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

[Vedant Kumar via llvm-dev]

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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