Compiler optimizations questions for ghc 6.10...

[Max Bolingbroke]

2009/2/19 Krasimir Angelov <kr.angelov at gmail.com>:
> I was surprised to see this case expression:
>
>>        case GHC.Prim.-# 9223372036854775807 ipv_s1bD
>>        of wild2_a1xi [ALWAYS Just L] {
>
> What is the purpose to compare the value with maxBound before the
> division? The case expression doesn't disappear even if I use quot
> instead of div.

Have a look at this snippet of the base library, file GHC/Real.lhs:

    a `quot` b
     | b == 0                     = divZeroError
     | a == minBound && b == (-1) = overflowError
     | otherwise                  =  a `quotInt` b

quotInt is defined in GHC/Base.lhs as:

(I# x) `quotInt`  (I# y) = I# (x `quotInt#` y)

And quotInt# is a primitive, which I guess is implemented via machine
division (though I don't work on the codegen stuff at all) - so your
offending case must come from those tests in GHC/Real.lhs.

In general, if you want to answer questions like this you can usually
find the answer by looking at the base code: it's all in Haskell, so
very readable! You can get it online at
http://darcs.haskell.org/libraries/base/GHC/

Cheers,
Max