[GHC] #10161: GHC does not relink if a library's code changed
#10161: GHC does not relink if a library's code changed -------------------------------------+------------------------------------- Reporter: nh2 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.4 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Blocked By: Test Case: | Related Tickets: Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Test case for reproducing: https://github.com/nh2/ghc-relinking-avoidance- bug/ ---- {{{myexe}}} is an executable that depends on {{{mylib}}}, whose {{{myfun = putStrLn "output 1"}}}. If we install {{{mylib}}}, compile {{{myexe}}}, then change mylib's code to {{{myfun = putStrLn "output 2"}}}, re-install it, and then compile {{{myexe}}}, then GHC does not notice that the library code changed. It avoids re-linking myexe, with the result that the program prints {{{output 1}}} when you told it to compile against code that prints {{{output 2}}}. ---- Note that I had to use {{{NOINLINE myfun}}} to trigger the bug, since otherwise {{{myfun}}}'s code would have ended up in the interface file, thus changing the package ID, which naturally forces GHC to relink (even re-compile). With the {{{NOINLINE}}}, the package IDs of the two different versions of {{{myfun}}} are completely identical. I think that is correct, since the package ID only hashes the API and ABI, not the actual implementation, right? How does GHC decide when to relink? I think in the present case, it doesn't notice that the object/archive file of the library changed. Does it check that somehow? Just looking at API and API can't be enough to make a decision for linking. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10161 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#10161: GHC does not relink if a library's code changed -------------------------------------+------------------------------------- Reporter: nh2 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.4 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: None/Unknown | Unknown/Multiple Blocked By: | Test Case: Related Tickets: | Blocking: | Differential Revisions: -------------------------------------+------------------------------------- Comment (by ezyang): Happens with the RTS too. One answer is, if your library changes, its version number should change (but this isn't very satisfactory!) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10161#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#10161: GHC does not relink if a library's code changed -------------------------------------+------------------------------------- Reporter: nh2 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Driver | Version: 7.8.4 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #10966 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by thomie): * component: Compiler => Driver * related: => #10966 Comment: Might have same underlying cause as #10966. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10161#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#10161: GHC does not relink if a library's code changed -------------------------------------+------------------------------------- Reporter: nh2 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Driver | Version: 7.8.4 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #10966 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by ezyang): So, I'm pretty sure the problem (in this bug) is this: 1. You made an ABI-compatible change to an upstream library. 2. Recompilation avoidance decides that myexe does not need to be recompiled (rightly so) 3. The decision whether to relink or not depends purely on whether or not any local modules got recompiled. Which they did not. It sort of sounds like we need to store some extra metadata in the final linked executable which talks about the precise objects involved, so we know when to relink. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10161#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#10161: GHC does not relink if a library's code changed -------------------------------------+------------------------------------- Reporter: nh2 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Driver | Version: 7.8.4 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #10966 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by ezyang): There are a few ways to go about fixing this, but we have to be careful to keep GHC's compilation results deterministic. One consequence of this is that we CANNOT store the paths that were linked to produce the final executable in the executable: we don't want to bake those paths into the build product. But then how does GHC know what to query in order to find out if linking is necessary? My conclusion is that GHC has to (1) somehow run ld in a dry run mode (I could see no flag which actually implemented this) or (2) reimplement ld's library finding logic ourselves, so that we can guess what the actual files we're going to depend on are and then make a linking decision. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10161#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#10161: GHC does not relink if we link against a new library with old timestamp -------------------------------------+------------------------------------- Reporter: nh2 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Driver | Version: 7.8.4 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #10966 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10161#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#10161: GHC does not relink if we link against a new library with old timestamp -------------------------------------+------------------------------------- Reporter: nh2 | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Driver | Version: 7.8.4 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #10966 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by ezyang): I think thomie is right and the base problem is the same: the way we decide to relink is if any of the inputs to the linker are newer than the executable. In the provided test case, both libraries are built before the initial link, so there's nothing newer. A workaround is to touch the library after you do an operation like this. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10161#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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