Thread status constants

[Jost Berthold]

> Message: 2
> Date: Sat, 19 Jul 2014 15:00:17 +1000
> From: Kyle Van Berendonck <kvanberendonck at gmail.com>
> To: "ghc-devs at haskell.org" <ghc-devs at haskell.org>
> Subject: Thread status constants
> Message-ID:
> 	<CAFSvR6A3maSWngY=GJdATvGPjTAyuPokXZ5=rLtLT=EbYdSLKA at mail.gmail.com>
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>
> Hi,
>
> I found these:
> https://github.com/ghc/ghc/blob/5f3c5384df59717ca8013c5df8d1f65692867825/includes/rts/Constants.h#L194
>
> They go only 0-14, so there's some long chains of branches and stuff in hot
> paths that could be cleaned up into single &-masked branches by changing
> these into a set of flags.
>
> And then I saw these:
> https://github.com/ghc/ghc/blob/master/libraries/base/GHC/Conc/Sync.lhs#L483
>
> Where does 16 and 17 come from -- I couldn't find them in the header files
> anywhere?
>
> Kyle

Hi Kyle,

I had a look at these constants just recently.

About 16 and 17, see here:
   https://github.com/ghc/ghc/blob/master/rts/PrimOps.cmm#L853

About the others:The block reasons received two updates just recently.
Turns out that the eventlog (GHC's post-mortem tracing for threadscope 
et al.) expects these states to have particular values (those in 
includes/rts/Constants.h, consider them a fixed ABI for the time being).
See here for the story:
   https://ghc.haskell.org/trac/ghc/ticket/9003

NB: Darn... base still uses GHC-7.8.2 constants. Thanks for the pointer!

About your proposal to use bit masks:

Did you find some of those hot paths?
I believe (believe, unchecked...) that there are not too many places 
where a complete switch over all possible reasons is done (apart from 
debug output maybe); the typical use is rather to compare to one single 
constant than to all. So the performance might not increase too much 
from optimisations. Again, just a hunch, not verified.

If the RTS contains code specialised to the current constants, these 
places need to be kept symbolic, to avoid future breakage when new 
states are added.

A fix which removes the tight implicit dependency of ghc-events from the 
constants in includes/rts/Constants.h is pending, btw.

/ Jost