#2271: floor, ceiling, round :: Double -> Int are awesomely slow
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Reporter: dons | Owner:
Type: bug | Status: new
Priority: low | Milestone: 7.0.1
Component: libraries/base | Version: 6.8.2
Keywords: performance, math, double | Testcase:
Blockedby: | Difficulty: Unknown
Os: Unknown/Multiple | Blocking:
Architecture: Unknown/Multiple | Failure: None/Unknown
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Comment(by daniel.is.fischer):
I have put together a package to test possible implementations of
the RealFrac methods for Double and Float (base-2 IEEE754).
On the one hand, pure Haskell implementations, on the other hand
implementations calling out to rint[f], trunc[f], floor[f] and ceil[f]
from math.h.
Both ways go via Integer by default, with a specialised faster
implementation
for Int (and narrower types, but those RULES haven't yet been written)
enabled by a rewrite rule.
Overall, the pure Haskell implementations don't fare badly on my computer.
All give a speedup compared to the current implementation, for most
conversions, pure Haskell is on par with or faster than the C-call
(although that would probably change if the C functions were made
primops).
The FFI calls are significantly faster for
properFraction :: Double -> (Integer, Double)
and for round (except round :: Integral a => Float -> a when compiled via
C,
then native and FFI are on par).
Sample results for the speedups against the current implementation (note:
for truncate :: x -> Int, the Prelude value is fst . properFraction, not
the rewritten float2Int or double2Int) are included in the tarball.
I would appreciate feedback from your tests/benchmarks on other platforms,
especially 64-bit platforms (mine is x86 linux, 32 bit).
To run the QuickCheck tests, you need QuickCheck-2.*, to run the
benchmarks,
criterion.
More instructions in the README.
Thanks,
Daniel
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2271#comment:13>
GHC <http://www.haskell.org/ghc/>
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