Hi Andrew, hi all,
I already saw that the old jit was (almost) deprecated. So, I'm
currently playing with the new jit and it's look very interesting.
(I'm working locally and I haven't pushed anything new on VMKit
because I'm also changing a little the design vmkit). For the moment,
MCJIT does not work with VMKit (but I haven't yet tested the
safepoint/stackmap patch), I don't know if it comes from what I'm
doing or if something is still missing in MCJIT.
Sorry, my mail will be too long, but I want first to explain what I'm
currently doing and then open the discussion to explain what I would
like to see in MCJIT :) Of course, I can help to develop something or
to give feed back, I'm hardly working on vmkit currently and I have
time to spend on that (but I'm far from an expert in compilation
stuffs!).
Basically, I want to compile lazily my functions (we have a Java
virtual machine built upon vmkit and compiling all the rt.jar during
the bootstrap is not very realistic:)). So, by lazy compilation, I
mean that I want to compile (and even resolve) a function only when it
is used. For example, during the compilation of
void f() { g() }
I don't want to compile g(). I will only compile g() when it will be called.
For the moment, I use a home-made stub (I have attached the asm code,
it can give you some ideas if you plan to integrate a stub generator
able to perform dynamic dispatch like virtual call in c++) because
MCJIT does not provide this facility. So, for each function, I define
a module. In each module, I have to define the runtime function (such
as gcmalloc, throwException and this kind of functions). They are
defined in a separated runtime module populated during the bootstrap.
I have thus this picture
MCJIT
|----------------------------------------------------------
| | |
Runtime module module for f module for g
When I see that I need g during the compilation of f, I define an
external symbol g in f's module and I add a global mapping between g
and its stub in MCJIT. Everything works perfectly in the old JIT, so
the code is correct in this case. The problem that I face with
multiple modules in the old JIT is that the symbol g defined in f's
module and the symbol g defined in g's module are not the same, I thus
have to define multiple symbol in the old JIT for the same entity,
which is far from perfect. Anyway, it works.
With MCJIT, I can call f from my C++ code (relatively easy as f is
defined in its own module), the stub of g is called and I can
generate its code (which use other functions). But I can not find a
way to compile g and to update the mapping between g and the new
function pointer. When I use recompileAndRelinkFunction, I see that it
is not implemented in MCJIT, and when I use getFunctionPointer, I
obtain.... a null pointer? I have not investigated further, but
probably, having two symbols g in the same MCJIT does not work. And I
don't see what I can do at this step? Maybe that I have missed
something?
Otherwise, I already predict that I will also have one big problem
latter: I would like to inline functions from the runtime module in f
or g, and I would like to inline the code of an already compiled
function h in g. So, I would like to inline functions that comes from
different modules. It means that I would like to see MCJIT working
like an llvm::Linker, able to resolve the h symbol during the
compilation of g. And for the moment, as MCJIT can not see that the g
defined in f's module and the g defined in g's module represent the
same function, I think that I will have a problem latter.
So, for the moment, I'm a little bit stuck with MCJIT. Something that
could be really useful could be a mcjit that acts as a linker. If
MCJIT could have a map like this (I give a pseudo-c++ code)
class FnDescriptor {
StringRef name;
FunctionType fnType;
LinkageType linkage;
};
class FnState {
llvm::Module* definedModule;
/* maybe a llvm::ObjectImage* ? */
List<RelocationTable*>* users,
void* currentPointer
};
map<FnDescriptor, FnState>
it could be really useful. Let's imagine the same scenario with f that
calls g while g is not yet compiled. At the beginning of this
scenario, "g" "void ()" could simply be associated to a
FnState with
<null, List<>, stubForG>.
After f's compilation, it could be something like that
<null, List<RelocationTable-of-module-f>, stubForG>.
And after the compilation of g, something like
<moduleOfG, List<Reloc-f, Reloc-g>, compiledCode-of-g>
with the relocation entries updated?
Otherwise, for safepoints, and for exception tables, it could be also
really useful to install call backs to let VMKit manages them itself
(but it's maybe provided by the safepoint/patchpoint patch?)? (with
something that can make the association between a MCSymbol and it's
actual address of course :) )
See you!
Gaël
PS: by the way, Yaron, we currently face almost the same problems
2013/11/13 Yaron Keren <yaron.keren at gmail.com>:> Hi Andy,
>
> We had previous discussions about this, I'd like to state more exactly
what
> features would make MCJIT a replacement for the JIT.
> After putting significant effort trying to move to MCJIT, I'm currently
back
> with the JIT. This is in a REPL environment where functions are added and
> removed dynamically and response time is important. The issue is the legacy
> JIT provides great flexibility for this use case which is currently missing
> from MCJIT because of their very different design and goals.
>
> With JIT, you can modify Function(s) in an already-compiled Module, unload
> the machine code and the JIT will automatically recompile and relink the
> function next time it is called. To make MCJIT work like that it would need
> at least :
>
> 1) Automatic module splitting into function-modules.
> 2) Module delete: from module list, from linker namespace, machine code
> unload, unregister EH and debuginfo.
> 3) Stub functions.
> 4) Relinking with stub functions so that new modules are relinked without
> changing already-finalized modules. This is critical to response time as
you
> may change just one function out of 1000.
> 5) Module addition should register EH and debuginfo (this is not done with
> current JIT but while at it...).
>
> REPL environments using the LLVM JIT would likely encounter great
difficulty
> moving to the current MCJIT without the above. 1) could be done by the
> programmer but the a helper function should provide this service. 2)-4)
> could be done only in the MCJIT. 5) is a bonus.
>
> Until MCJIT has this kind of flexibility, I hope the JIT would be kept
> alive.
>
> Yaron
>
>
>
>
>
>
> 2013/11/13 Kaylor, Andrew <andrew.kaylor at intel.com>
>>
>> Hi Gaël,
>>
>> I'm not familiar enough with the details of the old JIT engine and
its
>> event interface to comment on whether or not your changes are
appropriate,
>> but I'm not sure anyone is so the patch is probably OK as is. I
don't see
>> any obvious problems with it.
