[LLVMdev] Inline hint for methods defined in-class

[Xinliang David Li]

On Wed, Jun 24, 2015 at 1:59 PM, Robinson, Paul
<Paul_Robinson at playstation.sony.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.
>
>
>
> Sorry, why is that a problem?  Did you have some other reason to want to
> identify functions that either specify 'inline' or have an in-class
> definition?

yes, I think there is a reason. In-class definitions are another way
for user to give inline hint (which depends on coding style).  This
does not apply to other cases such as template function
instantiations.

>
> The inliner is not *prevented* from inlining a function that falls outside
> of those cases, as far as I know.  A static file-level function might be a
> good candidate, even if it is not explicitly marked 'inline'.

yes, but those other cases have explicit attribute for inliner to tell
them apart.

David


>
> --paulr
>
>
>
> From: Xinliang David Li [mailto:xinliangli at gmail.com]
> Sent: Wednesday, June 24, 2015 12:56 PM
> To: Robinson, Paul
> Cc: Easwaran Raman; Xinliang David Li; <llvmdev at cs.uiuc.edu> List
>
>
> Subject: Re: [LLVMdev] Inline hint for methods defined in-class
>
>
>
> 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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