[LLVMdev] Inline hint for methods defined in-class

[Easwaran Raman]

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.
>>
>> -----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.
>>
>> Thanks,
>> Easwaran
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