[LLVMdev] MCJIT and Lazy Compilation
Andrew Sorensen
digegoo at gmail.com
Sat Mar 9 01:26:11 PST 2013
Hi Andy/Albert,
Sorry for the slow reply, my day job caught up with me.
Two bits of progress. (a) MCJIT is working nicely for non-trivial
examples in Extempore on x86 and ARM, and (b) the page
permissions are now RO again. For your amusement a *very*
cheesy video of Extempore running on-the-fly with MCJIT on an
ARM Pandaboard. Viewer discretion is advised!
https://vimeo.com/60407237
Here is the overview of changes I promised a couple of weeks back.
These comments are based on the 3.3 trunk of about 3 weeks ago.
-- RuntimeDyld --
Relocations needs to be cleared between
each emitObject.
New method required: clearRelocations
void RuntimeDyldImpl::clearRelocations() {
ExternalSymbolRelocations.clear();
Relocations.clear();
}
void RuntimeDyld::clearRelocations() {
Dyld->clearRelocations();
}
-- RuntimeDyldImpl --
void clearRelocations();
-- SectionMemoryManager --
Removed option in allocateSection to use existing 'free' memory
regions - i.e. now always allocate new memory. This means that once a
module is emitted and its memory permissions are applied we don't have
to touch it again. This means we don't have to set exec sections
writable before a new emit. I'm sure there is a nicer way to achieve
this.
-- MCJIT --
MCJIT currently takes a single Module and only supports a single call
to emitObject. A number of assertion and conditional checks enforce
this. These checks can all be removed (assertions and conditionals),
making M obsolete.
At the moment there are a number of call sites for emitObject. I
removed all of them and replaced them with a single emitObject call
site accessible through the extant, but unused,
recompileAndRelinkFunction method. This then becomes the clients
access point to compile each individual module. This is obviously a
hack to maintain the integrity of the existing API (i.e. bad name,
evil type munging F->M etc..).
void *MCJIT::recompileAndRelinkFunction(Function *F) {
emitObject((Module*) F);
finalizeObject();
return NULL;
}
FinalizeObject should not call resolveRelocations. Instead it should
call the *new* clearRelocations.
void MCJIT::finalizeObject() {
// New Dyld call clearRelocations
Dyld.clearRelocations();
// Set page permissions.
MemMgr->applyPermissions();
}
At the moment I am just leaking allocated sections in SMM. i.e.
void MCJIT::freeMachineCodeForFunction(Function *F) {
dbgs() << "free machine code not yet supported in MCJIT\n";
return;
}
But it should be relatively straight forward to maintain some kind of
Module -> SMM Section map. Hotswapping currently works fine because
the relocations all update as expected. So this is *just* a leakage
problem.
-- AsmParser/LLParser --
Some forward refs in the LLParser need fixing. I'm working directly
from IR though, so C++ API people will need a different fix.
-------------------------
I might have missed a few things as it has been a couple of weeks
since I looked at the code but that is the general drift. As
previously mentioned there really is not much required to make all of
this function well for the individual module case.
Of course this does not address the issues that clients may face
moving from JIT to MCJIT as a result of these changes. However, I
expect that many have similar solutions to Extempore, and should not
have any major problems moving to a multi-module approach. Albert do
you see this being much of a problem for Pure?
There are obviously some things that will need fixing.
1) The current single Module case may need to be retained??
2) We need a more appropriate call than recompileAndRelinkFunction
3) We need proper section reclamation/erasure
4) Better solution for permissions re. freemem in section allocation.
Andy, aside from these 4 points does everything here sound OK to you,
and if so how would you like to proceed. I can put *some* time in but
you'll need to give me some direction about what you would like done.
Cheers,
Andrew.
On Sat, Feb 16, 2013 at 5:26 PM, Andrew Sorensen <digegoo at gmail.com> wrote:
> Hey Andy,
>
> Yep I've tested some non-trivial examples with loads of dependencies,
> both code and data, global, local and external symbol resolution etc..
>
> Actually this was truly a piece of cake, nothing to do, the memory manager
> is working really nicely so far as I can tell. Relocations to sections
> are all working
> as expected (aside from previously mentioned ARM issue which is probably
> just
> something that I'm doing wrong) with all global symbol relocs managed
> persistently
> by the MM between object injections. All in all it just works ;) I had
> to make a few
> minor adjustments to things like the LLParser for forward dependencies but
> overall
> really simple stuff.
>
> There certainly are some section management issues that will need to be
> addressed,
> but I don't see any major hurdles there. I was going to take a look into
> this next week?
