compiling huge HashSet hogs memory

Compiling a huge (~2.5k elements) set with GHC-7.10.1 or GHC-head and unordered-containers-0.2.5.1 or unordered-containers-head takes way too much memory. Here is a file which I am trying to compile. I've also set up a travis build which demonstrates the behaviour with different versions of GHC and unordered-containers. Further I would be referring to this build-job which uses GHC-7.10.1 and unordered-containers-0.2.5.1.

• Compilation with default optimization options takes around 12GB of memory.
• Compilation with -O0 takes around 400MB.
• Compilation with -O2 takes around 12GB of memory.
• Compilation with -O2 -fignore-interface-pragmas takes around 500MB of memory, which solves the problem.

When the build uses GHC-7.8.4, neither of this hogging occurs. Another interesting observation is that compiling HashMap of the same size, doesn't cause memory hogging even with -O2. This attracted my attention as HashSet is implemented in terms of HashMap.

I reported the issue to unordered-containers as well: link.

Trac metadata

Trac field	Value
Version	7.10.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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