>>
>> However, your description of the changes raises a bigger issue in my
mind.
>> I'm not sure if you are aware of this, but we're planning to
deprecate the
>> old JIT engine in a future release -- possibly as soon as LLVM 3.5. In
>> order to do so we need to make sure the MCJIT engine is capable of
meeting
>> the needs of current JIT users, and I'm not sure we've got your
case fully
>> covered yet.
>>
>> Can you tell me a little bit more about the details of how you are
using
>> the JIT engine? I'm putting together a document describing various
models
>> for MCJIT use and if your model isn't covered by one of the cases
I've got
>> now I'd like to add it.
>>
>> Also, have you looked at the recently added Stackmap and Patchpoint
>> intrinsics. Without knowing a lot about either your case or those
>> intrinsics, I think that there may be a possible match there. The
thing
>> that raised a red flag for me in your message was that MCJIT
doesn't
>> maintain mappings between the generated code and the LLVM classes from
which
>> it is produced, so we'll probably need a different way to handle
your
>> safepoints.
>>
>> (BTW, it's probably appropriate to move further discussion to the
LLVMDev
>> list rather than llvm-commits.)
>>
>> Thanks,
>> Andy
>>
>>
>> -----Original Message-----
>> From: llvm-commits-bounces at cs.uiuc.edu
>> [mailto:llvm-commits-bounces at cs.uiuc.edu] On Behalf Of Gaël Thomas
>> Sent: Wednesday, November 13, 2013 6:09 AM
>> To: llvm-commits at cs.uiuc.edu
>> Subject: (Very) small patch for the jit event listener
>>
>> Hi all,
>>
>> We have a small problem for vmkit. We rely on the JITEventListener to
>> register the safepoints generated for the garbage collector, and for
that
>> purpose, we have to use the JITCodeEmitter (the
>> MachineCodeEmitter) that was used to generate the MachineFunction in
order
>> to find the physical address of the safepoints (aka, the MCSymbols). A
long
>> time ago, it was not a problem as the JIT class was in the llvm
interface,
>> but today, the header is hidden inside the lib directory and not
installed
>> by llvm. Currently, we directly use this header, but it means that
during
>> the compilation of vmkit, we need the sources of llvm. But, as we are
>> currently developing a debian package of vmkit, we would like to avoid
the
>> installation of the llvm sources to compile vmkit.
>>
>> So, I made a small patch that just adds a new MachineCodeEmitter field
in
>> JITEvent_EmittedFunctionDetails and fill it in JITCodeEmitter. As the
patch
>> only adds a new field in the JITEvent_EmittedFunctionDetails, it should
not
>> break anything. At least, my llvm and my vmkit are still running :) (by
the
>> way, I had to execute a make clean before recompiling llvm because I
think
>> that a dependency is missing)
>>
>> As it is my first patch, I hope that I have used the llvm coding
style...
>>
>> See you,
>> Gaël
>>
>>
>>
>>
>>
>>
>> --
>> -------------------------------------------------------------------
>> Gaël Thomas, Associate Professor, UPMC
>> http://pagesperso-systeme.lip6.fr/Gael.Thomas/
>> -------------------------------------------------------------------
>>
>> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>
>
--
-------------------------------------------------------------------
Gaël Thomas, Associate Professor, UPMC
http://pagesperso-systeme.lip6.fr/Gael.Thomas/
-------------------------------------------------------------------
Gael, anyone who dynamically un/loads functions will face these problems. You have certainly gone much further than me trying to solve them! Yaron 2013/11/14 Gaël Thomas <gael.thomas at lip6.fr>> Hi Andrew, hi all, > > I already saw that the old jit was (almost) deprecated. So, I'm > currently playing with the new jit and it's look very interesting. > (I'm working locally and I haven't pushed anything new on VMKit > because I'm also changing a little the design vmkit). For the moment, > MCJIT does not work with VMKit (but I haven't yet tested the > safepoint/stackmap patch), I don't know if it comes from what I'm > doing or if something is still missing in MCJIT. > > Sorry, my mail will be too long, but I want first to explain what I'm > currently doing and then open the discussion to explain what I would > like to see in MCJIT :) Of course, I can help to develop something or > to give feed back, I'm hardly working on vmkit currently and I have > time to spend on that (but I'm far from an expert in compilation > stuffs!). > > Basically, I want to compile lazily my functions (we have a Java > virtual machine built upon vmkit and compiling all the rt.jar during > the bootstrap is not very realistic:)). So, by lazy compilation, I > mean that I want to compile (and even resolve) a function only when it > is used. For example, during the compilation of > void f() { g() } > I don't want to compile g(). I will only compile g() when it will be > called. > > For the moment, I use a home-made stub (I have attached the asm code, > it can give you some ideas if you plan to integrate a stub generator > able to perform dynamic dispatch like virtual call in c++) because > MCJIT does not provide this facility. So, for each function, I define > a module. In each module, I have to define the runtime function (such > as gcmalloc, throwException and this kind of functions). They are > defined in a separated runtime module populated during the bootstrap. > I have thus this picture > > MCJIT > |---------------------------------------------------------- > | | | > Runtime module module for f module for g > > When I see that I need g during the compilation of f, I define an > external symbol g in f's module and I add a global mapping between g > and its stub in MCJIT. Everything works perfectly in the old JIT, so > the code is correct in this case. The problem that I face with > multiple modules in the old JIT is that the symbol g defined in f's > module and the symbol g defined in g's module are not the same, I thus > have to define multiple symbol in the old JIT for the same entity, > which is far from perfect. Anyway, it works. > > With MCJIT, I can call f from my C++ code (relatively easy as f is > defined in its own module), the stub of g is called and I can > generate its code (which use other functions). But I can not find a > way to compile g and to update the mapping between g and the new > function pointer. When I use recompileAndRelinkFunction, I see that it > is not implemented in MCJIT, and when I use getFunctionPointer, I > obtain.... a null pointer? I have not investigated further, but > probably, having two symbols g in the same MCJIT