llvm dev - Mar 2013 - [LLVMdev] Memory clean for applications using LLVM for JIT compilation

Thanks for the reply, nice to have some validation.  I thought of
another approach which might be preferable:
generate relocatable code, use a JITEventListener to grab each
function and copy it to my own memory,
let all the LLVM stuff die normally then use my copy of the code.
However when I call setRelocationModel(Reloc::PIC_) on the engine
builder I get code that seg faults.
The assembly looks plausible at a glance, but I'm not really up to
speed on x86 assembly.
Is PIC supposed to work with JIT on X86-32?

On Sat, Mar 16, 2013 at 11:35 AM, Dirkjan Bussink <d.bussink at gmail.com>
wrote:
>
> On Mar 7, 2013, at 20:48 , Frank Henigman <fjhenigman at google.com>
wrote:
>
>> I derived a class from JITMemoryManager which delegates everything to
>> an instance made with CreateDefaultMemManager().  ExecutionEngine
>> destroys the wrapper, but I keep the inner instance which did the
>> actual work.  Works, but seems a bit ugly.  Did you find any other
>> solutions?
>
> I ended up implementing the exact same thing, it feels dirty but it has
worked great for us so far.
>
> --
> Dirkjan

I'm not sure I see why the delegating memory manager feels wrong to both of
you.  That's exactly the kind of usage model I would envision for clients
that needed to handle multiple ExecutionEngines that had reason to share a
memory manager.  I'm saying this as an invitation to discussion, not to be
argumentative.  We certainly could change things if it would make the design
better.

Basically, the intention of the current design is to keep the ExecutionEngine
ignorant of memory allocation details.  I'm not sure it always works that
way, but that's the intent.  I believe that the decision to have the
ExecutionEngine own the memory manager was made so that it would be
self-contained and capable of cleaning up after itself.

The JIT engine has a rather ad hoc history and is more complicated than anyone
would want.  It allocates and links together more things than it directly
exposes and the lifecycles of different bits of memory aren't always
obvious, especially if you get into re-JITing code.  I'm not sure
there's a lot that can be done in the general case to keep it clean other
than to have the engine own everything.

I don't believe that the JIT engine is designed to handle having its
function memory moved, so even if you could work out the relocation issues I
don't think it's a good idea.

The MCJIT engine, on the other hand, is designed to have its memory moved on the
fly (though at the granularity of sections, not functions).  The MCJIT engine
does not work with the PIC relocation model on 32-bit x86 targets, but it will
let you reapply relocations with other models after you move things. 
There's some code in the lli utility that does this if you're
interested.

I'm not sure what the current maintenance model is for the older JIT engine.
Near as I can tell when it grows at all it is driven by people who have a need
making specific contributions and as such it doesn't have a particularly
coherent direction.

I have more idealistic hopes for the MCJIT engine, and if you're interested
in using it I'd be happy to talk to you about future design directions.

-Andy



-----Original Message-----
From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Frank Henigman
Sent: Sunday, March 17, 2013 2:18 PM
To: Dirkjan Bussink
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Memory clean for applications using LLVM for JIT
compilation

Thanks for the reply, nice to have some validation.  I thought of another
approach which might be preferable:
generate relocatable code, use a JITEventListener to grab each function and copy
it to my own memory, let all the LLVM stuff die normally then use my copy of the
code.
However when I call setRelocationModel(Reloc::PIC_) on the engine builder I get
code that seg faults.
The assembly looks plausible at a glance, but I'm not really up to speed on
x86 assembly.
Is PIC supposed to work with JIT on X86-32?

