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I realized that the latest version of GHC (7.10.3) produces significantly slower code than an older version. My current version as of now:
$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.10.3
I have also two other old versions installed on my local machine.
My test code is taken from here (the collatz1.hs code):
import Data.Word
import Data.List
import System.Environment
collatzNext :: Word32 -> Word32
collatzNext a = (if even a then a else 3*a+1) `div` 2
-- new code
collatzLen :: Word32 -> Int
collatzLen a0 = lenIterWhile collatzNext (/= 1) a0
lenIterWhile :: (a -> a) -> (a -> Bool) -> a -> Int
lenIterWhile next notDone start = len start 0 where
len n m = if notDone n
then len (next n) (m+1)
else m
-- End of new code
main = do
[a0] <- getArgs
let max_a0 = (read a0)::Word32
print $ maximum $ map (\a0 -> (collatzLen a0, a0)) [1..max_a0]
Compiling with GHC 7.4, 7.6 and 7.10 yields the following times:
$ ~/Tools/ghc-7.4.2/bin/ghc -O2 Test.hs
[1 of 1] Compiling Main ( Test.hs, Test.o )
Linking Test ...
$ time ./Test 1000000
(329,837799)
real 0m1.879s
user 0m1.876s
sys 0m0.000s
$ ~/Tools/ghc-7.6.1/bin/ghc -O2 Test.hs
[1 of 1] Compiling Main ( Test.hs, Test.o )
Linking Test ...
$ time ./Test 1000000
(329,837799)
real 0m1.901s
user 0m1.896s
sys 0m0.000s
$ ~/Tools/ghc/bin/ghc -O2 Test.hs
[1 of 1] Compiling Main ( Test.hs, Test.o )
Linking Test ...
$ time ./Test 1000000
(329,837799)
real 0m10.562s
user 0m10.528s
sys 0m0.036s
We can tell there is no doubt that the latest version of GHC produces worse code than the older two versions. I can't reproduce the same efficiency as the blog though probably because I don't have LLVM and Idon't have the exact version the author used. But still, I believe the conclusion is obvious.
My question is, in general, why this could happen? Somehow GHC becomes worse than it used to be. And specifically, if I want to investigate, how should I get myself started?
310
Here's a comparison of both profiles (diff Test-GHC-7-8-4.prof Test-GHC-7-10-3.prof)
1c1
< Fri Mar 11 19:58 2016 Time and Allocation Profiling Report (Final)
---
> Fri Mar 11 19:59 2016 Time and Allocation Profiling Report (Final)
5,6c5,6
< total time = 2.40 secs (2400 ticks @ 1000 us, 1 processor)
< total alloc = 256,066,744 bytes (excludes profiling overheads)
---
> total time = 10.89 secs (10895 ticks @ 1000 us, 1 processor)
> total alloc = 15,713,590,808 bytes (excludes profiling overheads)
10,13c10,12
< lenIterWhile.len Main 93.8 0.0
< collatzMax Main 2.2 93.7
< collatzNext Main 2.0 0.0
< lenIterWhile Main 1.5 6.2
---
> collatzNext Main 79.6 89.4
> lenIterWhile.len Main 18.9 8.8
> collatzMax Main 0.8 1.5
There's something very strange going on. While in GHC lenIterWhile.len was taking most of the time, collatzNext is now the culprit. Let's have a look at the dumped core:
-- GHC 7.8.4
Rec {
Main.$wlen [Occ=LoopBreaker]
:: GHC.Prim.Word# -> GHC.Prim.Int# -> GHC.Prim.Int#
[GblId, Arity=2, Caf=NoCafRefs, Str=DmdType <S,1*U><L,U>]
Main.$wlen =
\ (ww_s4Mn :: GHC.Prim.Word#) (ww1_s4Mr :: GHC.Prim.Int#) ->
case ww_s4Mn of wild_XQ {
__DEFAULT ->
case GHC.Prim.remWord# wild_XQ (__word 2) of _ [Occ=Dead] {
__DEFAULT ->
Main.$wlen
(GHC.Prim.quotWord#
(GHC.Prim.narrow32Word#
(GHC.Prim.plusWord#
(GHC.Prim.narrow32Word# (GHC.Prim.timesWord# (__word 3) wild_XQ))
(__word 1)))
(__word 2))
(GHC.Prim.+# ww1_s4Mr 1);
__word 0 ->
Main.$wlen
