Access RTS flag values from inside Haskell programs

[ezyang]

Strictly speaking, it is already possible to do this since includes/rts/Flags.h exports RtsFlags, which can be accessed with a mini wrapper function. We should add this wrapper function so we can import it, and then write code which reads out the contents of all the various parameters so that users can introspect this at runtime. Maybe put them in the GHC.RTS module.

[thoughtpolice]

Moving to 7.10.1.

[thoughtpolice]

Moving over to new owning component 'Core Libraries'.

[Austin Seipp <austin@…>]

In 1617a10aaa75567b776d4a47200ddaa1267771db/ghc:

accessors to RTS flag values -- #5364

Summary: Implementation of #5364. Mostly boilerplate, reading FILE fields is missing.

Test Plan:
- Get some feedback on missing parts. (FILE fields)
- Get some feedback on module name.
- Get some feedback on other things.
- Get code reviewed.
- Make sure test suite is passing. (I haven't run it myself)

Reviewers: hvr, austin, ezyang

Reviewed By: ezyang

Subscribers: ekmett, simonmar, ezyang, carter, thomie

Differential Revision: https://phabricator.haskell.org/D306

GHC Trac Issues: #5364

Conflicts:
	includes/rts/Flags.h

[thoughtpolice]

Moving to 7.12.1 milestone; if you feel this is an error and should be addressed sooner, please move it back to the 7.10.1 milestone.

[Herbert Valerio Riedel <hvr@…>]

In 40561cd235f07d41904d2604ff7f0c942af4d35e/ghc:

Fix `heapSizeSuggesionAuto` typo (#9934)

This was introduced in 1617a10a (re #5364)

[hvr]

btw, I noticed that the API exposed by GHC.RTS.Flags varies depending on unsigned int:

-- | @'nat'@ defined in @rts/Types.h@
type Nat = #{type unsigned int}

data GCFlags = GCFlags
    { statsFile             :: Maybe FilePath
    , giveStats             :: GiveGCStats
    , maxStkSize            :: Nat
    , initialStkSize        :: Nat
    , stkChunkSize          :: Nat
...

Is this really a good idea rather simply using Word which I assume should always be large enough to contain the range of CULong?

Moreover, shouldn't most of those small-fields be !-ed to avoid thunks and unecessary pointer chasing?

[thoughtpolice]

Yes, we should make those cleanups. I've filed another ticket: #10022. However, this ticket is (in general) worth closing.