[LLVMdev] Inline hint for methods defined in-class
Xinliang David Li
davidxl at google.com
Wed Jun 24 14:44:38 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)
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.
>
> 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
>> >> >> > >>
>> >> >> > _______________________________________________
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>> >> >>
>> >> >> _______________________________________________
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>> >> >
>> >> >
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