[Haskell-cafe] Does GHC compare pointers when eval'ing (==)

[Alexander Kjeldaas]

On Wed, Aug 20, 2014 at 11:20 AM, Johan Tibell <johan.tibell at gmail.com>
wrote:

> On Wed, Aug 20, 2014 at 11:05 AM, Johan Holmquist <holmisen at gmail.com>
> wrote:
>
>> > I think the reason it isn't done is that it's not always an optimization
>>
>> Could you explain when it's not?
>>
>
> A simple example is when the two values aren't equal, as you'll be doing
> one more branch than you otherwise would.
>

True


>  The extra branch might also hurt the branch predictor even in the equal
> case, if the branch is hard to predict.
>
>
This cost I think we can completely eliminate.  If the pointer comparison
is done only for an Eq instance that is statically known to execute N
instructions, then as as long as the cost of all subsequent branch
mispredictions are less than N, then we win, always.

Alexander


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