does not work. And I > don't see what I can do at this step? Maybe that I have missed > something? > > Otherwise, I already predict that I will also have one big problem > latter: I would like to inline functions from the runtime module in f > or g, and I would like to inline the code of an already compiled > function h in g. So, I would like to inline functions that comes from > different modules. It means that I would like to see MCJIT working > like an llvm::Linker, able to resolve the h symbol during the > compilation of g. And for the moment, as MCJIT can not see that the g > defined in f's module and the g defined in g's module represent the > same function, I think that I will have a problem latter. > > So, for the moment, I'm a little bit stuck with MCJIT. Something that > could be really useful could be a mcjit that acts as a linker. If > MCJIT could have a map like this (I give a pseudo-c++ code) > > class FnDescriptor { > StringRef name; > FunctionType fnType; > LinkageType linkage; > }; > > class FnState { > llvm::Module* definedModule; > /* maybe a llvm::ObjectImage* ? */ > List<RelocationTable*>* users, > void* currentPointer > }; > > map<FnDescriptor, FnState> > > it could be really useful. Let's imagine the same scenario with f that > calls g while g is not yet compiled. At the beginning of this > scenario, "g" "void ()" could simply be associated to a FnState with > > <null, List<>, stubForG>. > > After f's compilation, it could be something like that > > <null, List<RelocationTable-of-module-f>, stubForG>. > > And after the compilation of g, something like > > <moduleOfG, List<Reloc-f, Reloc-g>, compiledCode-of-g> > > with the relocation entries updated? > > Otherwise, for safepoints, and for exception tables, it could be also > really useful to install call backs to let VMKit manages them itself > (but it's maybe provided by the safepoint/patchpoint patch?)? (with > something that can make the association between a MCSymbol and it's > actual address of course :) ) > > See you! > Gaël > > PS: by the way, Yaron, we currently face almost the same problems > > > 2013/11/13 Yaron Keren <yaron.keren at gmail.com>: > > Hi Andy, > > > > We had previous discussions about this, I'd like to state more exactly > what > > features would make MCJIT a replacement for the JIT. > > After putting significant effort trying to move to MCJIT, I'm currently > back > > with the JIT. This is in a REPL environment where functions are added and > > removed dynamically and response time is important. The issue is the > legacy > > JIT provides great flexibility for this use case which is currently > missing > > from MCJIT because of their very different design and goals. > > > > With JIT, you can modify Function(s) in an already-compiled Module, > unload > > the machine code and the JIT will automatically recompile and relink the > > function next time it is called. To make MCJIT work like that it would > need > > at least : > > > > 1) Automatic module splitting into function-modules. > > 2) Module delete: from module list, from linker namespace, machine code > > unload, unregister EH and debuginfo. > > 3) Stub functions. > > 4) Relinking with stub functions so that new modules are relinked without > > changing already-finalized modules. This is critical to response time as > you > > may change just one function out of 1000. > > 5) Module addition should register EH and debuginfo (this is not done > with > > current JIT but while at it...). > > > > REPL environments using the LLVM JIT would likely encounter great > difficulty > > moving to the current MCJIT without the above. 1) could be done by the > > programmer but the a helper function should provide this service. 2)-4) > > could be done only in the MCJIT. 5) is a bonus. > > > > Until MCJIT has this kind of flexibility, I hope the JIT would be kept > > alive. > > > > Yaron > > > > > > > > > > > > > > 2013/11/13 Kaylor, Andrew <andrew.kaylor at intel.com> > >> > >> Hi Gaël, > >> > >> I'm not familiar enough with the details of the old JIT engine and its > >> event interface to comment on whether or not your changes are > appropriate, > >> but I'm not sure anyone is so the patch is probably OK as is. I don't > see > >> any obvious problems with it. > >> > >> However, your description of the changes raises a bigger issue in my > mind. > >> I'm not sure if you are aware of this, but we're planning to deprecate > the > >> old JIT engine in a future release -- possibly as soon as LLVM 3.5. In > >> order to do so we need to make sure the MCJIT engine is capable of > meeting > >> the needs of current JIT users, and I'm not sure we've got your case > fully > >> covered yet. > >> > >> Can you tell me a little bit more about the details of how you are using > >> the JIT engine? I'm putting together a document describing various > models > >> for MCJIT use and if your model isn't covered by one of the cases I've > got > >> now I'd like to add it. > >> > >> Also, have you looked at the recently added Stackmap and Patchpoint > >> intrinsics. Without knowing a lot about either your case or those > >> intrinsics, I think that there may be a possible match there. The thing > >> that raised a red flag for me in your message was that MCJIT doesn't > >> maintain mappings between the generated code and the LLVM classes from > which > >> it is produced, so we'll probably need a different way to handle your > >> safepoints. > >> > >> (BTW, it's probably appropriate to move further discussion to the > LLVMDev > >> list rather than llvm-commits.) > >> > >> Thanks, > >> Andy > >> > >> > >> -----Original Message----- > >> From: llvm-commits-bounces at cs.uiuc.edu > >> [mailto:llvm-commits-bounces at cs.uiuc.edu] On Behalf Of Gaël Thomas > >> Sent: Wednesday, November 13, 2013 6:09 AM > >> To: llvm-commits at cs.uiuc.edu > >> Subject: (Very) small patch for the jit event listener > >> > >> Hi all, > >> > >> We have a small problem for vmkit. We rely on the JITEventListener to > >> register the safepoints generated for the garbage collector, and for > that > >> purpose, we have to use the JITCodeEmitter (the > >> MachineCodeEmitter) that was used to generate the MachineFunction in > order > >> to find the physical address of the safepoints (aka, the MCSymbols). A > long > >> time ago, it was not a problem as the JIT class was in the llvm > interface, > >> but today, the header is hidden inside the lib directory and not > installed > >> by llvm. Currently, we directly use this header, but it means that > during > >> the compilation of vmkit, we need the sources of llvm. But, as we are > >> currently developing a debian package of vmkit, we would like to avoid > the > >> installation of the llvm sources to compile vmkit. > >> > >> So, I made a small patch that just adds a new MachineCodeEmitter field > in > >> JITEvent_EmittedFunctionDetails and fill it in JITCodeEmitter. As the > patch > >> only adds a new field in the JITEvent_EmittedFunctionDetails, it should > not > >> break anything. At least, my llvm and my vmkit are still running :) (by > the > >> way, I had to execute a make clean before recompiling llvm because I > think > >> that a dependency is missing) > >> > >> As it is my first patch, I hope that I have used the llvm coding > style... > >> > >> See you, > >> Gaël > >> > >> > >> > >> > >> > >> > >> -- > >> ------------------------------------------------------------------- > >> Gaël Thomas, Associate Professor, UPMC > >> http://pagesperso-systeme.lip6.fr/Gael.Thomas/ > >> ------------------------------------------------------------------- > >> > >> _______________________________________________ > >> llvm-commits mailing list > >> llvm-commits at cs.uiuc.edu > >> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits > > > > > > > > -- > ------------------------------------------------------------------- > Gaël Thomas, Associate Professor, UPMC > http://pagesperso-systeme.lip6.fr/Gael.Thomas/ > ------------------------------------------------------------------- >-------------- next part -------------- An HTML attachment was scrubbed... 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Kaylor, Andrew
2013-Nov-14 00:09 UTC
[LLVMdev] (Very) small patch for the jit event listener
Hi Gaël,
Thank you for the detailed explanation. It's very helpful.
All of the things you describe could be done within MCJIT, but I'm not sure
that's where they belong. We had a discussion about lazy function
compilation at the LLVM Developers Meeting last week and the consensus among
those present was that it would be better to leave this sort of lazy compilation
to the MCJIT client rather than having MCJIT try to solve the problem because
each client knows more about how it should be implemented for their particular
case than MCJIT can possibly know. We could, perhaps, provide a reference
implementation (outside of MCJIT) but it would likely be a very simple solution
not well-suited for use in real-world programs.
The approach you describe with home-made stubs seems good. Now we just need to
figure out why it isn't working!
To begin with, there problem is an issue with naming. In order to get the
home-made stub solution to work, you'll need to distinguish between the name
of the function as it appears at the call site (which will result in a call to
your stub function) and the name of the function as it is defined in the
implementation module. For instance, you'll probably want the module for
'f' to look something like this:
----
declare i32 @g_stub()
define i32 @f() {
%r = call i32 @g_stub( )
ret i32 %r
}
----
and the module for 'g' will look something like this:
----
define i32 @g() {
ret i32 0
}
----
Now when you generate code for the 'f' module, MCJIT will call the
memory manager asking for the address of 'g_stub' and you'll give it
a pointer to your stub function. When your stub function gets called,
you'll call MCJIT::getFunctionAddress('g') and MCJIT will generate
code for the 'g' module and return the address of the real 'g'
function.
You'll need to keep the stub around as a pass-through because the address of
the stub is now baked into the code that was generated for 'f', but
that's a good thing is you might eventually want to replace the function
because it gives you a single point to redirect calls to 'g' from any
module that is calling it through 'g_stub'. If it's really
important to you to be able to modify the 'f' call site to go directly
to 'g' once its generated, you may be able to do that with the
patchpoint stuff, but I don't know the details.
Anyway, except for the stub naming, that sounds an awful lot like what you
described. If using the names as I describe doesn't fix things for you, we
may have a bug to fix.
The inlining problem is perhaps a bit trickier. What I would suggest in that
case is that you basically need to link your modules against the library module
that you want to use for inlining before you pass the modules to MCJIT.
There's an Intel product that does this, so I know it can be made to work.
I'm not sure there's a simple interface for it. Basically what you need
to do is extract the functions from your runtime library into the module you
want to optimize and then run it through whatever optimization passes you're
interested in. I'm sure there are some helper classes that could be
implemented to make this easier, but the details are ultimately implementation
specific. This is a problem with the old JIT too though, right?
Inlining the code of an already compiled module is an even tougher problem.
That's really something that would need to be addressed at the code
generation level, I think. If you had the generated code in a sufficiently
general form you could probably do this with an intrinsic (assuming you
definitely knew you wanted to inline it). Generally I think it's outside
the design space that MCJIT intends to target.
So, to summarize, I think that the current implementation of MCJIT is sufficient
to address your lazy compilation needs, though I'd be happy to continue the
discussion if you think something more is needed. That leaves me with your
"safepoint" issue, which I don't have a clear picture of.
-Andy
-----Original Message-----
From: Gaël Thomas [mailto:gael.thomas at lip6.fr]
Sent: Wednesday, November 13, 2013 2:46 PM
To: Yaron Keren
Cc: Kaylor, Andrew; llvm-commits at cs.uiuc.edu; LLVM Dev
Subject: Re: (Very) small patch for the jit event listener
Hi Andrew, hi all,
I already saw that the old jit was (almost) deprecated. So, I'm currently
playing with the new jit and it's look very interesting.
(I'm working locally and I haven't pushed anything new on VMKit because
I'm also changing a little the design vmkit). For the moment, MCJIT does not
work with VMKit (but I haven't yet tested the safepoint/stackmap patch), I
don't know if it comes from what I'm doing or if something is still
missing in MCJIT.
Sorry, my mail will be too long, but I want first to explain what I'm
currently doing and then open the discussion to explain what I would like to see
in MCJIT :) Of course, I can help to develop something or to give feed back,
I'm hardly working on vmkit currently and I have time to spend on that (but
I'm far from an expert in compilation stuffs!).
