[LLVMdev] Inline hint for methods defined in-class
Robinson, Paul
Paul_Robinson at playstation.sony.com
Wed Jun 24 14:49:29 PDT 2015
> -----Original Message-----
> From: Xinliang David Li [mailto:davidxl at google.com]
> Sent: Wednesday, June 24, 2015 2:45 PM
> To: David Blaikie
> Cc: Easwaran Raman; Robinson, Paul; <llvmdev at cs.uiuc.edu> List
> Subject: Re: [LLVMdev] Inline hint for methods defined in-class
>
> On Wed, Jun 24, 2015 at 2:35 PM, David Blaikie <dblaikie at gmail.com> wrote:
> >
> >
> > On Wed, Jun 24, 2015 at 2:20 PM, Easwaran Raman <eraman at google.com>
> wrote:
> >>
> >> On Wed, Jun 24, 2015 at 2:10 PM, Robinson, Paul
> >> <Paul_Robinson at playstation.sony.com> wrote:
> >> >> -----Original Message-----
> >> >> From: Easwaran Raman [mailto:eraman at google.com]
> >> >> Sent: Wednesday, June 24, 2015 1:27 PM
> >> >> To: Xinliang David Li
> >> >> Cc: Robinson, Paul; Xinliang David Li; <llvmdev at cs.uiuc.edu> List
> >> >> Subject: Re: [LLVMdev] Inline hint for methods defined in-class
> >> >>
> >> >> The method to identify functions with in-class definitions is one
> part
> >> >> of my question. Even if there is a way to do that without passing
> the
> >> >> hint, I'm interested in getting feedback on treating it at-par with
> >> >> functions having the inline hint in inline cost analysis.
> >> >
> >> > Well, personally I think having the 'inline' keyword mean "try
> harder"
> >> > is worth something, but that's intuition backed by no data
> whatsoever.
> >> > Your patch would turn 'inline' into noise, when applied to a function
> >> > with an in-class definition. Granted that the way the C++ standard
> >> > describes 'inline' it is effectively noise in that situation.
> >>
> >> The reason I started looking into this is that, for a suite of
> >> benchmarks we use internally, treating the in-class definitions
> >> equivalent to having an 'inline' keyword, when combined with a higher
> >> inlinehint-threshold, is a measurable win in performance. I am not
> >> making any claim that this is a universal truth, but intuitively, the
> >> description of 'inline' in C++ standard seems to influence what
> >> methods are defined in-class.
> >
> >
> > I'm not sure that's the case - in my experience (for my own code & the
> code
> > I see from others) people put stuff in headers that's "short enough"
> that
> > it's not worth the hassle of an external definition. I don't really
> think
> > authors are making an actual judgment about how much of a win inlining
> their
> > function is most of the time when they put a definition inline in a
> class.
> > (maybe a litttle more likely when it's a standalone function where you
> have
> > to write "inline" explicitly, but maybe not even then)
>
> Good observation, but not quite complete. It is true that a lot of the
> in-class definitions are small functions (as the author does not
> bother to provide standalone defintions). However those cases are not
> interesting, as regular inline heuristics can handle already.
>
> The interesting cases are those where user explicitly/deliberately
> puts relatively large bodies in class. They also really want the body
> to be visible to all TUs (so that inliner can do something) -- it is a
> strong hint.
I think in a template class, pretty much all methods have to be defined
in-class regardless of size or other suitability for inlining. Marking
such methods with inlinehint is not really appropriate.
--paulr
>
> >
> > It would seem that improving the inliner to do a better job of judging
> the
> > inlining benefit would be ideal (for this case and for LTO, etc - where
> > we'll pick up equivalently small non-inline function definitions that
> the
> > author had decided to define out of line (either because they used to be
> > longer or the author didn't find out of line definitions to be as
> > inconveniently verbose as someone else, etc)), if there's something more
> > useful to go on than "the user sort of maybe implied that this would be
> good
> > to inline". It seems like a very weak signal.
>
> That is ideal, but something we will improve independently. Assuming
> inliner can not do a perfect job, not differentiating functions with
> hints can result in large size growth.
>
> David
>
>
> >
> > - David
> >
> >>
> >>
> >> - Easwaran
> >>
> >> > --paulr
> >> >
> >> >>
> >> >> Thanks,
> >> >> Easwaran
> >> >>
> >> >>
> >> >> On Wed, Jun 24, 2015 at 12:56 PM, Xinliang David Li
> >> >> <xinliangli at gmail.com> wrote:
> >> >> > The problem is that the other way around is not true: a function
> >> >> > linkonce_odr linkage may be neither inline declared nor have in-
> class
> >> >> > definition.
> >> >> >
> >> >> > David
> >> >> >
> >> >> >
> >> >> > On Wed, Jun 24, 2015 at 11:53 AM, Robinson, Paul
> >> >> > <Paul_Robinson at playstation.sony.com> wrote:
> >> >> >>
> >> >> >>
> >> >> >>
> >> >> >> > -----Original Message-----
> >> >> >> > From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-
> >> >> bounces at cs.uiuc.edu]
> >> >> >> > On
> >> >> >> > Behalf Of Easwaran Raman
> >> >> >> > Sent: Wednesday, June 24, 2015 9:54 AM
> >> >> >> > To: Xinliang David Li
> >> >> >> > Cc: <llvmdev at cs.uiuc.edu> List
> >> >> >> > Subject: Re: [LLVMdev] Inline hint for methods defined in-class
> >> >> >> >
> >> >> >> > Ping.
