[LLVMdev] Inline hint for methods defined in-class

[Robinson, Paul]

> -----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.
--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
> >> > >> _______________________________________________
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> >> > >>
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