#14626: No need to enter a scrutinised value -------------------------------------+------------------------------------- Reporter: heisenbug | Owner: heisenbug Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.2.2 Resolution: | Keywords: performance Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #13861 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by heisenbug): Replying to [comment:5 simonpj]:
So something is definitely fishy here.
I think that there's something different about '''top-level''' binders. Consider {{{#!hs data T = MkT !Bool top_x = True
f True (MkT local_y) = local_y f False _ = top_x }}} Here I think that `local_y` gets bound to a correctly-tagged pointer, fetched out of the `MkT` constructor.
But, in contrast, I think that `top_x` is bound to the label for the top-level closure for `top_x`, which is 8-byte aligned. So the label isn't tagged; instead, the code generator has to tag it. Is that happening in your "return it instead of eantering it" path?
No, I am not returning top-level bindings, I would be okay with entering those. What I want is to avoid entering local bindings from the `StgCase`: {{{#!hs f = \inp -> case inp of wildBind { True -> ...; False -> wildBind } }}} `wildBind` is known to be evaled and properly tagged. I want to `ReturnIt`.
Other than that I have no idea why it crashes. Things you might try:
* Switch off the new optimisation when building stage2 and the
libraries. Use it only for the test suite: these are small programs and easier to debug.
* Maybe add an assertion in the Cmm: the claim is that on the "return
it" path, the thing beign returned is a correctly-tagged pointer. So, the assertion can follow the pointer and check that the thing pointed to is a value, with the right tag, etc. There must be some code in the RTS (or somewhere) that checks that. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14626#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler