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

[Sanjoy Das via llvm-dev]

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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