Branchless implementation for literal case – is it worth it?

[Ben Gamari]

Joachim Breitner <mail at joachim-breitner.de> writes:

> Dear devs,
>
> in  https://ghc.haskell.org/trac/ghc/ticket/10124 bgamari suggested that
> code like
>         
>         f :: Int -> Bool
>         f a = case a of
>                 1  -> True
>                 5  -> True
>                 8  -> True
>                 9  -> True
>                 11 -> True
>                 19 -> True
>                 _  -> False
>         {-# NOINLINE f #-}
>
> should not compile to a series of conditional jumps, but rather a
> branchless code akin to
>
>         let !p = a ==# 1 `orI#` a ==# 5 `orI#` a ==# 8 `orI#` a ==# 9
>                          `orI#` a ==# 11 `orI#` a ==# 19
>         case p of 
>           1 -> do_something
>           0 -> do_something_else
>
Hi Joachim,

Thanks for trying this and I'm sorry to hear that the optimization
hasn't yielded the improvement I would have expected. I've been a bit
silent on the issue as I've been working on finishing up my
PhD. Hopefully in a couple of months after I'm settled in Germany I'll
have time to delve into this more deeply.

> Subsequently, I refactored the way we produce Cmm code from STG, opening
> the possibility to implement this optimization at that stage¹.
>
> But when I then added this implementation, I could not measure any
> runtime improvement, see
> https://ghc.haskell.org/trac/ghc/ticket/10124#comment:15
>
> So my question to the general audience: Is such branchless code really
> better than the current, branching code? Can someone provide us with an
> example that shows that it is better? Do I need to produce different
> branchless assembly?
>
Have you had a look at the Intel Optimization manual? I'll admit that I
haven't looked myself but I'd imagine they'd probably have a few things
to say about this.

Cheers,

- Ben
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