[LLVMdev] Inline hint for methods defined in-class

Easwaran Raman eraman at google.com
Wed Jun 24 15:04:00 PDT 2015 

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)
Ok, that may very well be the case.

> 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.

I don't disagree with your ideal scenario. In the current non-ideal
state, LLVM does use a larger threshold for using the 'inline'
keyword. The question is whether using this larger threshold for
in-class definitions is a backward step.

- Easwaran

> - 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
>> >> >> > >>
>> >> >> > _______________________________________________
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>> >> >>
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>> >> >
>> >> >
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