[llvm-dev] JIT compiler and calls to existing functions
Philip Reames via llvm-dev
llvm-dev at lists.llvm.org
Tue Mar 29 16:51:41 PDT 2016
There is no documentation I know of. Rough sketch:
1) Create a subclass of SectionMemoryManager
2) Create an instance of this class and add it the EngineBuilder via
setMCJITMemoryManager
(Make sure everything runs without changes)
3) Override the getSymbolAddress method, have your implementation call
the base classes impl
(Make sure everything runs without changes)
4) Add handling to map name->address for any custom functions you want.
On 03/28/2016 09:18 PM, Russell Wallace wrote:
> True, I care more about how fast the code runs than how long it takes
> to compile it. So if the symbolic approach enables better code
> generation, that is a very significant advantage from my perspective.
>
> Is there any example code or documentation you can point to for
> details about how to implement the symbolic approach? Is it similar to
> any of the versions of Kaleidoscope or any other extant tutorial?
>
> On Tue, Mar 29, 2016 at 5:07 AM, Philip Reames
> <listmail at philipreames.com <mailto:listmail at philipreames.com>> wrote:
>
> Other advantages of keeping things symbolic:
> 1) You can use function attributes to provide optimization or
> semantic information.
> 2) Linking modules works as expected when one of them contains the
> definition.
> 3) You can get better code generation (i.e. pc relative addressing
> for local symbols, etc..)
>
> If the inttoptr scheme makes you happy, go for it. I'm not
> telling you its wrong, merely that there's another approach you
> should consider which has it's own advantages.
>
> Philip
>
> p.s. Your point about compiling faster is off base. Not because
> you're wrong, but because if you're trying to optimize for compile
> speed at that granularity, you really don't want to be using LLVM
> (or just about any framework.) LLVM does not make a good first
> tier JIT. It's makes a great high tier JIT, but if you really
> really care about compile time, it is not the appropriate answer.
>
> On 03/28/2016 08:57 PM, Russell Wallace wrote:
>> Ah! Okay then, so you are saying something substantive that I
>> think I disagree with, but that could be because there are
>> relevant issues I don't understand.
>>
>> My reasoning is, I've already got a pointer to the function I
>> want the generated code to call, so I just supply that pointer,
>> it looks ugly on a microscopic scale because there are a couple
>> of lines of casts to shepherd it through the type system, but on
>> an even slightly larger scale it's as simple and elegant as it
>> gets: I supply a 64-bit machine word that gets compiled into the
>> object code, there are no extra layers of machinery to go wrong,
>> no nonlocal effects to understand, no having to wonder about
>> whether anything depends on symbol information that might vary
>> between debug and release builds or between the vendor compiler
>> and clang or between Windows and Linux; if it works once, it will
>> work the same way every time. As a bonus, it will compile
>> slightly faster.
>>
>> Keeping things symbolic - you're saying the advantage is the
>> intermediate code is easier to debug because a dump will say
>> 'call print' instead of 'call function at 123456'? I can see that
>> being a real advantage but as of right now it doesn't jump out at
>> me as necessarily outweighing the advantages of the direct
>> pointer method.
>>
>>
>>
>> On Tue, Mar 29, 2016 at 4:37 AM, Philip Reames
>> <listmail at philipreames.com <mailto:listmail at philipreames.com>> wrote:
>>
>> I think our use cases are actually quite similar. Part of
>> generating the in memory executable code is resolving all the
>> symbolic symbols and relocations. The details of this are
>> mostly hidden from you by the MCJIT interface, but it's this
>> step I was referring to as "link time".
>>
>> The way to think of MCJIT: generate object file,
>> incrementally link, run dynamic loader, but do it all in
>> memory without round tripping through disk or explicit files.
>>
>> Philip
>>
>>
>>
>> On Mar 28, 2016, at 7:25 PM, Russell Wallace
>> <russell.wallace at gmail.com
>> <mailto:russell.wallace at gmail.com>> wrote:
>>
>>> Right, but when you say link time, the JIT compiler I'm
>>> writing works the way openJDK or v8 do, it reads a script,
>>> JIT compiles it into memory and runs the code in memory
>>> without ever writing anything to disk (an option for ahead
>>> of time compilation may come later, but that will be a while
>>> down the road), so we might be doing different things?
>>>
>>> On Tue, Mar 29, 2016 at 2:59 AM, Philip Reames
>>> <listmail at philipreames.com
>>> <mailto:listmail at philipreames.com>> wrote:
>>>
>>> The option we use is to have a custom memory manager,
>>> override the getPointerToNamedFunction function, and
>>> provide the pointer to the external function at link
>>> time. The inttoptr scheme works fairly well, but it
>>> does make for some pretty ugly and sometimes hard to
>>> analyze IR. I recommend leaving everything symbolic
>>> until link time if you can.
>>>
>>> Philip
>>>
>>>
>>> On 03/28/2016 06:33 PM, Russell Wallace via llvm-dev wrote:
>>>> That seems to work, thanks! The specific code I ended
>>>> up with to call int64_t print(int64_t) looks like:
>>>>
>>>> auto f = builder.CreateIntToPtr(
>>>> ConstantInt::get(builder.getInt64Ty(), uintptr_t(print)),
>>>> PointerType::getUnqual(FunctionType::get(
>>>> builder.getInt64Ty(), {builder.getInt64Ty()}, false)));
>>>> return builder.CreateCall(f, args);
>>>>
>>>>
>>>> On Mon, Mar 28, 2016 at 1:40 PM, Caldarale, Charles R
>>>> <Chuck.Caldarale at unisys.com
>>>> <mailto:Chuck.Caldarale at unisys.com>> wrote:
>>>>
>>>> > From: llvm-dev
>>>> [mailto:llvm-dev-bounces at lists.llvm.org
>>>> <mailto:llvm-dev-bounces at lists.llvm.org>]
>>>> > On Behalf Of Russell Wallace via llvm-dev
>>>> > Subject: [llvm-dev] JIT compiler and calls to
>>>> existing functions
>>>>
>>>> > In the context of a JIT compiler, what's the
>>>> recommended way to generate a call to an
>>>> > existing function, that is, not one that you are
>>>> generating on-the-fly with LLVM, but
>>>> > one that's already linked into your program? For
>>>> example the cosine function (from the
>>>> > standard math library); the Kaleidoscope tutorial
>>>> recommends looking it up by name with
>>>> > dlsym("cos"), but it seems to me that it should
>>>> be possible to use a more efficient and
>>>> > portable solution that takes advantage of the
>>>> fact that you already have an actual pointer
>>>> > to cos, even if you haven't linked with debugging
>>>> symbols.
>>>>
>>>> Perhaps not the most elegant, but we simply use the
>>>> IRBuilder.CreateIntToPtr() method to construct the
>>>> Callee argument for IRBuilder.CreateCall(). The
>>>> first argument for CreateIntToPtr() comes from
>>>> ConstantInt::get(I64, uintptr_t(ptr)), while the
>>>> second is a function type pointer defined by using
>>>> PointerType::get() on the result of
>>>> FunctionType::get() with the appropriate function
>>>> signature.
>>>>
>>>> - Chuck
>>>>
>>>>
>>>>
>>>>
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>
>
>
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