[LLVMdev] MC-JIT Design

Mon Nov 15 13:36:50 PST 2010

2010/11/15 Török Edwin <edwintorok at gmail.com>:
> On Mon, 15 Nov 2010 11:34:22 -0800
> Daniel Dunbar <daniel at zuster.org> wrote:
>
>> Quick follow up here:
>>
>> I talked to Eric for a bit about this proposal, and he convinced me
>> that we should take a slightly different tack. I'll write a bit more
>> about it later.
>>
>> The idea Eric convinced me was better is to not invent a FOO format,
>> but just use the native platform object format and focus on writing
>> essentially a runtime linker for that platforms object files.
>>
>> This has various pros and cons, but I hadn't given it enough weight
>> before.
>>
>> The pros and cons we discussed:
>>
>> Pro:
>>  - We reuse all existing MC output functionality.
>>  - We have a shorter path to working well with the external system
>> tools (the real runtime linker, the debugger, the unwinder).
>>
>> Cons:
>>  - Requires developing good object file libraries for LLVM. This is
>> also a pro, as it coalesces work other people (Michael Spencer, Nick
>> Kledzik) are already interested in doing.
>>  - Means JIT is slightly more platform dependent, as the runtime
>> linker could have ELF or Mach-O specific bugs that wouldn't show up on
>> another platform.
>
> Don't forget PE (or is it pecoff?), otherwise JIT won't work on win32.

Don't worry -- out of sight but not out of mind! :)

> But what does the obj format has to do with the JIT?
> Sure for debug info its good to have an ELF around to give to gdb, but
> otherwise can't you just emit the code to memory, with a small header
> to keep info you need, and thats it?

You are right, this was the line of thought following my initial proposal.

Ultimately I think we need both pieces. We need the native object
piece, because the reality is the JIT does need to interface with
other code on the platform. We want the FOO object piece, because the
code generator etc. shouldn't be constrained by what can be
represented in the native object format.

The question is which way gets us onto the right path faster...

