#13828: Re-linking is not avoided when -dynamic is used -------------------------------------+------------------------------------- Reporter: nh2 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.2 (Linking) | 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: | -------------------------------------+------------------------------------- Comment (by nh2): When using `cabal build -v`, we see these ghc invocations: {{{ Component build order: library, executable 'cabal-relink-test' creating dist/build creating dist/build/autogen Building cabal-relink-test-0.1.0.0... /path/to/ghc-8.0.2-with-packages/bin/ghc-pkg init dist/package.conf.inplace Preprocessing library cabal-relink-test-0.1.0.0... Building library... creating dist/build /path/to/ghc-8.0.2-with-packages/bin/ghc --make -fbuilding-cabal-package -O -j4 -static -dynamic-too -dynosuf dyn_o -dynhisuf dyn_hi -outputdir dist/build -odir dist/build -hidir dist/build -stubdir dist/build -i -idist/build -isrc -idist/build/autogen -Idist/build/autogen -Idist/build -optP-include -optPdist/build/autogen/cabal_macros.h -this-unit-id cabal- relink-test-0.1.0.0-9KTeoqstB5uArQE6VcRCGn -hide-all-packages -package-db dist/package.conf.inplace -package-id base-4.9.1.0 -XHaskell2010 Mymodule Linking... [(SimpleUnitId (ComponentId "base-4.9.1.0"),PackageIdentifier {pkgName = PackageName {unPackageName = "base"}, pkgVersion = Version {versionBranch = [4,9,1,0], versionTags = []}},ModuleRenaming True [])] /path/to/binutils-2.27/bin/ar -r dist/build/objs-18165/libHScabal-relink- test-0.1.0.0-9KTeoqstB5uArQE6VcRCGn.a dist/build/Mymodule.o /path/to/binutils-2.27/bin/ar: creating dist/build/objs-18165/libHScabal- relink-test-0.1.0.0-9KTeoqstB5uArQE6VcRCGn.a /path/to/ghc-8.0.2-with-packages/bin/ghc -shared -dynamic '-dynload deploy' -optl-Wl,-rpath,/path/to/ghc-8.0.2/lib/ghc-8.0.2/base-4.9.1.0 -optl-Wl,-rpath,/path/to/ghc-8.0.2/lib/ghc-8.0.2/ghc-prim-0.5.0.0 -optl- Wl,-rpath,/path/to/ghc-8.0.2/lib/ghc-8.0.2/integer-gmp-1.0.0.1 -optl- Wl,-rpath,/path/to/gmp-6.1.1/lib -optl- Wl,-rpath,/path/to/ghc-8.0.2/lib/ghc-8.0.2/rts -hide-all-packages -no- auto-link-packages -package-db dist/package.conf.inplace -package-id base-4.9.1.0 dist/build/Mymodule.dyn_o -o dist/build/libHScabal-relink- test-0.1.0.0-9KTeoqstB5uArQE6VcRCGn-ghc8.0.2.so /path/to/ghc-8.0.2-with-packages/bin/ghc-pkg update - --global --user '--package-db=dist/package.conf.inplace' Preprocessing executable 'cabal-relink-test' for cabal-relink- test-0.1.0.0... Building executable cabal-relink-test... creating dist/build/cabal-relink-test creating dist/build/cabal-relink-test/cabal-relink-test-tmp /path/to/ghc-8.0.2-with-packages/bin/ghc --make -no-link -fbuilding-cabal- package -O -j4 -static -outputdir dist/build/cabal-relink-test/cabal- relink-test-tmp -odir dist/build/cabal-relink-test/cabal-relink-test-tmp -hidir dist/build/cabal-relink-test/cabal-relink-test-tmp -stubdir dist/build/cabal-relink-test/cabal-relink-test-tmp -i -idist/build/cabal- relink-test/cabal-relink-test-tmp -iapp -idist/build/autogen -Idist/build/autogen -Idist/build/cabal-relink-test/cabal-relink-test-tmp -optP-include -optPdist/build/autogen/cabal_macros.h -hide-all-packages -package-db dist/package.conf.inplace -package-id base-4.9.1.0 -package-id cabal-relink-test-0.1.0.0-9KTeoqstB5uArQE6VcRCGn -XHaskell2010 app/Main.hs -dynamic Linking... /path/to/ghc-8.0.2-with-packages/bin/ghc --make -fbuilding-cabal-package -O -static -outputdir dist/build/cabal-relink-test/cabal-relink-test-tmp -odir dist/build/cabal-relink-test/cabal-relink-test-tmp -hidir dist/build /cabal-relink-test/cabal-relink-test-tmp -stubdir dist/build/cabal-relink- test/cabal-relink-test-tmp -i -idist/build/cabal-relink-test/cabal-relink- test-tmp -iapp -idist/build/autogen -Idist/build/autogen -Idist/build /cabal-relink-test/cabal-relink-test-tmp -optP-include -optPdist/build/autogen/cabal_macros.h -hide-all-packages -package-db dist/package.conf.inplace -package-id base-4.9.1.0 -package-id cabal- relink-test-0.1.0.0-9KTeoqstB5uArQE6VcRCGn -XHaskell2010 app/Main.hs -o dist/build/cabal-relink-test/cabal-relink-test -dynamic Linking dist/build/cabal-relink-test/cabal-relink-test ... }}} The following 2 GHC invocations are of interest: 1. Building the shared library `.so` file for the cabal `library`: `ghc -shared -dynamic '-dynload deploy' -optl- Wl,-rpath,/path/to/lib/ghc-8.0.2/base-4.9.1.0 -optl- Wl,-rpath,/path/to/lib/ghc-8.0.2/ghc-prim-0.5.0.0 -optl- Wl,-rpath,/path/to/lib/ghc-8.0.2/integer-gmp-1.0.0.1 -optl- Wl,-rpath,/path/to/gmp-6.1.1/lib -optl- Wl,-rpath,/path/to/lib/ghc-8.0.2/rts -hide-all-packages -no-auto-link- packages -package-db dist/package.conf.inplace -package-id base-4.9.1.0 dist/build/Mymodule.dyn_o -o dist/build/libHScabal-relink-test-0.1.0.0 -9KTeoqstB5uArQE6VcRCGn-ghc8.0.2.so` 2. Building the dynamically linked executable: `ghc --make -fbuilding-cabal-package -O -static -outputdir dist/build /cabal-relink-test/cabal-relink-test-tmp -odir dist/build/cabal-relink- test/cabal-relink-test-tmp -hidir dist/build/cabal-relink-test/cabal- relink-test-tmp -stubdir dist/build/cabal-relink-test/cabal-relink-test- tmp -i -idist/build/cabal-relink-test/cabal-relink-test-tmp -iapp -idist/build/autogen -Idist/build/autogen -Idist/build/cabal-relink-test /cabal-relink-test-tmp -optP-include -optPdist/build/autogen/cabal_macros.h -hide-all-packages -package-db dist/package.conf.inplace -package-id base-4.9.1.0 -package-id cabal- relink-test-0.1.0.0-9KTeoqstB5uArQE6VcRCGn -XHaskell2010 app/Main.hs -o dist/build/cabal-relink-test/cabal-relink-test -rtsopts -dynamic` The first invocation always touches (updates the mtime of) `dist/build/libHScabal-relink-test-0.1.0.0-9KTeoqstB5uArQE6VcRCGn- ghc8.0.2.so` and when the second one sees the mtime updated, it relinks. From a quick look at the code, I think the reason is this: In `linkingNeeded` in https://github.com/ghc/ghc/blob/ghc-8.0.2-release/compiler/main/DriverPipeli... {{{ let pkg_hslibs = [ (collectLibraryPaths dflags [c], lib) | Just c <- map (lookupPackage dflags) pkg_deps, lib <- packageHsLibs dflags c ] pkg_libfiles <- mapM (uncurry (findHSLib dflags)) pkg_hslibs if any isNothing pkg_libfiles then return True else do e_lib_times <- mapM (tryIO . getModificationUTCTime) (catMaybes pkg_libfiles) }}} I suspect that when linking statically, `pkg_libfiles` is empty, and when linking dynamically, it contains the above mentioned `.so` file; then, if the `.so` file was touched `linkingNeeded` returns `True`, and it relinks. Now the question is: Why does the mtime of the `.so` file change at all? It's simply because ghc unconditionally invokes `ld` (through `gcc`, or more precisely, whatever is configured as the linker program `pgm_l`) here in `linkDynLib`: https://github.com/ghc/ghc/blob/ghc-8.0.2-release/compiler/main/SysTools.hs#... {{{ runLink dflags ( map Option verbFlags ++ [ Option "-o" , FileOption "" output_fn , Option "-shared" ] ++ ... ... }}} And `ld` will always touch the `-o` output file. What's not clear to me is where the chain should have ended: * Should `linkDynLib` have not been called in the first place? * Or should `linkDynLib` only touch the file when the contents change (e.g. by writing it somewhere else, and doing a `mv` after a contents comparison; cabal does that in a couple places)? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13828#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler