inlining

[JAnderson]

Don wrote:
> JAnderson wrote:
>> JAnderson wrote:
>>> bobef wrote:
>>>> This has probably been asked many times before. If someone knows of 
>>>> such discussion please paste me a link. Why not an 'inline' 
>>>> attribute? We all know that the compiler can be stupid some times, 
>>>> and even if it is not people may want to inline something that is 
>>>> normally not appropriate for inlining. Auto inlining is fine, but 
>>>> people should have control over such kind of things I believe.
>>>>
>>>> Regards, bobef
>>>
>>> May C++ compilers ignore the inline attribute because it has a better 
>>> handle on when to inline.  There have been some studies (does anyone 
>>> have the links to these) where they've shown that most of the time 
>>> the compiler can make a more intelligent guess then the average 
>>> engineer.
>>>
>>> But that's C++.  D does this automatic virtual's thing so its 
>>> difficult to say whether the compiler can always make a good choice.
>>>
>>> -Joel
>>
>> I was working with MSVC++ the other day and found a couple of places 
>> where it wasn't inlining the code and was running slow.  So I placed a 
>> few inlines around and *bam* that code started running faster.  Then I 
>> profiled the code as a whole to see how much of an improvement I'd 
>> gained.  However the game was actually running slower.  It turned out 
>> that inlining had simply shifted the bottneck into memory and the 
>> program file size had got bigger, so the program cache was stalling 
>> all the time.
>>
>> I'm not against inlining, I just think that you have to be really 
>> careful when using it and understand its implications (ie use a 
>> profiler), otherwise you could be making things worse.
>>
>> -Joel
> 
> Yup. Code cache can easily become a bottleneck. Similarly, turning on 
> 'optimise for speed' for a whole program is almost always a bad idea.

I've had some good results with whole program optimisation.  However not 
always.

> 
> More useful than 'inline', would be some way to tell the compiler 'this 
> function is speed-critical'. If nothing else, it would have some 
> documentation value.

I think that's a good idea.  The compiler could also spin more cycles to 
optimize that piece of code.

-Joel