Improve `mkInteger` interface
mkInteger
is the current operation provided by the Integer
libraries to construct (large) integer values. The current type-signature is
mkInteger :: Bool -- sign-bit
-> [Int] -- absolute value in 31 bit chunks, least significant first
-> Integer
A somewhat pathological example of why this representation is not so nice is the following simple CAF
c :: Integer
c = 0xf000000000000000
that (this is for a 64bit arch!) gets compiled into the following STG:
==================== STG syntax: ====================
sat_sJQ :: GHC.Types.Int
[LclId, Str=DmdType] =
NO_CCS GHC.Types.I#! [3];
sat_sJR :: [GHC.Types.Int]
[LclId, Str=DmdType] =
NO_CCS :! [sat_sJQ GHC.Types.[]];
sat_sJP :: GHC.Types.Int
[LclId, Str=DmdType] =
NO_CCS GHC.Types.I#! [1610612736];
sat_sJS :: [GHC.Types.Int]
[LclId, Str=DmdType] =
NO_CCS :! [sat_sJP sat_sJR];
sat_sJO :: GHC.Types.Int
[LclId, Str=DmdType] =
NO_CCS GHC.Types.I#! [0];
sat_sJT :: [GHC.Types.Int]
[LclId, Str=DmdType] =
NO_CCS :! [sat_sJO sat_sJS];
Foo.c :: GHC.Integer.Type.Integer
[GblId, Str=DmdType] =
\u srt:SRT:[0Y :-> GHC.Integer.Type.mkInteger, sJT :-> sat_sJT] []
GHC.Integer.Type.mkInteger GHC.Types.True sat_sJT;
Moreover, determining how much space to allocate for the resulting Integer
is a bit work as it for one, you need to traverse the list twice, and then you can't simply derive the exact allocation amount by the list-length alone due to the odd 31-bit chunks.
Instead a more "natural" mkInteger
would be desirable, possibly in the style of unpackCString#
, in terms of a packed/unboxed vector of machine-word-sized chunks. A better mkInteger
could then take a ByteArray#
or a Addr#
+ length instead, e.g.
mkInteger :: Int# -- signum(n) = sign of encoded Integer
-- abs(n) = number of machine-word sized chunks
-> Addr# -- pointer to start of machine-word sized chunks,
-- least-significant chunk first
-> Integer