[Haskell-cafe] GHC optimisations

Stefan O'Rear stefanor at cox.net
Sun Aug 19 15:14:20 EDT 2007

On Sun, Aug 19, 2007 at 12:53:07PM +0100, Andrew Coppin wrote:
> Does GHC do stuff like converting (2*) into (shift 1) or converting x + x 
> into 2*x?

For a good time, compile some code which uses even or odd :: Int -> Bool
using -O2 -fasm -ddump-asm...  The compiler *really* shouldn't be using 'idivl'.

(If you use -fvia-C -optc-O2, the C compiler will notice the operations
and optimize it itself.  This is one of the very few areas where -fvia-C
is still better.)

> If I do x * sin 12, is GHC likely to compute sin 12 at compile-time?

Also try -ddump-simpl-stats and -ddump-simpl-iterations if you want to
know *why*.  (The extremely obscure 'full laziness' transformation
performed by GHC has a fundamental effect on the compilation of x * sin

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