#7450: Regression in optimisation time of functions with many patterns (6.12 to
7.4)?
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Reporter: iustin | Owner:
Type: bug | Status: new
Priority: normal | Component: Compiler
Version: 7.6.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: Compile-time performance bug | Blockedby:
Blocking: | Related:
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In our project, we build (via TH) some not-trivial data structures,
including custom serialisation. After growing the number of constructors
significantly, compiling same code with ghc 7.4/7.6 is about 10 times
slower than with ghc 6.12.
Narrowing this down on a simpler test case, it turns out that TH is not
the cause, just the optimisation of the generated code.
The attached Generator.hs file generates a simple data structure (our
project is using a much smaller number of constructors, but with more
complex data types, and the result is the same):
{{{
data F
= F1 {fldF1 :: Int} |
F2 {fldF2 :: Int} |
F3 {fldF3 :: Int} |
F4 {fldF4 :: Int} |
F5 {fldF5 :: Int} |
… deriving (Eq, Show, Read)
prettyF :: F -> String
prettyF (F1 v) = show v
prettyF (F2 v) = show v
…
}}}
The main.hs file just calls this splice and then does a trivial use of
prettyF.
Compiling this with a variable number of constructors, with -O:
||Constructors||6.12||7.6||
||50||2.45s||4.10s||
||100||4.40s||10.60s||
||200||8.45s||33.30s||
||400||16.90s||121.00s||
||800||35.95s||514.50s||
As you can see, 6.12 looks liniar in the number of constructors, whereas
7.6 is not anymore (at least above 100 constructors).
Compiling without -O makes things sane again:
||Constructors||6.12||7.6||
||50||1.40s||1.97s||
||100||2.45s||2.70s||
||200||4.50s||4.95s||
||400||8.95s||9.55s||
||800||18.25s||19.10s||
After some more investigation, it turns out that just our function
function (prettyF) with no automatic instances is cheap,
compiling/deriving Eq/Show is also cheap, but Read is very expensive (all
numbers with ghc 7.6):
||Constructors||No instances||Eq||Show||Read||
||50||0.75s||0.90s||1.20s||2.95s||
||100||0.85s||1.00s||1.70s||6.80s||
||200||1.20s||1.45s||2.85s||19.15s||
||400||2.05s||2.50s||5.40s||64.45s||
||800||4.30s||5.40s||11.65s||259.40s||
So I believe this is about optimisation of complex functions with many
patterns/case statements. 6.12 seems nicely liniar, whereas 7.4/7.6
exhibit a much worse behaviour.
This might be a known issue, or even a non-issue ("Don't use Read for
complex types"), but I thought it's better to report it. I don't have the
skills to look at the core result, and see if it's different or not
between 6.12/7.6, sorry.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7450>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler
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