[LLVMdev] Inline hint for methods defined in-class

[Robinson, Paul]

> -----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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