One possible use of unboxed booleans is branchless search. There are some algorithms where you can replace branches (e.g. case statements) with bit twiddling operators. I believe Gregory Collins used one such trick in

#6135: Unboxed Booleans ---------------------------------+------------------------------------------ Reporter: benl | Owner: jstolarek Type: feature request | Status: patch Priority: normal | Milestone: 7.8.1 Component: Compiler | Version: 7.4.1 Keywords: | Os: Unknown/Multiple Architecture: Unknown/Multiple | Failure: None/Unknown Difficulty: Unknown | Testcase: Blockedby: | Blocking: Related: #605 | ---------------------------------+------------------------------------------ Comment(by gregorycollins): the hashtables package. Didn't see this before now. Yes, this is true, part of the reason I wrote the cache-line-sized hash code search is that I couldn't get efficient straight-line code out of GHC for doing it. One very useful bit-twiddling hack that the processor can do but GHC can't: int mask(int a, int b) { return -(a == b); } The C code gives me this: {{{ xorl %eax, %eax cmpl %esi, %edi sete %al negl %eax }}} The Haskell code gives me code with branches because of the case on `Bool`, which I couldn't figure out how to get rid of. I did manage to get straight-line code for a pure Haskell function with the same semantics, but at a cost of about twice as many instructions. Interestingly, getting rid of the branch here, even in the roundabout way the Haskell version did it, was worth O(30%) on hash table lookup (it's in an inner loop). The C version I wrote for cache line-sized hash code lookups was good for another 20-30% besides that, and I chalked up the difference to this comparison issue with Bool and much better register allocation on the C side. (GHC was spilling even when it seemed there were many free 64-bit registers). So a solid +1 for me for implementing something like this, although maybe not with the names in the patch? -- Ticket URL: http://hackage.haskell.org/trac/ghc/ticket/6135#comment:23 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler