[cfe-dev] always_inline and noinline attributes

[Chris Lattner]

On Jul 18, 2008, at 2:09 PM, Matthijs Kooijman wrote:
>> Once the information is encoded in the IR, the remaining issue is  
>> how to
>> invoke the LLVM inlining pass when at least one function is marked as
>> always_inline. There are two possible approaches:
>>
>> 1) Teach FE tools (e.g. clang, llvm-gcc) to insert inlining pass in  
>> the
>> PassManager while requesting (opt + code generation) when it least  
>> one
>> function with attribute always_inline is seen.
> Perhaps there could be a specific inliner that does only this? This  
> prevents
> surprises when people didn't expect inlining?

I'd strongly prefer to do this in the LLVM IR level instead of on  
clang ASTs.

>> 2) Teach the LLVM PassManager to sniff always_inline property  
>> encoded in
>> the LLVM IR and do the right thing.
> This sounds like it might also surprise people, if they only add a  
> single pass
> and still things are inlined. Might not be a big problem, though.

GCC runs its inliner, even at -O0.  I agree it is ugly, but seems  
necessary.

> Another reason why I think encoding in the IR is necessary: If I  
> have an
> always inline function defined in one module and referenced in  
> another, I
> think it should still be inlined after I link the two modules  
> together, right?

No, it should only happen within a translation unit.  The semantics of  
the program should not change based on whether you're using LTO or  
not, and inlining can sometimes change semantics for (arguably very  
broken) uses.

-Chris