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

Wed Oct 21 15:20:39 PDT 2015

I feel OK about this trade-off.  The vast majority of C++ TUs have < 65k
functions.  Sanjoy?

On Wed, Oct 21, 2015 at 2:25 PM, Vedant Kumar <vsk at apple.com> wrote:

> I've done some measurements on this.
>
> The test program I have just calls Function::Create(),
> F->setPersonalityFn(), and then F->eraseFromParent() in a loop 2^20 times.
>
> Results:
>
>     pre-patch  --- min: 1.10s    max: 1.13s    avg: 1.11s
>     post-patch --- min: 1.26s    max: 1.35s    avg: 1.29s
>
> So we expect to lose 0.2 seconds per 1 million functions (with personality
> functions) in a project. I've only tested on my machine, and I haven't
> accounted for performance variations in other parts of the codebase where
> code was removed.
>
> I'm happy enough with the patch and think that this is a reasonable
> tradeoff. Should we go with this?
>
> http://reviews.llvm.org/D13829
>
> vedant
>
>
> > On Oct 16, 2015, at 5:56 PM, Duncan P. N. Exon Smith <
> dexonsmith at apple.com> wrote:
> >
> >
> >> On 2015-Oct-16, at 13:54, Vedant Kumar <vsk at apple.com> wrote:
> >>
> >> 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 the overhead isn't bad, then this SGTM.  (I haven't looked at the
> > patch.)
> >
> > I guess I was thinking about large applications that use C++ exceptions
> > and build with LTO, but even there it's a per-function overhead.  I
> > doubt it really matters.
> >
> >> If not, maybe we could leave personality function handling untouched
> and add a "Constant **OptionalData" array to Function.
> >
> > The `IndirectUser` approach you outlined initially might be better
> > than this?
> >
> >> 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
> >>>>
> >>>>>
> >>>>> _______________________________________________
> >>>>> 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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> >>>> llvm-dev at lists.llvm.org
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> >>>
> >>
> >
>
>
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