Basically, I want to compile lazily my functions (we have a Java virtual machine
built upon vmkit and compiling all the rt.jar during the bootstrap is not very
realistic:)). So, by lazy compilation, I mean that I want to compile (and even
resolve) a function only when it is used. For example, during the compilation of
void f() { g() } I don't want to compile g(). I will only compile g() when
it will be called.
For the moment, I use a home-made stub (I have attached the asm code, it can
give you some ideas if you plan to integrate a stub generator able to perform
dynamic dispatch like virtual call in c++) because MCJIT does not provide this
facility. So, for each function, I define a module. In each module, I have to
define the runtime function (such as gcmalloc, throwException and this kind of
functions). They are defined in a separated runtime module populated during the
bootstrap.
I have thus this picture
MCJIT
|----------------------------------------------------------
| | |
Runtime module module for f module for g
When I see that I need g during the compilation of f, I define an external
symbol g in f's module and I add a global mapping between g and its stub in
MCJIT. Everything works perfectly in the old JIT, so the code is correct in this
case. The problem that I face with multiple modules in the old JIT is that the
symbol g defined in f's module and the symbol g defined in g's module
are not the same, I thus have to define multiple symbol in the old JIT for the
same entity, which is far from perfect. Anyway, it works.
With MCJIT, I can call f from my C++ code (relatively easy as f is defined in
its own module), the stub of g is called and I can generate its code (which use
other functions). But I can not find a way to compile g and to update the
mapping between g and the new function pointer. When I use
recompileAndRelinkFunction, I see that it is not implemented in MCJIT, and when
I use getFunctionPointer, I obtain.... a null pointer? I have not investigated
further, but probably, having two symbols g in the same MCJIT does not work. And
I don't see what I can do at this step? Maybe that I have missed something?
Otherwise, I already predict that I will also have one big problem
latter: I would like to inline functions from the runtime module in f or g, and
I would like to inline the code of an already compiled function h in g. So, I
would like to inline functions that comes from different modules. It means that
I would like to see MCJIT working like an llvm::Linker, able to resolve the h
symbol during the compilation of g. And for the moment, as MCJIT can not see
that the g defined in f's module and the g defined in g's module
represent the same function, I think that I will have a problem latter.
So, for the moment, I'm a little bit stuck with MCJIT. Something that could
be really useful could be a mcjit that acts as a linker. If MCJIT could have a
map like this (I give a pseudo-c++ code)
class FnDescriptor {
StringRef name;
FunctionType fnType;
LinkageType linkage;
};
class FnState {
llvm::Module* definedModule;
/* maybe a llvm::ObjectImage* ? */
List<RelocationTable*>* users,
void* currentPointer
};
map<FnDescriptor, FnState>
it could be really useful. Let's imagine the same scenario with f that calls
g while g is not yet compiled. At the beginning of this scenario, "g"
"void ()" could simply be associated to a FnState with
<null, List<>, stubForG>.
After f's compilation, it could be something like that
<null, List<RelocationTable-of-module-f>, stubForG>.
And after the compilation of g, something like
<moduleOfG, List<Reloc-f, Reloc-g>, compiledCode-of-g>
with the relocation entries updated?
Otherwise, for safepoints, and for exception tables, it could be also really
useful to install call backs to let VMKit manages them itself (but it's
maybe provided by the safepoint/patchpoint patch?)? (with something that can
make the association between a MCSymbol and it's actual address of course :)
)
See you!
Gaël
PS: by the way, Yaron, we currently face almost the same problems
2013/11/13 Yaron Keren <yaron.keren at gmail.com>:> Hi Andy,
>
> We had previous discussions about this, I'd like to state more exactly
> what features would make MCJIT a replacement for the JIT.
> After putting significant effort trying to move to MCJIT, I'm
> currently back with the JIT. This is in a REPL environment where
> functions are added and removed dynamically and response time is
> important. The issue is the legacy JIT provides great flexibility for
> this use case which is currently missing from MCJIT because of their very
different design and goals.
>
> With JIT, you can modify Function(s) in an already-compiled Module,
> unload the machine code and the JIT will automatically recompile and
> relink the function next time it is called. To make MCJIT work like
> that it would need at least :
>
> 1) Automatic module splitting into function-modules.
> 2) Module delete: from module list, from linker namespace, machine
> code unload, unregister EH and debuginfo.
> 3) Stub functions.
> 4) Relinking with stub functions so that new modules are relinked
> without changing already-finalized modules. This is critical to
> response time as you may change just one function out of 1000.
> 5) Module addition should register EH and debuginfo (this is not done
> with current JIT but while at it...).
>
> REPL environments using the LLVM JIT would likely encounter great
> difficulty moving to the current MCJIT without the above. 1) could be
> done by the programmer but the a helper function should provide this
> service. 2)-4) could be done only in the MCJIT. 5) is a bonus.
>
> Until MCJIT has this kind of flexibility, I hope the JIT would be kept
> alive.
>
> Yaron
>
>
>
>
>
>
> 2013/11/13 Kaylor, Andrew <andrew.kaylor at intel.com>
>>
>> Hi Gaël,
>>
>> I'm not familiar enough with the details of the old JIT engine and
>> its event interface to comment on whether or not your changes are
>> appropriate, but I'm not sure anyone is so the patch is probably OK
>> as is. I don't see any obvious problems with it.
>>
>> However, your description of the changes raises a bigger issue in my
mind.
>> I'm not sure if you are aware of this, but we're planning to
>> deprecate the old JIT engine in a future release -- possibly as soon
>> as LLVM 3.5. In order to do so we need to make sure the MCJIT engine
>> is capable of meeting the needs of current JIT users, and I'm not
>> sure we've got your case fully covered yet.
>>
>> Can you tell me a little bit more about the details of how you are
>> using the JIT engine? I'm putting together a document describing
>> various models for MCJIT use and if your model isn't covered by one
>> of the cases I've got now I'd like to add it.
