[LLVMdev] Inline hint for methods defined in-class

David Blaikie dblaikie at gmail.com
Wed Jun 24 14:35:15 PDT 2015 

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)

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.

- 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
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150624/7e946c16/attachment.html>

More information about the llvm-dev mailing list