[LLVMdev] Inline hint for methods defined in-class

[Xinliang David Li]

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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