On Sat, Mar 16, 2013 at 11:35 AM, Dirkjan Bussink <d.bussink at gmail.com>
wrote:
>
> On Mar 7, 2013, at 20:48 , Frank Henigman <fjhenigman at google.com>
wrote:
>
>> I derived a class from JITMemoryManager which delegates everything to 
>> an instance made with CreateDefaultMemManager().  ExecutionEngine 
>> destroys the wrapper, but I keep the inner instance which did the 
>> actual work.  Works, but seems a bit ugly.  Did you find any other 
>> solutions?
>
> I ended up implementing the exact same thing, it feels dirty but it has
worked great for us so far.
>
> --
> Dirkjan
_______________________________________________
LLVM Developers mailing list
LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

It seems wrong to delegate a dozen or more methods when I only want to
change the behavior in a couple small ways.  It would have been easier
to derive a class, but the base class is inaccessible (in an anonymous
namespace).  And I was afraid to roll my own memory manager because I
didn't understand what all those methods do.
So MCJIT is future - will JITMemoryManager remain relevant?  If so I
can send a patch to make it possible to derive from the default
implementation.
In my case (compiling shaders in mesa, a graphics library) an engine
is used once then thrown away so I don't think there's a risk of
confusing it by moving the code.
Thanks for the tip about relocating with MCJIT.


On Mon, Mar 18, 2013 at 8:56 PM, Kaylor, Andrew <andrew.kaylor at
intel.com> wrote:
> I'm not sure I see why the delegating memory manager feels wrong to
both of you.  That's exactly the kind of usage model I would envision for
clients that needed to handle multiple ExecutionEngines that had reason to share
a memory manager.  I'm saying this as an invitation to discussion, not to be
argumentative.  We certainly could change things if it would make the design
better.
>
> Basically, the intention of the current design is to keep the
ExecutionEngine ignorant of memory allocation details.  I'm not sure it
always works that way, but that's the intent.  I believe that the decision
to have the ExecutionEngine own the memory manager was made so that it would be
self-contained and capable of cleaning up after itself.
>
> The JIT engine has a rather ad hoc history and is more complicated than
anyone would want.  It allocates and links together more things than it directly
exposes and the lifecycles of different bits of memory aren't always
obvious, especially if you get into re-JITing code.  I'm not sure
there's a lot that can be done in the general case to keep it clean other
than to have the engine own everything.
>
> I don't believe that the JIT engine is designed to handle having its
function memory moved, so even if you could work out the relocation issues I
don't think it's a good idea.
>
> The MCJIT engine, on the other hand, is designed to have its memory moved
on the fly (though at the granularity of sections, not functions).  The MCJIT
engine does not work with the PIC relocation model on 32-bit x86 targets, but it
will let you reapply relocations with other models after you move things. 
There's some code in the lli utility that does this if you're
interested.
>
> I'm not sure what the current maintenance model is for the older JIT
engine.  Near as I can tell when it grows at all it is driven by people who have
a need making specific contributions and as such it doesn't have a
particularly coherent direction.
>
> I have more idealistic hopes for the MCJIT engine, and if you're
interested in using it I'd be happy to talk to you about future design
directions.
>
> -Andy
>
>
>
> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu] On Behalf Of Frank Henigman
> Sent: Sunday, March 17, 2013 2:18 PM
> To: Dirkjan Bussink
> Cc: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Memory clean for applications using LLVM for JIT
compilation
>
> Thanks for the reply, nice to have some validation.  I thought of another
approach which might be preferable:
> generate relocatable code, use a JITEventListener to grab each function and
copy it to my own memory, let all the LLVM stuff die normally then use my copy
of the code.
> However when I call setRelocationModel(Reloc::PIC_) on the engine builder I
get code that seg faults.
> The assembly looks plausible at a glance, but I'm not really up to
speed on x86 assembly.
> Is PIC supposed to work with JIT on X86-32?
>
> On Sat, Mar 16, 2013 at 11:35 AM, Dirkjan Bussink <d.bussink at
gmail.com> wrote:
>>
>> On Mar 7, 2013, at 20:48 , Frank Henigman <fjhenigman at
google.com> wrote:
>>
>>> I derived a class from JITMemoryManager which delegates everything
to
>>> an instance made with CreateDefaultMemManager().  ExecutionEngine
>>> destroys the wrapper, but I keep the inner instance which did the
>>> actual work.  Works, but seems a bit ugly.  Did you find any other
>>> solutions?
>>
>> I ended up implementing the exact same thing, it feels dirty but it has
worked great for us so far.
>>
>> --
>> Dirkjan
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Hi Andy,

One of the issues that I found not intuitive, is that when an ExecutionEngine is
deallocated, the memory manager's destructor is also called. This resulted
in having to write two objects in my case, one as a per JIT request memory
manager and one global JIT memory manager.

