[LLVMdev] Inline hint for methods defined in-class

Robinson, Paul Paul_Robinson at playstation.sony.com
Wed Jun 24 15:10:16 PDT 2015 

> -----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?)
--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
> >> >> > >>
> >> >> > _______________________________________________
> >> >> > 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