#12733: Orphan hashes are not being propagated correctly -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Driver | Version: 8.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by ezyang: @@ -75,1 +75,1 @@ - The orphan hash didn't get updated. + The ABI hash didn't get updated. @@ -77,1 +77,1 @@ - At the moment, when we compute the orphan hash, we only consider the + At the moment, when we compute the export hash, we only consider the New description: In Phab:D2607, simonpj asked why we didn't also put class instances in the export hash. Why indeed. In the course of investigating, I discovered another bug in how we handle orphan recompilation. Unfortunately, this bug needs two packages to manifest. {{{ -- p/P.hs module P where instance Show (a -> b) where --instance Show (IO a) where -- q/Q.hs module Q where import P -- q/Q2.hs module Q2 where import Q }}} Put in some default Cabal files, with q depending on p, and then build the entire kaboodle. Then edit `p/P.hs` to uncomment the second orphan instance. Rebuild. {{{ ezyang@sabre:~/Dev/labs/orph$ cabal new-build q -w ghc-head In order, the following will be built (use -v for more details): - p-0.1.0.0 (lib) (file P.hs changed) - q-0.1.0.0 (lib) (dependency rebuilt) Preprocessing library p-0.1.0.0... [1 of 1] Compiling P ( P.hs, /srv/code/labs/orph/dist- newstyle/build/x86_64-linux/ghc-8.1.20161016/p-0.1.0.0/build/P.o ) P.hs:2:10: warning: [-Wmissing-methods] • No explicit implementation for either ‘showsPrec’ or ‘show’ • In the instance declaration for ‘Show (a -> b)’ P.hs:3:10: warning: [-Wmissing-methods] • No explicit implementation for either ‘showsPrec’ or ‘show’ • In the instance declaration for ‘Show (IO a)’ module Q2 where Preprocessing library q-0.1.0.0... [1 of 2] Compiling Q ( Q.hs, /srv/code/labs/orph/dist- newstyle/build/x86_64-linux/ghc-8.1.20161016/q-0.1.0.0/build/Q.o ) [P changed] ezyang@sabre:~/Dev/labs/orph$ }}} Notice: Q2 never got rebuilt. This is wrong wrong wrong: P changed, and we need to rebuild Q2! Here are the hashes for Q.hi before and after: {{{ -- Before interface Q 80120161016 interface hash: 2ffc06a1552565a73c888110ab4cf8df ABI hash: b2768862866252ea5a63401d2aa05aaa export-list hash: 092acc280445a8bcc96904afb8b766b0 orphan hash: 693e9af84d3dfcc71e640e005bdc5e2e flag hash: f01bb78d3c47152bc5253f256b2a60d3 -- After interface Q 80120161016 interface hash: 009db9e5ecbe49e36b7bf38e02087e75 ABI hash: b2768862866252ea5a63401d2aa05aaa export-list hash: 092acc280445a8bcc96904afb8b766b0 orphan hash: 693e9af84d3dfcc71e640e005bdc5e2e flag hash: f01bb78d3c47152bc5253f256b2a60d3 }}} The ABI hash didn't get updated. At the moment, when we compute the export hash, we only consider the orphan hashes from the home package. Here's the comment justifying it: {{{ -- the export hash of a module depends on the orphan hashes of the -- orphan modules below us in the dependency tree. This is the way -- that changes in orphans get propagated all the way up the -- dependency tree. We only care about orphan modules in the current -- package, because changes to orphans outside this package will be -- tracked by the usage on the ABI hash of package modules that we import. }}} It is true that we know to rebuild Q.hs when P.hs changes. But that is not much solace for Q2.hs... -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12733#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler