[LLVMdev] How will OrcJIT guarantee thread-safety when a function is asked to be re generated?

[Lang Hames]

Hi Sanjoy,

>> (1) Replacing function bodies at the same address is impossible if the
>> function is already on the stack: You'd be replacing a definition that
>> you're later going to return through.
> 
> If the function you wish to replace is active on the stack, you can
> replace the return PC that was going to return into that active frame
> with a PC pointing into a stub that knows how to replace the active
> stack frame with something that would let the new code continue
> executing.  The stub will then have to branch into a suitable position
> in the new generated code.  Once you have done this for all "pending
> returns" into the old bit of generated code, you can throw the old
> code away, since nothing will ever return into it.
> 
> This can be tricky to get right but if you have built OSR support
> already for some other reason then this is a viable option.  This
> scheme is very similar to throwing an exception, and the semantics of
> "catching" an exception is to branch to a newly generated block of
> code.

That all makes sense. What are your thoughts on the trade-offs of this vs the patchpoint approach though? If you can modify previously executable memory it seems like the patchpoint approach would have lower overhead, unless you have a truly huge number of callsites to update?
 
>> So, if you want to replace functions
>> at the same address you'll have to have some sort of safe-point concept
>> where you know the function you want to replace isn't on the stack.
> 
> That will work, but can be very hard to make happen.  For instance,
> the method you want to replace may have called a function that has an
> infinite loop in it.

Agreed. This *might* find a home in simple REPLs where calling an infinite-loop would be undesirable/unexpected behavior, but that's also an environment where you are unlikely to want reoptimization.
 
>> (2) Replacing function bodies at the same address isn't the only way to
>> avoid the overhead of a trampoline. I haven't implemented this yet, but I
>> really want to add llvm.patchpoint support to Orc. In that case you can lay
>> down your replacement definition at a different address, update all your
>> callsites, then delete your old definition after you're done executing it.
>> Relative to using trampolines this lowers your execution cost (calls are
>> direct rather than indirect), but increases your update cost (you have to
>> update many callsites, rather than a single trampoline).

FWIW, Pete Cooper and I have tossed around ideas about adding utilities to Orc for injecting frame-residence counting and automatic cleanup in to functions to facilitate this 2nd approach. The rough idea was that each function would increment a counter on entry and decrement it on exit. Every time the counter hits zero it would check whether it has been "deleted" (presumably due to being replaced), and if so it would free its memory. This scheme should be easy to implement, but hasn't gone past speculation on our part.

- Lang.

Sent from my iPad

> On Mar 19, 2015, at 3:00 PM, Sanjoy Das <sanjoy at playingwithpointers.com> wrote:
> 
>> On Wed, Mar 18, 2015 at 6:39 PM, Lang Hames <lhames at gmail.com> wrote:
>> Hi Hayden,
>> 
>> Dave's answer covers this pretty well. Neither Orc nor MCJIT currently
>> reason about replacing function bodies. They may let you add duplicate
>> definitions, but how they'll behave if you do that isn't specified in their
>> contracts. They definitely won't replace old definitions unless you provide
>> a custom memory manager that's rigged to lay new definitions down on top of
>> old ones.
>> 
>> I suspect that existing clients of MCJIT have tackled this by adding thread
>> safety into their wrappers around MCJIT, or into the JIT'd code itself, but
>> I'm just guessing. (CC'ing Keno and Philip, in case they have insights).
>> 
>> I think this would be cool to build in to Orc though. Two quick thoughts:
>> 
>> (1) Replacing function bodies at the same address is impossible if the
>> function is already on the stack: You'd be replacing a definition that
>> you're later going to return through.
> 
> If the function you wish to replace is active on the stack, you can
> replace the return PC that was going to return into that active frame
> with a PC pointing into a stub that knows how to replace the active
> stack frame with something that would let the new code continue
> executing.  The stub will then have to branch into a suitable position
> in the new generated code.  Once you have done this for all "pending
> returns" into the old bit of generated code, you can throw the old
> code away, since nothing will ever return into it.
> 
> This can be tricky to get right but if you have built OSR support
> already for some other reason then this is a viable option.  This
> scheme is very similar to throwing an exception, and the semantics of
> "catching" an exception is to branch to a newly generated block of
> code.
> 
>> So, if you want to replace functions
>> at the same address you'll have to have some sort of safe-point concept
>> where you know the function you want to replace isn't on the stack.
> 
> That will work, but can be very hard to make happen.  For instance,
> the method you want to replace may have called a function that has an
> infinite loop in it.
> 
>> 
>> (2) Replacing function bodies at the same address isn't the only way to
>> avoid the overhead of a trampoline. I haven't implemented this yet, but I
>> really want to add llvm.patchpoint support to Orc. In that case you can lay
>> down your replacement definition at a different address, update all your
>> callsites, then delete your old definition after you're done executing it.
>> Relative to using trampolines this lowers your execution cost (calls are
>> direct rather than indirect), but increases your update cost (you have to
>> update many callsites, rather than a single trampoline).
>> 
>> Out of interest, why the desire to avoid trampolines? They do make life a
>> lot easier here. :)
>> 
>> Cheers,
>> Lang.
>> 
>>> On Wed, Mar 18, 2015 at 3:13 AM, David Blaikie <dblaikie at gmail.com> wrote:
>>> 
>>> [+Lang, keeper of JITs, designer of ORCs]
>>> 
>>> On Tue, Mar 17, 2015 at 1:27 AM, Hayden Livingston
>>> <halivingston at gmail.com> wrote:
>>>> 
>>>> I've been playing with OrcJIT a bit, and from the looks of it I can (like
>>>> in the previous JIT I suppose?) ask for a function to be re generated.
>>>> 
>>>> If I've given the address of the function that LLVM gave me to an
>>>> external party, do "I" need to ensure thread-safety?
>>>> 
>>>> Or is it safe to ask OrcJIT to re generate code at that address and
>>>> everything will work magically?
>>> 
>>> 
>>> As I understand it, Orc won't regenerate the function at the same location
>>> unless your memory manager returns the same memory twice - so if you know
>>> you've successfully migrated all callers off a certain chunk of allocated
>>> memory, you might be able to recycle it back into Orc (but I think on MacOS,
>>> the way page permissions work, this would be impossible - once a memory page
>>> is marked executable, it's no longer writable and can't be set back - you
>>> need a new page).
>>> 
>>>> 
>>>> I'm thinking it won't because it's quite possible some thread might be
>>>> executing code, and we'll be asking LLVM to write bytes there.
>>>> 
>>>> How does one generally go do such updates? I'm looking for some guidance
>>>> without adding a trampoline in front of it. Do runtimes that support
>>>> re-generation of code have an if check or something before entering the
>>>> method?
>>> 
>>> 
>>> Without a trampoline you're probably going to have to be constrained in
>>> some other ways - possibly (& I'm really out of my depth at this point) the
>>> kind of safe/pause points used for GC - but perhaps more constrained than
>>> that, such that you have safe places where your JIT'd code (or at least the
>>> replaceable functions) isn't running.
>>> 
>>> But again, still depends on platform - writing to executable memory isn't
>>> possible on MacOS so far as I know (as mentioned above) so there would be no
>>> way to replace a function there without a trampoline or at least a global
>>> variable to load/jump to.
>>> 
>>> - David
>>> 
>>>> 
>>>> 
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>>> 
>> 
>> 
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