#10678: integer-gmp's runS seems unnecessarily expensive
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Reporter: rwbarton | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Compiler | Version: 7.10.1
(CodeGen) |
Keywords: | Operating System: Unknown/Multiple
Architecture: | Type of failure: Runtime
Unknown/Multiple | performance bug
Test Case: | Blocked By:
Blocking: | Related Tickets:
Differential Revisions: |
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integer-gmp uses an unsafePerformIO-like operation to work with mutable
BigNats (unsafePerformIO and even the IO type are not yet available, since
integer-gmp is a dependency of base):
{{{
type S s a = State# s -> (# State# s, a #)
-- NB: equivalent of GHC.IO.unsafeDupablePerformIO, see notes there
runS :: S RealWorld a -> a
runS m = lazy (case m realWorld# of (# _, r #) -> r)
{-# NOINLINE runS #-}
}}}
It's tempting to think of such an operation as "free" like an
unsafeCoerce, but it is actually somewhat expensive.
Consider `plusBigNat` for instance. (Most BigNat operations have a similar
structure.)
{{{
plusBigNat :: BigNat -> BigNat -> BigNat
plusBigNat x y
| isTrue# (eqBigNatWord# x 0##) = y
| isTrue# (eqBigNatWord# y 0##) = x
| isTrue# (nx# >=# ny#) = go x nx# y ny#
| True = go y ny# x nx#
where
go (BN# a#) na# (BN# b#) nb# = runS $ do
mbn@(MBN# mba#) <- newBigNat# na#
(W# c#) <- liftIO (c_mpn_add mba# a# na# b# nb#)
case c# of
0## -> unsafeFreezeBigNat# mbn
_ -> unsafeSnocFreezeBigNat# mbn c#
nx# = sizeofBigNat# x
ny# = sizeofBigNat# y
}}}
The assembly for `go` begins
{{{
00000000000001d0