[llvm-dev] [RFC] Clean up the way we store optional Function data

[Vedant Kumar via llvm-dev]

Here is a WIP patch as promised:

    http://reviews.llvm.org/D13829

It uses a hungoff uselist to store optional data as needed.

Some early objections from Duncan:

    - An extra one-time malloc() is required to set personality functions.
    - We get and set personality functions frequently. This patch introduces a level of indirection which slows the common case down.

Is this overhead acceptable?

If not, maybe we could leave personality function handling untouched and add a "Constant **OptionalData" array to Function.

vedant


> On Oct 14, 2015, at 3:12 PM, Vedant Kumar <vsk at apple.com> wrote:
> 
> I like the idea of using hung off uses.
> 
> We can keep using SubclassData to indicate whether or not some optional data is present.
> 
> Benefits: zero space overhead until some optional data is set, we can get rid of the DenseMaps in LLVMContextImpl, and RAUW just works (so no clang changes are needed).
> 
> I'll have a patch up before the end of the week.
> 
> thanks
> vedant
> 
> 
>> On Oct 12, 2015, at 12:15 PM, Sanjoy Das via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> 
>> 
>> 
>> David Majnemer wrote:
>>> 
>>> 
>>> On Mon, Oct 12, 2015 at 11:00 AM, Duncan P. N. Exon Smith via llvm-dev
>>> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>> 
>>> 
>>>> On 2015-Oct-12, at 10:41, Sanjoy Das
>>>   <sanjoy at playingwithpointers.com
>>>   <mailto:sanjoy at playingwithpointers.com>> wrote:
>>>> 
>>>> 
>>>> 
>>>> Vedant Kumar wrote:
>>>>>>> That's a neat idea. To summarize: make Function have 3 optional
>>>   operands. (For context -- Function currently has 1 optional operand,
>>>   and my proposal is to move to 0.)
>>>>>>> 
>>>>>>> Could someone else chime in on what they'd like to see?
>>>>>> Sanjoy's idea makes sense to me, but only if we never need to add
>>>>>> prefix/prologue data after functions are created.  Are there any
>>>   places
>>>>>> where we need/want to add them after the fact?
>>>>> 
>>>>> I think so. I see:
>>>>> 
>>>>> LinkModules.cpp:
>>>   Dst.setPrefixData(MapValue(Src.getPrefixData(), ValueMap,
>>>>> BitcodeReader.cpp:
>>>   FunctionPrologueWorklist.back().first->setPrologueData(C);
>>>>> InlineFunction.cpp: Caller->setPersonalityFn(CalledPersonality);
>>>>> 
>>>>> Some of these sites could be refactored so that the Functions are
>>>   created with the prefix/prologue data they need. I don't think
>>>   that's possible for personality functions (see my third example).
>>>>> 
>>>>> Would we inhibit any future patches which add prefix/prologue
>>>   data to Functions on the fly by taking this approach?
>>>> 
>>>> You should always be able to create a new `llvm::Function`
>>>   instance (and RAUW it in) if you want to add prefix/prologue data to
>>>   functions after they've been created; just like you have to do today
>>>   for any other `llvm::User`s that do not have hung off uses.
>>> 
>>>   It's possible, but a lot more involved with `Function`s.  Besides
>>>   RAUW, you need to transfer over all the basic blocks.
>>> 
>>>   This seems kind of wrong to me, if we expect it to happen.
>>> 
>>>>  Which brings me to -- can you use hung off uses for this?  These
>>>   use lists can be resized on the fly, so you should be able to add
>>>   and remove prologue data on the fly.  If you're using hung off uses,
>>>   you'll probably still need a descriptor to remember whether / which
>>>   operands are prologue data etc.
>>> 
>>>   Sure, this is another option.  It might be simplest.  I'd be
>>>   tempted to start with a 0/3 choice (if we allocate any hung-off
>>>   uses, allocate enough for all three operands) to simplify the
>>>   logic.  Although I can't remember right now whether that's
>>>   legal (having nullptr operands followed by real ones)...
>>> 
>>>>>>>>> Personalities are stored as ``optional`` Function operands.
>>>   We actually always
>>>>>>>>> allocate the space for this ``optional`` operand: there's a
>>>   FIXME in the
>>>>>>>>> destructor for Function about this.
>>> 
>>>   Makes me wonder, why didn't we use hung off uses to begin with?
>>>   Do functions "usually" have personality functions, for some
>>>   definition of?
>>> 
>>> 
>>> Depends. In C++? It's pretty common to have objects which have
>>> non-trivial destructors on the stack which means calling a function will
>>> be an invoke which will require the function to have a personality. In
>>> C? It's pretty rare. You'd need something like __attribute__((cleanup))
>>> to do it, the most common source of this will be something
>>> like pthread_cleanup_push. If I recall correctly, Julia sets the
>>> personality on functions regardless of whether or not there are any
>>> invokes, they need the AsmPrinter to scribble something down.  I can't
>>> say for other languages (Rust, etc.).  From what I understand, Swift
>>> doesn't use landingpad for EH so they wouldn't need the personality set.
>> 
>> Most functions we emit from our Java frontend have personalities.
>> 
>> -- Sanjoy
>> 
>>> 
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