Compiler optimizations questions for ghc 6.10...

[Bertram Felgenhauer]

Krasimir Angelov wrote:
> Well I actually did, almost. I added this function:
> 
> quotX :: Int -> Int -> Int
> a `quotX` b
>    | b == 0                     = error "divZeroError"
>    | b == (-1) && a == minBound = error "overflowError"
>    | otherwise                  =  a `quotInt` b
> 
> It does the right thing. However to be sure that this doesn't
> interfere with some other GHC magic the real quot function have to be
> changed and tested. I haven't build GHC from source for 2-3 years now
> and I don't have the time to do it just to test whether this works.

It works, and it has the desired effect. It's not always a win though;
the nofib prime sieve benchmark suffers.

For a patch, see
  http://int-e.home.tlink.de/haskell/ghc-libraries-base-tune-division.patch

Nofib results extract:

------------------------------------------------------------------------
        Program           Size    Allocs   Runtime   Elapsed
------------------------------------------------------------------------
           fish          -0.7%     -5.3%      0.05      0.04
         primes          -0.0%    +28.5%    +25.6%    +25.5%
   wheel-sieve2          -0.0%     -0.3%    -17.9%    -18.6%
------------------------------------------------------------------------
            Min          -0.9%     -5.3%    -17.9%    -18.6%
            Max          +0.1%    +28.5%    +25.6%    +25.5%
 Geometric Mean          -0.2%     +0.2%     -0.0%     +0.2%

'primes' is an outlier - the other slowdowns are below 3%

What happens in 'primes' is that 'mod' no longer gets inlined;
apparently it now looks bigger to the compiler than before.

Using -funfolding-use-threshold=10 brings the benchmark back to its
original speed, despite the extra comparison before doing the
division.

In 'wheel-sieve2', the first different optimization choice I see is
again a 'mod' that is no longer inlined; this leads to a change in other
inlining choices that result in a speedup for reasons that I have not
investigated.

Bertram