of
doing this?
You could delete the body of the function that's being jitted after you call
getPointerToFunction.

-eric