>
> The biggest issue for multi-module is probably going to be client side not
> LLVM side,
> although this has not been a huge problem for me as most of this
> bookkeeping is
> already managed "client side" in extempore.
>
> I'm happy to send you code although it might be more useful for me to
> write a
> followup email outlining exactly what changes were made and then let the
> experts
> decide how best to proceed ;) Tomorrows a little hectic but I'll try to
> send a note
> through on Monday.
>
>
> Cheers,
> Andrew.
>
>
>
>
>
>
> On Sat, Feb 16, 2013 at 8:13 AM, Kaylor, Andrew <andrew.kaylor at intel.com>wrote:
>
>> This is great news.****
>>
>> ** **
>>
>> Do you have any dependencies between your modules? For instance, one
>> calling a function in another? If so, how did you handle that?****
>>
>> ** **
>>
>> Any chance you could share some code snippets or the relevant portions?**
>> **
>>
>> ** **
>>
>> -Andy****
>>
>> ** **
>>
>> *From:* Andrew Sorensen [mailto:digegoo at gmail.com]
>> *Sent:* Thursday, February 14, 2013 11:48 PM
>>
>> *To:* Kaylor, Andrew
>> *Cc:* llvmdev at cs.uiuc.edu
>> *Subject:* Re: [LLVMdev] MCJIT and Lazy Compilation****
>>
>> ** **
>>
>> OK, so I have some *preliminary* results, which are on the whole quite
>> encouraging!****
>>
>> ** **
>>
>> I haven't had a great deal of time, but I have managed to get Extempore
>> up and ****
>>
>> running with function (actually lexical closures so composed of quite a
>> bit of additional****
>>
>> guff) level compilation using Andy's multi module suggestion. I also have
>> on-the-fly ****
>>
>> recompilation of existing closures working (caveats below) so from an
>> end-user ****
>>
>> perspective this means that Extempore appears functionally equivalent
>> with MCJIT ****
>>
>> and the old legacy JIT - hot-swapping audio signal processing code
>> on-the-fly using ****
>>
>> MCJIT for example.****
>>
>> ** **
>>
>> Firstly multi-module definitely proved to be considerably easier than
>> attempting to hack****
>>
>> solutions for incremental *monolithic* module builds - which I also
>> investigated.****
>>
>> ** **
>>
>> So the only major obstacle that I have run into so far are page
>> permissions in relation****
>>
>> to code relocations. I have a *safe* hack which is to toggle section
>> permissions between****
>>
>> rw and exec/ro in-between new object injections - however this is
>> obviously problematic ****
>>
>> for code that is executing concurrently (i.e. secondary threads). I also
>> have an *unsafe*****
>>
>> hack, (purely for experimentation :-) whereby exec sections are left rw,
>> and although ****
>>
>> very evil it works for test purposes (i.e. the audio example mentioned
>> above). These ****
>>
>> solutions are obviously both inappropriate and I will investigate a
>> *real* solution when ****
>>
>> I find some time.****
>>
>> ** **
>>
>> Also I didn't bother to implement section erasure, at the moment I'm just
>> allocating ****
>>
>> new sections for each compile regardless of whether the new code replaces
>> existing****
>>
>> functionality. Having said that I don't see this as much of an issue, I
>> was just to****
>>
>> lazy to bother implementing it. I'll check this when I have some further
>> free time.****
>>
>> ** **
>>
>> FYI this is all under x86. I did try to run under ARM but bombed out on
>> an assertion error ****
>>
>> in the ARM ELF relocation code - specifically assert((*TargetPtr &
>> 0x000F0FFF) == 0);****
>>
>> I assume this is a result of something evil that I have done but I
>> haven't yet had time to ****
>>
>> investigate any further. Again I'll let you know when I have some more
>> time.****
>>
>> ** **
>>
>> Just a quick heads up but In general my initial thoughts are that MCJIT
>> is really not ****
>>
>> that far off.****
>>
>> ** **
>>
>> Cheers,****
>>
>> Andrew.****
>>
>> ** **
>>
>> ** **
>>
>> ** **
>>
>> ****
>>
>> ** **
>>
>> ****
>>
>> ** **
>>
>> ** **
>>
>> ** **
>>
>> ** **
>>
>> On Fri, Feb 8, 2013 at 7:36 AM, Kaylor, Andrew <andrew.kaylor at intel.com>
>> wrote:****
>>
>> That’s awesome!****
>>
>> ****
>>
>> I think at this point having people try out various approaches and seeing
>> what works and what doesn’t is our biggest need in this area. Please do
