[LLVMdev] Inline hint for methods defined in-class
Easwaran Raman
eraman at google.com
Wed Jun 24 15:22:37 PDT 2015
On Wed, Jun 24, 2015 at 3:10 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 2:21 PM
>> To: Robinson, Paul
>> Cc: Xinliang David Li; Xinliang David Li; <llvmdev at cs.uiuc.edu> List
>> Subject: Re: [LLVMdev] Inline hint for methods defined in-class
>>
>> 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.
>
> Manipulating the thresholds is much more significant than which threshold
> applies to defined-in-class methods. Sorry I missed this point earlier.
> (Is the inlinehint-threshold the only one you can fiddle with?)
Sorry, I wasn't very clear there. What I observe for the benchmarks I looked at:
* Benefits of applying inline hint to in-class definition (without
altering the thresholds): x%
* Benefits of changing the thresholds (without adding inlinehint): ~x%
* Combined effects of these two changes: >> 2x%
Changing the thresholds had some adverse impact on some SPEC
benchmarks which I have't fully understood, so I wanted to look only
at the inline hint for now.
Let me take a step back and restate my question. If I want to tune the
inline thresholds, what are good set of benchmarks to consider, given
that tuning parameters invariably will hurt in some cases. If those
set of benchmarks also benefit from adding inline hint to in-class
definitions, is that a reasonable change even though programmers may
not have intended to use in-class definitions for the purpose of
boosting inlining?
Thanks,
Easwaran
> --paulr
>
>>
>> - 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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