The per request JIT memory manager gets memory from the global manager, but both
ended up implementing (almost) the whole memory manager api, because I
couldn't find another way to do it. Also see the code here:

https://github.com/rubinius/rubinius/blob/master/vm/llvm/jit_memory_manager.hpp
https://github.com/rubinius/rubinius/blob/master/vm/llvm/jit_memory_manager.cpp

If there are better ways to clean up the code, that would be great, but having
to things like that is what made me state feeling dirty on how it was
implemented.

-- 
Dirkjan


On Mar 19, 2013, at 01:56 , "Kaylor, Andrew" <andrew.kaylor at
intel.com> wrote:
> I'm not sure I see why the delegating memory manager feels wrong to
both of you.  That's exactly the kind of usage model I would envision for
clients that needed to handle multiple ExecutionEngines that had reason to share
a memory manager.  I'm saying this as an invitation to discussion, not to be
argumentative.  We certainly could change things if it would make the design
better.
> 
> Basically, the intention of the current design is to keep the
ExecutionEngine ignorant of memory allocation details.  I'm not sure it
always works that way, but that's the intent.  I believe that the decision
to have the ExecutionEngine own the memory manager was made so that it would be
self-contained and capable of cleaning up after itself.
> 
> The JIT engine has a rather ad hoc history and is more complicated than
anyone would want.  It allocates and links together more things than it directly
exposes and the lifecycles of different bits of memory aren't always
obvious, especially if you get into re-JITing code.  I'm not sure
there's a lot that can be done in the general case to keep it clean other
than to have the engine own everything.
> 
> I don't believe that the JIT engine is designed to handle having its
function memory moved, so even if you could work out the relocation issues I
don't think it's a good idea.
> 
> The MCJIT engine, on the other hand, is designed to have its memory moved
on the fly (though at the granularity of sections, not functions).  The MCJIT
engine does not work with the PIC relocation model on 32-bit x86 targets, but it
will let you reapply relocations with other models after you move things. 
There's some code in the lli utility that does this if you're
interested.
> 
> I'm not sure what the current maintenance model is for the older JIT
engine.  Near as I can tell when it grows at all it is driven by people who have
a need making specific contributions and as such it doesn't have a
particularly coherent direction.
> 
> I have more idealistic hopes for the MCJIT engine, and if you're
interested in using it I'd be happy to talk to you about future design
directions.
> 
> -Andy
> 
> 
> 
> -----Original Message-----
> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu] On Behalf Of Frank Henigman
> Sent: Sunday, March 17, 2013 2:18 PM
> To: Dirkjan Bussink
> Cc: llvmdev at cs.uiuc.edu
> Subject: Re: [LLVMdev] Memory clean for applications using LLVM for JIT
compilation
> 
> Thanks for the reply, nice to have some validation.  I thought of another
approach which might be preferable:
> generate relocatable code, use a JITEventListener to grab each function and
copy it to my own memory, let all the LLVM stuff die normally then use my copy
of the code.
> However when I call setRelocationModel(Reloc::PIC_) on the engine builder I
get code that seg faults.
> The assembly looks plausible at a glance, but I'm not really up to
speed on x86 assembly.
> Is PIC supposed to work with JIT on X86-32?
> 
> On Sat, Mar 16, 2013 at 11:35 AM, Dirkjan Bussink <d.bussink at
gmail.com> wrote:
>> 
>> On Mar 7, 2013, at 20:48 , Frank Henigman <fjhenigman at
google.com> wrote:
>> 
>>> I derived a class from JITMemoryManager which delegates everything
to
>>> an instance made with CreateDefaultMemManager().  ExecutionEngine 
>>> destroys the wrapper, but I keep the inner instance which did the 
>>> actual work.  Works, but seems a bit ugly.  Did you find any other 
>>> solutions?
>> 
>> I ended up implementing the exact same thing, it feels dirty but it has
worked great for us so far.
>> 
>> --
>> Dirkjan
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev