-O in 6.8.3

Serge D. Mechveliani mechvel at botik.ru
Fri Jun 20 10:14:33 EDT 2008


This is on the recent topic of very eager inlining in ghc-6.8.3. 

I wrote recently that
(1) 6.8.3 takes 7 times more of heap than 6.8.2 to complile under -O 
    a certain small-size module  RsePol_ (let us call it  M.hs),
(2) even the needed ~70 Mb in 6.8.2 looks unnaturally large.

After thinking a bit, I take back (2).

For `making' M under -Onot -- all right, it must suffice about 20 Mb. 
But -O allows deeper optimization, this needs more inlining, inlining 
pieces between modules, inlining instances (to some extent).
I expect that -O needs for compilation not more than about 5 times 
greater expence in memory and time than -Onot.
ghc-6.8.2 satisfies this. 
And 6.8.3 does not, its ratio is about  35  on the example of  

What is the developer's idea abot -O/-Onot cost ratio for RAM and 
What is the difference between ghc-6.8.2 and ghc-6.8.3 ?
The former does not inline instances under -O and the latter does 

As to the 10-20 % of the result code increase, I am calm about this
-- if something it bought by this. But I am not an expert and do not
see in what point 6.8.3 is better, at least the DoCon performance is 
not greater. I do not know, maybe some bugs are fixed, and a bug fix 
often costs some code increase, and a bug fix is good.


Serge Mechveliani
mechvel at botik.ru

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