Pattern matching against sets of strings sharing a prefix blows up pattern checker

hvr stumbled upon this issue while attempting to bootstrap GHC with GHC HEAD. In so doing he found that GHC HEAD required more than 10 GB of memory while compiling genprimopcode (and never completed).

It appears that this blow-up is due to the new pattern checker. In particular it appears that the pattern checker is affected quite adversely by sets of patterns sharing a prefix. For instance, this example,

import System.Environment

main :: IO ()
main = do
  args <- getArgs
  print $ case head args of
                      "--primop-primop-info"  -> "turtle"
                      "--primop-tag" -> "asdf"
                      "--primop-list"  -> "casdhf"
                      "--primop-vector-uniques"  -> "this"
                      "--primop-vector-tys"  -> "is"
                      "--primop-vector-tys-exports"  -> "silly"
                      "--primop-vector-tycons"  -> "hmmm"
                      "--make-haskell-wrappers" -> "123512"
                      "--make-haskell-source"  -> "as;dg"
                      "--make-latex-doc" -> "adghiw"
                      _ -> error "Should not happen, known_args out of sync?"

As written GHC requires over ten gigabytes of heap and several minutes to compile the example. If one perform s/--primop-// to this example it takes 500ms to compile. Alternatively, if on replace the first - in each of the --primop strings with a unique character (thus breaking the shared prefixes) compilation time is a bit shy of a second.

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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