(GHC.Prim.quotWord# wild_XQ (__word 2)) (GHC.Prim.+# ww1_s4Mr 1)
};
__word 1 -> ww1_s4Mr
}
end Rec }
Seems more or less reasonable. Now about GHC 7.10.3:
Rec {
$wlen_r6Sy :: GHC.Prim.Word# -> GHC.Prim.Int# -> GHC.Prim.Int#
[GblId, Arity=2, Str=DmdType <S,U><L,U>]
$wlen_r6Sy =
\ (ww_s60s :: GHC.Prim.Word#) (ww1_s60w :: GHC.Prim.Int#) ->
case ww_s60s of wild_X1Z {
__DEFAULT ->
case even
@ Word32 GHC.Word.$fIntegralWord32 (GHC.Word.W32# wild_X1Z)
of _ [Occ=Dead] {
False ->
$wlen_r6Sy
(GHC.Prim.quotWord#
(GHC.Prim.narrow32Word#
(GHC.Prim.plusWord#
(GHC.Prim.narrow32Word# (GHC.Prim.timesWord# (__word 3) wild_X1Z))
(__word 1)))
(__word 2))
(GHC.Prim.+# ww1_s60w 1);
True ->
$wlen_r6Sy
(GHC.Prim.quotWord# wild_X1Z (__word 2)) (GHC.Prim.+# ww1_s60w 1)
};
__word 1 -> ww1_s60w
}
end Rec }
Allright, seems like it's the same. Except for the call of even. Let's replace even with one of the inline variants of Integral, e.g. x :rem 2 == 0
import Data.Word
import Data.List
import System.Environment
collatzNext :: Word32 -> Word32
collatzNext a = (if a `rem` 2 == 0 then a else 3*a+1) `div` 2
-- rest of code the same
Let's compile it again with profiling and check:
$ stack --resolver=ghc-7.10 ghc -- Test.hs -O2 -fforce-recomp -prof -fprof-auto -auto-all
$ ./Test +RTS -s -p -RTS
(329,837799)
416,119,240 bytes allocated in the heap
69,760 bytes copied during GC
59,368 bytes maximum residency (2 sample(s))
21,912 bytes maximum slop
1 MB total memory in use (0 MB lost due to fragmentation)
Tot time (elapsed) Avg pause Max pause
Gen 0 800 colls, 0 par 0.000s 0.002s 0.0000s 0.0001s
Gen 1 2 colls, 0 par 0.000s 0.000s 0.0002s 0.0003s
INIT time 0.000s ( 0.019s elapsed)
MUT time 2.500s ( 2.546s elapsed)
GC time 0.000s ( 0.003s elapsed)
RP time 0.000s ( 0.000s elapsed)
PROF time 0.000s ( 0.000s elapsed)
EXIT time 0.000s ( 0.000s elapsed)
Total time 2.500s ( 2.567s elapsed)
%GC time 0.0% (0.1% elapsed)
Alloc rate 166,447,696 bytes per MUT second
Productivity 100.0% of total user, 97.4% of total elapsed
$ cat Test.prof
Fri Mar 11 20:22 2016 Time and Allocation Profiling Report (Final)
Test.exe +RTS -s -p -RTS 1000000
total time = 2.54 secs (2535 ticks @ 1000 us, 1 processor)
total alloc = 256,066,984 bytes (excludes profiling overheads)
COST CENTRE MODULE %time %alloc
lenIterWhile.len Main 94.4 0.0
main Main 1.9 93.7
collatzNext Main 1.8 0.0
lenIterWhile Main 1.3 6.2
individual inherited
COST CENTRE MODULE no. entries %time %alloc %time %alloc
MAIN MAIN 44 0 0.0 0.0 100.0 100.0
main Main 89 0 1.9 93.7 100.0 100.0
main.\ Main 92 1000000 0.4 0.0 98.1 6.2
collatzLen Main 93 1000000 0.2 0.0 97.8 6.2
lenIterWhile Main 94 1000000 1.3 6.2 97.5 6.2
lenIterWhile.len Main 95 88826840 94.4 0.0 96.2 0.0
collatzNext Main 96 87826840 1.8 0.0 1.8 0.0
main.max_a0 Main 90 1 0.0 0.0 0.0 0.0
CAF GHC.IO.Encoding.CodePage 73 0 0.0 0.0 0.0 0.0
CAF System.Environment 64 0 0.0 0.0 0.0 0.0
CAF GHC.IO.Handle.Text 62 0 0.0 0.0 0.0 0.0
CAF GHC.IO.Encoding 61 0 0.0 0.0 0.0 0.0
Seems like that fixed it. So the problem is that GHC-7.8 inlines even, while GHC-7.10 doesn't. This happens due to added {-# SPECIALISE even :: x -> x -> Bool #-} rules for Int and Integer, which don't allow inlining.
As issue's discussion documents making even and odd {-# INLINEABLE ... #-} would resolve this issue. Note that the specialisation itself was added for perfomance reasons.
167
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