 - Daniel

>
>>  - Doesn't acknowledge that the JIT is a separate target. Constraints
>> the code generator to only doing what is actually supported on the
>> platform.
>>
>> The last con is the main thing I wanted to not preclude in a new JIT
>> design, but Eric convinced me that if we start by using the native
>> format, we can always introduce my new FOO format transparently if we
>> realize there is a concrete need for it.
>>
>> I'll try and sketch up some more of what this design would look like
>> this evening...
>>
>>  - Daniel
>>
>> On Mon, Nov 15, 2010 at 10:15 AM, Daniel Dunbar <daniel at zuster.org>
>> wrote:
>> > Hi all,
>> >
>> > As promised, here is the rough design of the upcoming MC-JIT*.
>> > Feedback appreciated!
>> >
>> > (*) To be clear, we are only calling it the MC-JIT until we have
>> > finished killing the old one. When I say JIT below, I mean the
>> > MC-JIT. I basically am ignoring completely the existing JIT. I will
>> > keep things API compatible whenever possible, of course.
>> >
>> > I see two main design directions for the JIT:
>> >
>> > --
>> >
>> > #1 (aka MCJIT) - We make a new MCJITStreamer which communicates with
>> > the JIT engine to arrange to plop code in the right place and update
>> > various state information.
>> >
>> > This is the most obvious approach, is roughly similar to the way the
>> > existing JIT works, and this is the way the proposed MC-JIT patches
>> > work (see MCJITState object).
>> >
>> > It also happens to not be the approach I want to take. :)
>> >
>> >
>> > #2 (aka FOOJIT) - MC grows a new "pure" backend, which is designed
>> > around representing everything that "can be run" on a target
>> > platform. This is very connected to the inherent capabilities of
>> > the hardware / OS, and is usually a superset** of what the native
>> > object format (Mach-O, ELF, COFF) can represent.
>> >
>> > The "pure" backend defines a hard (but non-stable) object file
>> > format which is more or less a direct encoding of the native MC
>> > APIs (it is not stable, so it can directly encode things like
>> > FixupKind enum values).
>> >
>> > I don't have a name for this format, so for now I will call it FOO.
>> >
>> > The "MC-JIT" then becomes something more like a "FOO-JIT". It is
>> > architected as a consumer of "FOO" object files over time. The basic
>> > architecture is quite simple:
>> >  (a) Load a module, emit it as a "FOO" object.
>> >  (b) Load the object into a worklist, scan for undefined symbols,
>> > dynamically emit more "FOO" modules.
>> >  (c) Iterate until no undefined symbols remain.
>> >  (d) Execute code -- if we hit a lazy compilation callback, go back
>> > to (a).
>> >
>> > (**) It more or less *must* be a superset, since object formats
>> > usually don't bother to represent things which can't be run.
>> > Features which require OS emulation is an obvious exception. As
>> > concrete example, consider the implementation of thread local
>> > storage. Each platform typically will chose an implementation
>> > approach and limit its format to supporting that, but the hardware
>> > itself supports many more implementation approaches.
>> >
>> > --
>> >
>> > I apologize if my description is a bit terse, but I hope the basic
>> > infrastructure comes through. I will make some pretty diagrams for
>> > it at some point (hopefully before the next dev mtg, hahaha
>> > hmmm....).
>> >
>> > Here are the reasons I want to follow approach #2:
>> >
>> > 1. It makes the JIT process look much more like the standard
>> > compilation process. In fact, from the FOOJIT's perspective, it
>> > could even run the compiler out of process to produce "FOO" object
>> > files, with no real change in behavior.
>> >
>> > This has two main implications:
>> >  a. We are leveraging much more of the existing infrastructure.
>> >  b. We can use more of the existing tools to test and debug the JIT.
>> >
>> > 2. It forces us to treat the JIT as a separate "subtarget".
>> >  a. In reality, this is already true. The compiler needs to know it
>> > is targeting a JIT in terms of what features are available (indirect
>> > stubs? exception tables? thread local storage?), but the current
>> > design papers over this. This design forces us to acknowledge that
>> > fact up front, and should make the architecture more understandable.
>> >
>> > 3. It eases testing and debugging.
>> >  a. We can build new tools to test the FOOJIT, for example, a tool
>> > that just loads a couple FOO object files and runs them, but without
>> > needing to do codegen. Since we can already use the existing tools
>> > to work with the FOO objects, this basically gives us a new testing
>> > entry point into the JIT.
>> >
>> > --
>> >
>> > Some caveats of this design:
>> >
>> > 1. The initial implementation will probably work very much as
>> > described, it will actually write "FOO" object files to memory and
>> > load them.
>> >
>> > In practice, we would like to avoid the performance overhead of this
>> > copy. My plan here is that eventually we would have multiple
>> > implementations of the FOO object writer, one of which would write
>> > to the serialized form, but another would splat directly into the
>> > process memory.
>> >
>> > We would allow other fancy things following the same approach, for
>> > example allow the JIT to pin symbols to their actual addresses, so
>> > that the assembler can do the optimal relaxation for where the code
>> > is actually landing in memory.
>> >
>> > 2. It requires some more up front work, in that there is more stuff
>> > to build. However, I feel it is a much stronger design, so I expect
>> > this to pay off relatively quickly.
>> >
>> > 3. Some JIT-tricks become a bit less obvious. For example, in a JIT,
>> > it is natural when seeing a symbol undefined "bar" to go ahead and
>> > see if you can find "bar" and immediately generated code for it.
>> > You can't do that in the FOOJIT model, because you won't know "bar"
>> > is undefined until you read the object back.
>> >
>> > However, in practice one needs to be careful about recursion and
>> > reentrancy, so you have to take care when trying to do things like
>> > this. The FOOJIT forces such tricks to go through a proper API (the
>> > FOO object) which I end up seeing as a feature, not a bug.
>> > --
>> >
>> > And, a final word on API compatibility:
>> >
>> > As mentioned before, I have no *plan* to break any existing public
>> > interface to the JIT. The goal is that we eventually have a strict
>> > superset of the current functionality.
>> >
>> > The actual plan will be to roll out the FOOJIT in tree with some
>> > option to allow clients to easily pick the implementation. A
>> > tentative goal would be to have the FOOJIT working well enough in
>> > 2.9 so that clients can test it against the released LLVM, and that
>> > for 2.10 (*grin*) we can make it the default.
>> >
>> > Thoughts?
>> >
>> >  - Daniel
>> >
>>
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