#2463: unsafePerformIO in unused record field affects optimisations
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Reporter: claus | Owner:
Type: run-time performance bug | Status: new
Priority: normal | Component: Compiler
Version: 6.9 | Severity: normal
Keywords: | Testcase:
Architecture: Unknown | Os: Unknown
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I have some code that runs 2x slower or worse than a previous version, for
no obvious reasons. In an attempt to root out the cause, I made a copy and
started deleting parts of the code, until I noticed that I could trigger
the performance difference by whether or not I initialise a record field
that is no longer even used in this reduced source!
Further refinement shows that much of the performance difference depends
on whether that ''unused'' field is initialised using 'unsafePerformIO'
(needed to use 'typeRepKey') or 'undefined'.
To reproduce, switch the lines with commented with '-- slow' and '--fast':
{{{
$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.9.20080514
$ ghc --make -O2 Main
[2 of 3] Compiling GPS ( GPS.hs, GPS.o )
GPS.hs:29:8:
Warning: Pattern match(es) are overlapped
In the definition of `typeRepKey'': typeRepKey' tr = ...
Linking Main.exe ...
$ ./Main.exe >dump
5553600000
"Data.Generics.GPS countStuff: 20 secs"
$ ghc --make -O2 Main
[2 of 3] Compiling GPS ( GPS.hs, GPS.o )
GPS.hs:29:8:
Warning: Pattern match(es) are overlapped
In the definition of `typeRepKey'': typeRepKey' tr = ...
[3 of 3] Compiling Main ( Main.hs, Main.o )
Linking Main.exe ...
$ ./Main.exe >dump
5553600000
"Data.Generics.GPS countStuff: 11 secs"
}}}
'Main' uses 'GPS' and 'CompanyDatatypes' for some benchmarking of a Syb
variant, hence the blown-up data structures and Syb imports. Only the code
in 'GPS' seems to be directly involved, so perhaps one could achieve the
same effect by constructing a non-Syb loop, but I'm reluctant to reduce
this further, as the performance impact has started to disappear with
shrinking code.
Since the remaining code doesn't actually do much anymore, the issue
doesn't seem to be in my code, which is nice. But in the original version,
I need that record field, and I need to use 'typeRepKey' via
'unsafePerformIO', so any suggestions for helpful compiler options, etc
would be welcome.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/2463>
GHC <http://www.haskell.org/ghc/>
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