[LLVMdev] GSOC Adaptive Compilation Framework for LLVM JIT Compiler

[Eric Christopher]

On Apr 1, 2011, at 9:38 AM, Reid Kleckner wrote:

> On Thu, Mar 31, 2011 at 11:15 PM, Eric Christopher <echristo at apple.com> wrote:
>>> 
>>> 
>>> Then we would always have the location of the br B instruction in A, as it is pushed onto the stack or saved in link register when A calls B.
>>> 
>> 
>> Right, unless you wanted to go with direct calls in the JIT. I don't know that inspecting a running program's stack for return values while compiling on a separate thread is sane :)
>> 
>> That said, the adaptive compilation strategy can always enforce the use of a stub for calls into jitted functions.
> 
> I think you said what I was going to say, but here's how I see it:
> 
> getPointerToFunction always returns the address of the stub, so any
> indirect calls to the function hit the stub.  The stub will attempt to
> backpatch the callsite, and should fail if it is a) outside the JIT or
> b) an indirect call.  Indirect calls will always go through the stub.
> Direct calls can be backpatched.
> 
> Alternatively, everything would go through the stub.
> 
> Finally, you have to be careful about the memory model of the
> architecture you're dealing with if you care about threads.  On x86,
> if you can align the operand of the call instruction, you can do an
> atomic update.  You will also have to make sure you don't free the old
> machine code while there are still functions executing in it.  An easy
> (yet inefficient) way to deal with this is to update a refcount on
> function entry/exit.

Well said :)

-eric