#10712: Regression: make TEST=exceptionsrun001 WAY=optasm is failing -------------------------------------+------------------------------------- Reporter: thomie | Owner: Type: bug | Status: new Priority: high | Milestone: 7.12.1 Component: Compiler | Version: 7.11 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: | base/tests/exceptionsrun001 Blocked By: | Blocking: Related Tickets: | Differential Revisions: -------------------------------------+------------------------------------- Changes (by simonpj): * cc: simonmar (added) Comment: Bother. I totally missed that. I think it's reasonable that this should allow the exception to be raised somewhere else (imprecisely): {{{ catch# (\s -> (raise# blah) `seq` blah2) (...) st }}} because `raise#` is in the pure world. But `raiseIO#` is specifically intended to raise an exception precisely at the specified moment, so the new behaviour is unacceptable. Now I think about this more I'm also worried about {{{ let r = \st -> raiseIO# blah st in catch (\st -> ...(r st)..) handler st }}} Now that I'm given `catch` a more aggressive strictness, I'll get a demand `C(S)` for `r`; that is, it is definitly called with one argument. And so it is! But the danger is that we'll feed `C(S)` into `r`'s rhs as the demand of the body, and say that that whole `let` will definitely diverge (which isn't true). However, we ''really'' want this function to be strict in `x`: {{{ f x st = catch (\s -> case x of I# x' -> ...) handler st }}} Getting this strictness was the whole point of the offending commit: {{{ Program Size Allocs Runtime Elapsed TotalMem -------------------------------------------------------------------------------- hpg -0.4% -2.9% -0.9% -1.0% +0.0% reverse-complem -0.4% -10.9% +10.7% +10.9% +0.0% simple -0.3% -0.0% +26.2% +26.2% +3.7% sphere -0.3% -6.3% 0.09 0.09 +0.0% -------------------------------------------------------------------------------- Min -0.7% -10.9% -4.6% -4.7% -1.7% Max -0.2% +0.0% +26.2% +26.2% +6.5% Geometric Mean -0.4% -0.3% +2.1% +2.1% +0.1% }}} There's something very special about `catch`: it turns divergence into non-divergence. (The strictness analyser treats divergence and exceptions identically.) I think #8598 is relevant. Bother bother. I'm really not sure what to do. Even if we revert, we should not revert all, just the strictness signatures for the `catch` primops. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10712#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler