long compilation time for module with large data type and partial record selectors

in both ghc 7.6 and 7.8rc, the Constants module in LLVM-General-Pure (and a few other similar modules) have surprisingly long (and noticable!) compilation times.

This seems to related to how GHC handles a many constructor data type with overlapping field names.

the time is nearly the same for building a static module with O1 and O2

carter repoScratcher/bad-ast » time ghc Constant.hs -O2                                                                      1 ↵
[1 of 1] Compiling Constant         ( Constant.hs, Constant.o )
ghc Constant.hs -O2  7.52s user 0.23s system 99% cpu 7.755 total

carter repoScratcher/bad-ast » time ghc Constant.hs -O1
[1 of 1] Compiling Constant         ( Constant.hs, Constant.o )
ghc Constant.hs -O1  6.62s user 0.20s system 100% cpu 6.768 total

heck even with -O0 its slow (though not terrible)

carter repoScratcher/bad-ast » time ghc Constant.hs -O0
[1 of 1] Compiling Constant         ( Constant.hs, Constant.o )
ghc Constant.hs -O0  2.76s user 0.14s system 101% cpu 2.873 total

Seems like there might something wrong in how we handle data types like this, the complexity need not

Trac metadata

Trac field	Value
Version	7.8.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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