>>
>> Also, have you looked at the recently added Stackmap and Patchpoint
>> intrinsics. Without knowing a lot about either your case or those
>> intrinsics, I think that there may be a possible match there. The
>> thing that raised a red flag for me in your message was that MCJIT
>> doesn't maintain mappings between the generated code and the LLVM
>> classes from which it is produced, so we'll probably need a
different
>> way to handle your safepoints.
>>
>> (BTW, it's probably appropriate to move further discussion to the
>> LLVMDev list rather than llvm-commits.)
>>
>> Thanks,
>> Andy
>>
>>
>> -----Original Message-----
>> From: llvm-commits-bounces at cs.uiuc.edu
>> [mailto:llvm-commits-bounces at cs.uiuc.edu] On Behalf Of Gaël Thomas
>> Sent: Wednesday, November 13, 2013 6:09 AM
>> To: llvm-commits at cs.uiuc.edu
>> Subject: (Very) small patch for the jit event listener
>>
>> Hi all,
>>
>> We have a small problem for vmkit. We rely on the JITEventListener to
>> register the safepoints generated for the garbage collector, and for
>> that purpose, we have to use the JITCodeEmitter (the
>> MachineCodeEmitter) that was used to generate the MachineFunction in
>> order to find the physical address of the safepoints (aka, the
>> MCSymbols). A long time ago, it was not a problem as the JIT class
>> was in the llvm interface, but today, the header is hidden inside the
>> lib directory and not installed by llvm. Currently, we directly use
>> this header, but it means that during the compilation of vmkit, we
>> need the sources of llvm. But, as we are currently developing a
>> debian package of vmkit, we would like to avoid the installation of the
llvm sources to compile vmkit.
>>
>> So, I made a small patch that just adds a new MachineCodeEmitter
>> field in JITEvent_EmittedFunctionDetails and fill it in
>> JITCodeEmitter. As the patch only adds a new field in the
>> JITEvent_EmittedFunctionDetails, it should not break anything. At
>> least, my llvm and my vmkit are still running :) (by the way, I had
>> to execute a make clean before recompiling llvm because I think that
>> a dependency is missing)
>>
>> As it is my first patch, I hope that I have used the llvm coding
style...
>>
>> See you,
>> Gaël
>>
>>
>>
>>
>>
>>
>> --
>> -------------------------------------------------------------------
>> Gaël Thomas, Associate Professor, UPMC
>> http://pagesperso-systeme.lip6.fr/Gael.Thomas/
>> -------------------------------------------------------------------
>>
>> _______________________________________________
>> llvm-commits mailing list
>> llvm-commits at cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
>
>
--
-------------------------------------------------------------------
Gaël Thomas, Associate Professor, UPMC
http://pagesperso-systeme.lip6.fr/Gael.Thomas/
-------------------------------------------------------------------
Hi Andy, Thanks for the answer. I'm currently reading the internal code of MCJIT and it's really a great work (I was only using the ExecutionEngine interface for the moment). So, I agree, all what I need is already in the code (see below) :) 2013/11/14 Kaylor, Andrew <andrew.kaylor at intel.com>:> Hi Gaël, > > Thank you for the detailed explanation. It's very helpful. > > All of the things you describe could be done within MCJIT, but I'm not sure that's where they belong. We had a discussion about lazy function compilation at the LLVM Developers Meeting last week and the consensus among those present was that it would be better to leave this sort of lazy compilation to the MCJIT client rather than having MCJIT try to solve the problem because each client knows more about how it should be implemented for their particular case than MCJIT can possibly know. We could, perhaps, provide a reference implementation (outside of MCJIT) but it would likely be a very simple solution not well-suited for use in real-world programs.I understand the point. Probably that providing a small example that describes how using advanced features of MCJIT could help. If I can manage to make MCJIT works with VMKit, I'll be happy to send you an example of lazy compilation that highlight some of the features of MCJIT.> > The approach you describe with home-made stubs seems good. Now we just need to figure out why it isn't working! > > To begin with, there problem is an issue with naming. In order to get the home-made stub solution to work, you'll need to distinguish between the name of the function as it appears at the call site (which will result in a call to your stub function) and the name of the function as it is defined in the implementation module.I don't know if I understand, you say that having two different names is required, or simply that it's more easy to manage? Because I can have to generate a lot of calls to g before compiling it and it means that I will have to generate a lot of different names (not a big deal, but it takes time during the execution)...> For instance, you'll probably want the module for 'f' to look something like this: > > ---- > > declare i32 @g_stub() > > define i32 @f() { > %r = call i32 @g_stub( ) > ret i32 %r > } > > ---- > > and the module for 'g' will look something like this: > > ---- > > define i32 @g() { > ret i32 0 > } > > ---- > > Now when you generate code for the 'f' module, MCJIT will call the memory manager asking for the address of 'g_stub' and you'll give it a pointer to your stub function. When your stub function gets called, you'll call MCJIT::getFunctionAddress('g') and MCJIT will generate code for the 'g' module and return the address of the real 'g' function. > > You'll need to keep the stub around as a pass-through because the address of the stub is now baked into the code that was generated for 'f',But the location of the call-site (the pointer to g_stub in the generated code) should be in the relocation table of the module f? You don't think that a client could reuse this relocation table directly? (as we can do to dynamically modify a function pointer in a shared library)> but that's a good thing is you might eventually want to replace the function because it gives you a single point to redirect calls to 'g' from any module that is calling it through 'g_stub'.For performance, it's not perfect (two calls to reach a function), but I agree, it simplifies the management.> If it's really important to you to be able to modify the 'f' call site to go directly to 'g' once its generated, you may be able to do that with the patchpoint stuff, but I don't know the details.Yes, I'm pretty sure that with patchpoint, I can handle this problem.> > Anyway, except for the stub naming, that sounds an awful lot like what you described. If using the names as I describe doesn't fix things for you, we may have a bug to fix.cool :) I will try this week-end and I will tell you if it works.