Branchless implementation for literal case – is it worth it?

[Joachim Breitner]

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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