[LLVMdev] inlining hint

[David Vandevoorde]

On Aug 26, 2009, at 10:14 PM, Chris Lattner wrote:

> On Aug 26, 2009, at 7:02 PM, David Vandevoorde wrote:
>>> It's actually the other way around. llvm has always ignored the
>>> "inline" keyword and now we are finding out we are missing some
>>> important cases.
>>
>> Okay.  It's the "other way around" in terms of history, but it looks
>> like the conclusion might be the same: Purely heuristics-based
>> inlining hasn't been proven entirely competitive yet (for C++)?
>
> That is what is unclear.  We have no empirical evidence that  
> listening to inline will improve the situation more than just  
> increasing the inlining threshold across the board.  Even if we did,  
> I would still be uncomfortable with this: "hasn't been proven  
> entirely competitive yet" implies that we've pursued the current  
> inliner and tried hard to make it better without listening to  
> inline.  In fact, the current inliner is almost completely untuned  
> and has many known problems.


Definitely: Efforts in this area are almost certainly not a waste.   
Even if it turns out you need to take "inline" into account, a better  
(back end) inliner is likely to be an appreciable improvement.

My point is mostly that I'm not aware of an existing, competitive C++  
compiler that always ignores the "this is a C++ inline function" bit.   
That doesn't mean it's not possible, and in fact I'm all for pursuing  
it.  I _do_ believe that it means that current C++ programs should  
only define functions inline if there is good reason to think that  
inlining the typical call to that function is a plus.

(P.S., I _have_ worked with a compiler that had a separate flag to  
ignore "inline" because a customer has gone inline-crazy and ignoring  
inline became a competitive advantage at that point.)


> I am concerned that "listening to inline" will paper over other  
> things that could be solved in much better ways.  If the inlining  
> heuristics were already considered to be well tuned and a study  
> showed that 'listening to inline' was better than increasing the  
> threshold, then I'd be a believer.


Skepticism is good...


> I am also much more pessimistic about the world's C++ code than  
> some.  It turns out that not all c++ programmers are totally  
> awesome :)


?? Shocked!  I'm shocked!!

>
>> My GCC installation here appears to inline g in the following  
>> example:
>>
>> 	inline int g(int x) { return x; }
>>
>> 	int main() {
>> 	        return g(3);
>> 	}
>>
>> with just "-O1", but it doesn't do so when I drop the "inline"  
>> keyword.
>
> LLVM already does that at -O1 without listening to inline because  
> the code is obviously smaller with the inline.


I just wanted to make sure that the observation that GCC ignores  
"inline" (implicit or explicit) until -O2 is double-checked (if it  
turns out to matter).

	Daveed