[llvm-dev] [RFC] Clean up the way we store optional Function data
Sanjoy Das via llvm-dev
llvm-dev at lists.llvm.org
Mon Oct 12 12:15:28 PDT 2015
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
> > >> 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:
> > > 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.
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