20x slow down with -O or -O2

This program:

http://www.cse.unsw.edu.au/~dons/code/nobench/spectral/calendar/

Suffers a misoptimisation when compiled with -O or -O2. When compiled with -Onot:

serenity$ ghc -Onot --make -o calendar calendar.hs -no-recomp
[1 of 1] Compiling Main             ( calendar.hs, calendar.o )
Linking calendar ...

serenity$ time ./calendar 1993 10000 > /dev/null
./calendar 1993 10000 > /dev/null  0.02s user 0.00s system 99% cpu 0.021 total

When compiled with -O:

serenity$ ghc -O --make -o calendar calendar.hs -no-recomp
[1 of 1] Compiling Main             ( calendar.hs, calendar.o )
Linking calendar ...

serenity$ time ./calendar 1993 10000 > /dev/null
./calendar 1993 10000 > /dev/null  5.00s user 0.01s system 99% cpu 5.011 total

And with -O2:

serenity$ time ./calendar 1993 10000 > /dev/null
./calendar 1993 10000 > /dev/null  4.95s user 0.01s system 100% cpu 4.953 total

Reproduced in amd64 and x86 linux.

Trac metadata

Trac field	Value
Version	6.6
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture	Multiple