> > The inlining problem is perhaps a bit trickier. What I would suggest in that case is that you basically need to link your modules against the library module that you want to use for inlining before you pass the modules to MCJIT. There's an Intel product that does this, so I know it can be made to work. I'm not sure there's a simple interface for it. Basically what you need to do is extract the functions from your runtime library into the module you want to optimize and then run it through whatever optimization passes you're interested in. > I'm sure there are some helper classes that could be implemented to make this easier, but the details are ultimately implementation specific. This is a problem with the old JIT too though, right? > > Inlining the code of an already compiled module is an even tougher problem. That's really something that would need to be addressed at the code generation level, I think. If you had the generated code in a sufficiently general form you could probably do this with an intrinsic (assuming you definitely knew you wanted to inline it). Generally I think it's outside the design space that MCJIT intends to target.Yes, I also think so, it's a problem of code generation. I will try to adapt the current SimpleInline pass of llvm and use the internal state of VMKit/J3 to retrieve the IR of an already generated function. As I have to keep a map that associates names to llvm::Function in VMKit, I just have to use it to find the code.> > So, to summarize, I think that the current implementation of MCJIT is sufficient to address your lazy compilation needs, though I'd be happy to continue the discussion if you think something more is needed.Yes, after your answer and after having taken a look inside the MCJIT code, I'm pretty sure that I can use MCJIT for lazy compilation.> That leaves me with your "safepoint" issue, which I don't have a clear picture of.I'm not at this point for the moment :) As soon as I'm able to make VMKit runs with MCJIT (without the garbage collector), I will try to find exactly what is missing for the GC (and I will also explore the safepoint/patchpoint patch to see if I can use it in VMKit for the GC, but it's not related to this topic). Thank you for everything, I'll tell you soon if it works, Gaël> > -Andy > > > -----Original Message----- > From: Gaël Thomas [mailto:gael.thomas at lip6.fr] > Sent: Wednesday, November 13, 2013 2:46 PM > To: Yaron Keren > Cc: Kaylor, Andrew; llvm-commits at cs.uiuc.edu; LLVM Dev > Subject: Re: (Very) small patch for the jit event listener > > Hi Andrew, hi all, > > I already saw that the old jit was (almost) deprecated. So, I'm currently playing with the new jit and it's look very interesting. > (I'm working locally and I haven't pushed anything new on VMKit because I'm also changing a little the design vmkit). For the moment, MCJIT does not work with VMKit (but I haven't yet tested the safepoint/stackmap patch), I don't know if it comes from what I'm doing or if something is still missing in MCJIT. > > Sorry, my mail will be too long, but I want first to explain what I'm currently doing and then open the discussion to explain what I would like to see in MCJIT :) Of course, I can help to develop something or to give feed back, I'm hardly working on vmkit currently and I have time to spend on that (but I'm far from an expert in compilation stuffs!). > > Basically, I want to compile lazily my functions (we have a Java virtual machine built upon vmkit and compiling all the rt.jar during the bootstrap is not very realistic:)). So, by lazy compilation, I mean that I want to compile (and even resolve) a function only when it is used. For example, during the compilation of void f() { g() } I don't want to compile g(). I will only compile g() when it will be called. > > For the moment, I use a home-made stub (I have attached the asm code, it can give you some ideas if you plan to integrate a stub generator able to perform dynamic dispatch like virtual call in c++) because MCJIT does not provide this facility. So, for each function, I define a module. In each module, I have to define the runtime function (such as gcmalloc, throwException and this kind of functions). They are defined in a separated runtime module populated during the bootstrap. > I have thus this picture > > MCJIT > |---------------------------------------------------------- > | | | > Runtime module module for f module for g > > When I see that I need g during the compilation of f, I define an external symbol g in f's module and I add a global mapping between g and its stub in MCJIT. Everything works perfectly in the old JIT, so the code is correct in this case. The problem that I face with multiple modules in the old JIT is that the symbol g defined in f's module and the symbol g defined in g's module are not the same, I thus have to define multiple symbol in the old JIT for the same entity, which is far from perfect. Anyway, it works. > > With MCJIT, I can call f from my C++ code (relatively easy as f is defined in its own module), the stub of g is called and I can generate its code (which use other functions). But I can not find a way to compile g and to update the mapping between g and the new function pointer. When I use recompileAndRelinkFunction, I see that it is not implemented in MCJIT, and when I use getFunctionPointer, I obtain.... a null pointer? I have not investigated further, but probably, having two symbols g in the same MCJIT does not work. And I don't see what I can do at this step? Maybe that I have missed something? > > Otherwise, I already predict that I will also have one big problem > latter: I would like to inline functions from the runtime module in f or g, and I would like to inline the code of an already compiled function h in g. So, I would like to inline functions that comes from different modules. It means that I would like to see MCJIT working like an llvm::Linker, able to resolve the h symbol during the compilation of g. And for the moment, as MCJIT can not see that the g defined in f's module and the g defined in g's module represent the same function, I think that I will have a problem latter. > > So, for the moment, I'm a little bit stuck with MCJIT. Something that could be really useful could be a mcjit that acts as a linker. If MCJIT could have a map like this (I give a pseudo-c++ code) > > class FnDescriptor { > StringRef name; > FunctionType fnType; > LinkageType linkage; > }; > > class FnState { > llvm::Module* definedModule; > /* maybe a llvm::ObjectImage* ? */ > List<RelocationTable*>* users, > void* currentPointer > }; > > map<FnDescriptor, FnState> > > it could be really useful. Let's imagine the same scenario with f that calls g while g is not yet compiled. At the beginning of this scenario, "g" "void ()" could simply be associated to a FnState with > > <null, List<>, stubForG>. > > After f's compilation, it could be something like that > > <null, List<RelocationTable-of-module-f>, stubForG>. > > And after the compilation of g, something like > > <moduleOfG, List<Reloc-f, Reloc-g>, compiledCode-of-g> > > with the relocation entries updated? > > Otherwise, for safepoints, and for exception tables, it could be also really useful to install call backs to let VMKit manages them itself (but it's maybe provided by the safepoint/patchpoint patch?)