#5444: Slow 64-bit primops on 32 bit system
---------------------------------+------------------------------------------
Reporter: Khudyakov | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.2.1 | Keywords:
Testcase: | Blockedby:
Os: Unknown/Multiple | Blocking:
Architecture: x86 | Failure: Runtime performance bug
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GHC primops for 64-bit arithmetic are implemented as FFI calls. It leads
to serious performance penalty for 32 bit code which heavily uses 64-bit
arithmetics.
I found this while investigating poor performance of mwc-random on 32-bit
systems. 32-bit build runs 3-4 times slower than 64-bit build on the same
hardware. It's difficult to estimate how faster would run optimal
implementation since it doesn't exist. But it's probably at least 2x
slowdown.
Here is simple program to demonstrate issue
{{{
sqr64 :: Int32 -> Int64
sqr64 x = y * y where y = fromIntegral x
}}}
Here is optimized core
{{{
$wsqr64 :: Int# -> Int64
$wsqr64 =
\ (ww_sGO :: Int#) ->
case {__pkg_ccall ghc-prim hs_intToInt64 Int#
-> State# RealWorld -> (# State#
RealWorld, Int64# #)}_aFY
ww_sGO realWorld#
of _ { (# _, ds2_aG2 #) ->
case {__pkg_ccall ghc-prim hs_timesInt64 Int64#
-> Int64# -> State# RealWorld -> (#
State# RealWorld, Int64# #)}_aGc
ds2_aG2 ds2_aG2 realWorld#
of _ { (# _, ds4_aGi #) ->
I64# ds4_aGi
}
}
sqr64 :: Int32 -> Int64
sqr64 = \ (w_sGM :: Int32) ->
case w_sGM of _ { I32# ww_sGO -> $wsqr64 ww_sGO }
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/5444>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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