> >> >> >> >
> >> >> >> > On Wed, Jun 17, 2015 at 4:13 PM, Xinliang David Li
> >> >> <davidxl at google.com>
> >> >> >> > wrote:
> >> >> >> > > that looks like a different fix. The case mentioned by
> Easwaran
> >> >> >> > > is
> >> >> >> > >
> >> >> >> > > class A{
> >> >> >> > > int foo () { return 1; }
> >> >> >> > > ...
> >> >> >> > > };
> >> >> >> > >
> >> >> >> > > where 'foo' is not explicitly declared with 'inline' keyword.
> >> >> >> > >
> >> >> >> > > David
> >> >> >> > >
> >> >> >> > > On Wed, Jun 17, 2015 at 4:07 PM, Balaram Makam
> >> >> <bmakam at codeaurora.org>
> >> >> >> > wrote:
> >> >> >> > >> AFAIK, this was fixed in r233817.
> >> >> >>
> >> >> >> That was later reverted.
> >> >> >>
> >> >> >> > >>
> >> >> >> > >> -----Original Message-----
> >> >> >> > >> From: llvmdev-bounces at cs.uiuc.edu
> >> >> >> > >> [mailto:llvmdev-bounces at cs.uiuc.edu]
> >> >> >> > On
> >> >> >> > >> Behalf Of Easwaran Raman
> >> >> >> > >> Sent: Wednesday, June 17, 2015 6:59 PM
> >> >> >> > >> To: llvmdev at cs.uiuc.edu
> >> >> >> > >> Cc: David Li
> >> >> >> > >> Subject: [LLVMdev] Inline hint for methods defined in-class
> >> >> >> > >>
> >> >> >> > >> Clang adds the InlineHint attribute to functions that are
> >> >> explicitly
> >> >> >> > marked
> >> >> >> > >> inline, but not if they are defined in the class body. I
> tried
> >> >> >> > >> the
> >> >> >> > following
> >> >> >> > >> patch, which I believe handles the in-class definition
> >> >> >> > >> case:
> >> >> >> > >>
> >> >> >> > >> --- a/lib/CodeGen/CodeGenFunction.cpp
> >> >> >> > >> +++ b/lib/CodeGen/CodeGenFunction.cpp
> >> >> >> > >> @@ -630,7 +630,7 @@ void
> >> >> >> > >> CodeGenFunction::StartFunction(GlobalDecl
> >> >> >> > >> GD,
> >> >> >> > >> if (const FunctionDecl *FD =
> >> >> >> > >> dyn_cast_or_null<FunctionDecl>(D))
> >> >> {
> >> >> >> > >> if (!CGM.getCodeGenOpts().NoInline) {
> >> >> >> > >> for (auto RI : FD->redecls())
> >> >> >> > >> - if (RI->isInlineSpecified()) {
> >> >> >> > >> + if (RI->isInlined()) {
> >> >> >> > >> Fn->addFnAttr(llvm::Attribute::InlineHint);
> >> >> >> > >> break;
> >> >> >> > >> }
> >> >> >> > >>
> >> >> >> > >> I tried this on C++ benchmarks in SPEC 2006. There is no
> >> >> noticeable
> >> >> >> > >> performance difference and the maximum text size increase is
> <
> >> >> 0.25%.
> >> >> >> > >> I then built clang with and without this change. This
> increases
> >> >> the
> >> >> >> > text
> >> >> >> > >> size by 4.1%. For measuring performance, I compiled a large
> >> >> >> > >> (4.8
> >> >> >> > million
> >> >> >> > >> lines) preprocessed file. This change improves runtime
> >> >> >> > >> performance
> >> >> by
> >> >> >> > 0.9%
> >> >> >> > >> (average of 10 runs) in O0 and O2.
> >> >> >> > >>
> >> >> >> > >> I think knowing whether a function is defined inside a class
> >> >> >> > >> body
> >> >> is
> >> >> >> > >> a
> >> >> >> > >> useful hint to the inliner. FWIW, GCC's inliner doesn't
> >> >> differentiate
> >> >> >> > these
> >> >> >> > >> from explicit inline functions. If the above results doesn't
> >> >> justify
> >> >> >> > this
> >> >> >> > >> change, are there other benchmarks that I should evaluate?
> >> >> >> > >> Another
> >> >> >> > >> possibility is to add a separate hint for this instead of
> using
> >> >> the
> >> >> >> > existing
> >> >> >> > >> inlinehint to allow for better tuning in the inliner.
> >> >> >>
> >> >> >> A function with an in-class definition will have linkonce_odr
> >> >> >> linkage,
> >> >> >> so it should be possible to identify such functions in the
> inliner
> >> >> >> without introducing the inlinehint attribute.
> >> >> >> --paulr
> >> >> >>
> >> >> >> > >>
> >> >> >> > >> Thanks,
> >> >> >> > >> Easwaran
> >> >> >> > >> _______________________________________________
> >> >> >> > >> LLVM Developers mailing list
> >> >> >> > >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> >> >> >> > >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >> >> >> > >>
> >> >> >> > _______________________________________________
> >> >> >> > LLVM Developers mailing list
> >> >> >> > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> >> >> >> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >> >> >>
> >> >> >> _______________________________________________
> >> >> >> LLVM Developers mailing list
> >> >> >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> >> >> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >> >> >
> >> >> >
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu
> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
> >
> >
More information about the llvm-dev
mailing list