? (with something that can make the association between a MCSymbol and it's actual address of course :) ) > > See you! > Gaël > > PS: by the way, Yaron, we currently face almost the same problems > > > 2013/11/13 Yaron Keren <yaron.keren at gmail.com>: >> Hi Andy, >> >> We had previous discussions about this, I'd like to state more exactly >> what features would make MCJIT a replacement for the JIT. >> After putting significant effort trying to move to MCJIT, I'm >> currently back with the JIT. This is in a REPL environment where >> functions are added and removed dynamically and response time is >> important. The issue is the legacy JIT provides great flexibility for >> this use case which is currently missing from MCJIT because of their very different design and goals. >> >> With JIT, you can modify Function(s) in an already-compiled Module, >> unload the machine code and the JIT will automatically recompile and >> relink the function next time it is called. To make MCJIT work like >> that it would need at least : >> >> 1) Automatic module splitting into function-modules. >> 2) Module delete: from module list, from linker namespace, machine >> code unload, unregister EH and debuginfo. >> 3) Stub functions. >> 4) Relinking with stub functions so that new modules are relinked >> without changing already-finalized modules. This is critical to >> response time as you may change just one function out of 1000. >> 5) Module addition should register EH and debuginfo (this is not done >> with current JIT but while at it...). >> >> REPL environments using the LLVM JIT would likely encounter great >> difficulty moving to the current MCJIT without the above. 1) could be >> done by the programmer but the a helper function should provide this >> service. 2)-4) could be done only in the MCJIT. 5) is a bonus. >> >> Until MCJIT has this kind of flexibility, I hope the JIT would be kept >> alive. >> >> Yaron >> >> >> >> >> >> >> 2013/11/13 Kaylor, Andrew <andrew.kaylor at intel.com> >>> >>> Hi Gaël, >>> >>> I'm not familiar enough with the details of the old JIT engine and >>> its event interface to comment on whether or not your changes are >>> appropriate, but I'm not sure anyone is so the patch is probably OK >>> as is. I don't see any obvious problems with it. >>> >>> However, your description of the changes raises a bigger issue in my mind. >>> I'm not sure if you are aware of this, but we're planning to >>> deprecate the old JIT engine in a future release -- possibly as soon >>> as LLVM 3.5. In order to do so we need to make sure the MCJIT engine >>> is capable of meeting the needs of current JIT users, and I'm not >>> sure we've got your case fully covered yet. >>> >>> Can you tell me a little bit more about the details of how you are >>> using the JIT engine? I'm putting together a document describing >>> various models for MCJIT use and if your model isn't covered by one >>> of the cases I've got now I'd like to add it. >>> >>> Also, have you looked at the recently added Stackmap and Patchpoint >>> intrinsics. Without knowing a lot about either your case or those >>> intrinsics, I think that there may be a possible match there. The >>> thing that raised a red flag for me in your message was that MCJIT >>> doesn't maintain mappings between the generated code and the LLVM >>> classes from which it is produced, so we'll probably need a different >>> way to handle your safepoints. >>> >>> (BTW, it's probably appropriate to move further discussion to the >>> LLVMDev list rather than llvm-commits.) >>> >>> Thanks, >>> Andy >>> >>> >>> -----Original Message----- >>> From: llvm-commits-bounces at cs.uiuc.edu >>> [mailto:llvm-commits-bounces at cs.uiuc.edu] On Behalf Of Gaël Thomas >>> Sent: Wednesday, November 13, 2013 6:09 AM >>> To: llvm-commits at cs.uiuc.edu >>> Subject: (Very) small patch for the jit event listener >>> >>> Hi all, >>> >>> We have a small problem for vmkit. We rely on the JITEventListener to >>> register the safepoints generated for the garbage collector, and for >>> that purpose, we have to use the JITCodeEmitter (the >>> MachineCodeEmitter) that was used to generate the MachineFunction in >>> order to find the physical address of the safepoints (aka, the >>> MCSymbols). A long time ago, it was not a problem as the JIT class >>> was in the llvm interface, but today, the header is hidden inside the >>> lib directory and not installed by llvm. Currently, we directly use >>> this header, but it means that during the compilation of vmkit, we >>> need the sources of llvm. But, as we are currently developing a >>> debian package of vmkit, we would like to avoid the installation of the llvm sources to compile vmkit. >>> >>> So, I made a small patch that just adds a new MachineCodeEmitter >>> field in JITEvent_EmittedFunctionDetails and fill it in >>> JITCodeEmitter. As the patch only adds a new field in the >>> JITEvent_EmittedFunctionDetails, it should not break anything. At >>> least, my llvm and my vmkit are still running :) (by the way, I had >>> to execute a make clean before recompiling llvm because I think that >>> a dependency is missing) >>> >>> As it is my first patch, I hope that I have used the llvm coding style... >>> >>> See you, >>> Gaël >>> >>> >>> >>> >>> >>> >>> -- >>> ------------------------------------------------------------------- >>> Gaël Thomas, Associate Professor, UPMC >>> http://pagesperso-systeme.lip6.fr/Gael.Thomas/ >>> ------------------------------------------------------------------- >>> >>> _______________________________________________ >>> llvm-commits mailing list >>> llvm-commits at cs.uiuc.edu >>> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits >> >> > > > > -- > ------------------------------------------------------------------- > Gaël Thomas, Associate Professor, UPMC > http://pagesperso-systeme.lip6.fr/Gael.Thomas/ > --------------------------------------------------------------------- ------------------------------------------------------------------- Gaël Thomas, Associate Professor, UPMC http://pagesperso-systeme.lip6.fr/Gael.Thomas/ -------------------------------------------------------------------
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