[LLVMdev] Inline hint for methods defined in-class

[Easwaran Raman]

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