Re: Branchless implementation for literal case – is it worth it?

[Gregory Collins]

I've gotten substantial speedups (>30%) in the past (e.g. in the hashtables
package) by replacing search loops with branchless code, but the technique
works best in situations where you have a long run of straight-line integer
code for the CPU to chew on; for branchless to be a win you need to feed
enough instructions into the pipeline such that the savings of getting 3
instructions per cycle and avoiding paying for branch mispredictions
outweighs the savings you would get from not doing the work. I don't think
the use case you posted falls into that category.

On Sun, Apr 19, 2015 at 12:44 AM, Joachim Breitner <mail at joachim-breitner.de
> wrote:

> 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
>
> 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?
>
> If it is not better, I can again refactor the switch generation and
> simplify it a bit again.
>
> Hmm, only now I see that rwbarton has replied there. Not sure why I
> missed that. Anyways, more voices are welcome :-)
>
> Greetings,
> Joachim
>
> ¹ This should not preclude an implementation on the Core level, which
> SPJ preferred. But I improved other aspects of the code generation as
> well, so this is worthwhile on its own.
>
> --
> Joachim “nomeata” Breitner
>   mail at joachim-breitner.de • http://www.joachim-breitner.de/
>   Jabber: nomeata at joachim-breitner.de  • GPG-Key: 0xF0FBF51F
>   Debian Developer: nomeata at debian.org
>
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>


-- 
Gregory Collins <greg at gregorycollins.net>
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