>> keep me informed about what you find out.****
>>
>> ****
>>
>> -Andy****
>>
>> ****
>>
>> *From:* Andrew Sorensen [mailto:digegoo at gmail.com]
>> *Sent:* Wednesday, February 06, 2013 4:33 PM
>> *To:* Kaylor, Andrew
>> *Cc:* llvmdev at cs.uiuc.edu
>> *Subject:* Re: [LLVMdev] MCJIT and Lazy Compilation****
>>
>> ****
>>
>> Thanks for the update Andy.****
>>
>> ****
>>
>> I'm very happy to be involved in anyway that is helpful. If you would
>> like me to test ideas, or contribute to further discussions, then please
>> let me know.****
>>
>> ****
>>
>> I currently have extempore running nicely with MCJIT for the "monolithic"
>> case and am working on various LLVM hacks to better understand the issues
>> involved with non-monolithic approaches - in particular I'm starting with
>> your multi-module approach. I will report back when (and if) I have
>> something useful to contribute.****
>>
>> ****
>>
>> Cheers,****
>>
>> Andrew.****
>>
>> ****
>>
>> On Wed, Feb 6, 2013 at 4:08 AM, Kaylor, Andrew <andrew.kaylor at intel.com>
>> wrote:****
>>
>> Hi Andrew,****
>>
>> ****
>>
>> I was about to write a belated reply to this message (sorry for the
>> delay), but then I realized that pretty much everything useful that I have
>> to say on the subject is contained in this message (which is in a thread
>> Albert Graef already linked to):****
>>
>> ****
>>
>> https://groups.google.com/d/msg/llvm-dev/Rk9cWdRX0Wg/Fa1Mn6cyS9UJ****
>>
>> ****
>>
>> Generally, I do hope that MCJIT will be capable of replacing the old JIT
>> someday soon, though obviously it cannot do so until it provides equivalent
>> functionality. I doubt it will ever be a “drop-in” replacement, but I hope
>> that minimal rework will be needed. Most significantly, as can be seen in
>> earlier discussions, things will need to be made Module-centric rather than
>> Function-centric. It ought to be possible to write a utility class that
>> takes a monolithic Module and breaks it up into sub-Modules for individual
>> functions, but I think that would need to happen outside of the MCJIT
>> engine because not all clients would want that kind of granularity.****
>>
>> ****
>>
>> There’s definitely a lot of work to be done here to get this right, and
>> hopefully we’ll get active participation in any design discussions to make
>> sure the solution meets everyone’s needs. I don’t have a time table for
>> this right now. I will file a Bugzilla report as soon as the LLVM server
>> is ready.****
>>
>> ****
>>
>> -Andy****
>>
>> ****
>>
>> *From:* llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu]
>> *On Behalf Of *Andrew Sorensen
>> *Sent:* Thursday, January 31, 2013 7:56 PM
>> *To:* llvmdev at cs.uiuc.edu
>> *Subject:* [LLVMdev] MCJIT and Lazy Compilation****
>>
>> ****
>>
>> Does anyone have a roadmap for MCJIT with what I think people are ****
>>
>> calling lazy compilation.****
>>
>> ****
>>
>> Is this even on the cards?****
>>
>> ****
>>
>> I spent the last few hours moving my project (extempore.moso.com.au) ****
>>
>> over to MCJIT (particularly for ARM), and am a little horrified to
>> discover ****
>>
>> no ability to compile, and just as importantly to recompile, at a
>> function level. ****
>>
>> This is absolutely mandatory for my project. ****
>>
>> ****
>>
>> I have been looking enviously at MCJIT's ARM+DWARF support for a ****
>>
>> couple of years and was under the misapprehension that MCJIT was ****
>>
>> attempting to be a *drop-in* replacement for JIT. So I wasn't overly****
>>
>> concerned about the primary JIT being largely neglected. This is obviously
>> ****
>>
>> my fault, I wasn't paying close enough attention.****
>>
>> ****
>>
>> I am now wondering what the LLVM project, in the large, plans regarding *
>> ***
>>
>> just-in-time compilation moving forward. Is MCJIT the future, and****
>>
>> if so what kind of roadmap is there to replicate current JIT
>> functionality. ****
>>
>> In my case in relation to function level (re)compilation.****
>>
>> ****
>>
>> I appreciate everyones efforts, and that we all have our own agendas.****
>>
>> I'm just trying to put my own roadmap in place.****
>>
>> ****
>>
>> Cheers,****
>>
>> Andrew.****
>>
>> ****
>>
>> ** **
>>
>
>
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