#9136: Constant folding in Core could be better --------------------------------------------+------------------------------ Reporter: simonpj | Owner: nomeata Type: bug | Status: patch Priority: normal | Milestone: Component: Compiler | Version: 7.8.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime performance bug | Unknown/Multiple Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: --------------------------------------------+------------------------------ Comment (by carter): @nomeata .... so I thought a bit about your reply... and part of my concern stems from the fact that the wrapping behavior of Int in GHC is actually a choice of "undefined" behavior by the various Haskell language standards. Point being, while wrap around is "folklore behavior", its actually deemed undefined by the standard. And it constitutes moving that undefined "folklore" into being a strong assumption in ghc. how would GCC come into play? theres no peephole optimization by the system assembler level in -fasm to my knowledge. most of the examples i've seen where arithmetic optimization makes a HUGE win are in the context of heavy bit fiddling sequences (I have some example that are 40 instructions long in -fasm, but optimized to NOOP on -fllvm), and various sophisticated array indexing computations. If nofib doesn't show a measurable difference and theres no motivating example program that shows a strong win.. I'd argue its not worth adding the rules. I'm happy to share some more interesting examples, but I'm not sure if they actually can be handled by a rewrite rules discipline. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9136#comment:12 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler