#16305: When should -Wmissed-specializations fire? -------------------------------------+------------------------------------- Reporter: crockeea | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by crockeea: Old description:

While working on a fix for #16282, I noticed:

1. that `-Wmissed-specializations` doesn't fire when the docs say it should 2. that the "probable fix" suggested when a warning does fire is highly suspect

{{{ import Data.Map main :: IO () main = do let m = [] :: [Map String Bool] mapM_ print m }}}

With GHC 8.4.3, 8.6.3, and HEAD: `ghc -O2 -Wmissed-specializations Main.hs` does *not* issue any warnings.

However, `ghc -O2 -Wall-missed-specializations Main.hs` *does* create a warning: {{{ Main.hs: warning: [-Wall-missed-specialisations] Could not specialise imported function ‘Data.Map.Internal.$w$cshowsPrec’ when specialising ‘Data.Map.Internal.$fShowMap_$cshowsPrec’ Probable fix: add INLINABLE pragma on ‘Data.Map.Internal.$w$cshowsPrec’ }}}

The docs for `-Wmissed-specializations` say, "warn when specialisation of an imported, overloaded function fails." Since `showsPrec` is an imported function, it seems that a warning should have been issued with `-Wmissed- specs`. My reading of the docs is that `-Wall-missed-specs` should output everything `-Wmissed-specs` does, along with any *local* overloaded and unspecialized functions.

Moreover, the "Probable fix" is suspect. A warning recommending an `INLINABLE` pragma is issued depending on the output of `warnMissedSpec` in specialise/Specialise.hs. For `-Wall-missed-specs`, `warnMissedSpec` doesn't check if an INLINABLE pragma is already present, so the fix could be redundant. For `-Wmissed-specs`, `warnMissedSpecs` *only* issues a warning if there *is* any inline pragma (of one sort of another) on all the callers, making the `probable fix` definitely redundant.

New description: While working on a fix for #16282, I noticed: 1. that `-Wmissed-specializations` doesn't fire when the docs say it should 2. that the "probable fix" suggested when a warning does fire is highly suspect {{{ import Data.Map main :: IO () main = do let m = [] :: [Map String Bool] mapM_ print m }}} With GHC 8.4.3, 8.6.3, and HEAD: `ghc -O2 -Wmissed-specializations Main.hs` does *not* issue any warnings. However, in 8.6.3 and HEAD, `ghc -O2 -Wall-missed-specializations Main.hs` *does* create a warning: {{{ Main.hs: warning: [-Wall-missed-specialisations] Could not specialise imported function ‘Data.Map.Internal.$w$cshowsPrec’ when specialising ‘Data.Map.Internal.$fShowMap_$cshowsPrec’ Probable fix: add INLINABLE pragma on ‘Data.Map.Internal.$w$cshowsPrec’ }}} The docs for `-Wmissed-specializations` say, "warn when specialisation of an imported, overloaded function fails." Since `showsPrec` is an imported function, it seems that a warning should have been issued with `-Wmissed- specs`. My reading of the docs is that `-Wall-missed-specs` should output everything `-Wmissed-specs` does, along with any *local* overloaded and unspecialized functions. Moreover, the "Probable fix" is suspect. A warning recommending an `INLINABLE` pragma is issued depending on the output of `warnMissedSpec` in specialise/Specialise.hs. For `-Wall-missed-specs`, `warnMissedSpec` doesn't check if an INLINABLE pragma is already present, so the fix could be redundant. For `-Wmissed-specs`, `warnMissedSpecs` *only* issues a warning if there *is* any inline pragma (of one sort of another) on all the callers, making the `probable fix` definitely redundant. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16305#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#16305: When should -Wmissed-specializations fire? -------------------------------------+------------------------------------- Reporter: crockeea | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by crockeea): Nearly. `-Wmissed-specs` warns when an *imported* function which has *any* "inline" pragma (haven't dug into which are allowed) doesn't get specialized. `-Wall-missed-specs` warns whenever any overloaded function fails to specialize, exactly as you say. Questions are: 1. Should `-Wmissed-specs` warn in the example above? 2. What should the "Probable fix" be for either flag? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16305#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#16305: When should -Wmissed-specializations fire? -------------------------------------+------------------------------------- Reporter: crockeea | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Poor/confusing | Unknown/Multiple error message | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): As I read the code it is trying to do this: * `-Wall-missed-specs`: warn about ''any'' overloaded imported functions that cannot be specialised. Usually this means we don't have its RHS. * `-Wmissed-specs`: warn about any overloaded imported functions that can't be specialised, and is transitively called by a chain of imported functions each with an INLINEABLE pragma. For the latter consider {{{ module A where import B foo = f (3::Int) + g (4::Int) module B where f :: Num a => a -> a {-# INLINABLE f #-} f x = h x g x = Num a => a -> a g x = blah h x :: Num a => a -> a h x = blah }}} Then when compiling A, `-Wmissed-specs` is supposed to * Not warn about not-specialising `g`, because `g` didn't have an INLINABLE pragma. * Warn about non-specialising `h` because `f` (which is directly called from A) has a careful `INLINABLE` pragma, but `h` (perhaps due to an oversight) does not. In contrast `-Wall-missed-specs` should warn about both. Does that make sense? Is it what we want? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16305#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler