#13851: Change in specialisation(?) behaviour since 8.0.2 causes 6x slowdown -------------------------------------+------------------------------------- Reporter: mpickering | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.1-rc2 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- I was investigating some benchmarks and I noticed some odd results if I duplicated one of my tests. Reproduction: https://github.com/mpickering/probable-eureka Looking at the core, it seems that repeating the definition means that one of the key functions doesn't get specialised as expected which leads to a much slower program. Observe that in the first two benchmarks there is a worker function `go :: Int# -> Int -> ReaderT Int (StateT Int Identity Int` but in the third benchmark this is specialised to `$sgo :: Int# -> Int -> Int -> Int# -> Int# -> Identity (Int, Int)`. Removing the duplicate benchmark means that specialisation happens properly in the first case as well. The proper specialisation also happens in 8.0.2. This causes the first two cases to be 6x slower than the last case. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13851 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler