Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC Commits: 7250fc0c by Matthew Pickering at 2025-04-22T16:24:04-04:00 Move -fno-code note into Downsweep module This note was left behind when all the code which referred to it was moved into the GHC.Driver.Downsweep module - - - - - d2dc89b4 by Matthew Pickering at 2025-04-22T16:24:04-04:00 Apply editing notes to Note [-fno-code mode] suggested by sheaf These notes were suggested in https://gitlab.haskell.org/ghc/ghc/-/merge_requests/14241 - - - - - 2 changed files: - compiler/GHC/Driver/Downsweep.hs - compiler/GHC/Driver/Make.hs Changes: ===================================== compiler/GHC/Driver/Downsweep.hs ===================================== @@ -947,6 +947,71 @@ enableCodeGenWhen logger tmpfs staticLife dynLife unit_env mod_graph = do hostFullWays in dflags_c +{- Note [-fno-code mode] +~~~~~~~~~~~~~~~~~~~~~~~~ +GHC offers the flag -fno-code for the purpose of parsing and typechecking a +program without generating object files. This is intended to be used by tooling +and IDEs to provide quick feedback on any parser or type errors as cheaply as +possible. + +When GHC is invoked with -fno-code, no object files or linked output will be +generated. As many errors and warnings as possible will be generated, as if +-fno-code had not been passed. The session DynFlags will have +backend == NoBackend. + +-fwrite-interface +~~~~~~~~~~~~~~~~ +Whether interface files are generated in -fno-code mode is controlled by the +-fwrite-interface flag. The -fwrite-interface flag is a no-op if -fno-code is +not also passed. Recompilation avoidance requires interface files, so passing +-fno-code without -fwrite-interface should be avoided. If -fno-code were +re-implemented today, there would be no need for -fwrite-interface as it +would considered always on; this behaviour is as it is for backwards compatibility. + +================================================================ +IN SUMMARY: ALWAYS PASS -fno-code AND -fwrite-interface TOGETHER +================================================================ + +Template Haskell +~~~~~~~~~~~~~~~~ +A module using Template Haskell may invoke an imported function from inside a +splice. This will cause the type-checker to attempt to execute that code, which +would fail if no object files had been generated. See #8025. To rectify this, +during the downsweep we patch the DynFlags in the ModSummary of any home module +that is imported by a module that uses Template Haskell to generate object +code. + +The flavour of the generated code depends on whether `-fprefer-byte-code` is enabled +or not in the module which needs the code generation. If the module requires byte-code then +dependencies will generate byte-code, otherwise they will generate object files. +In the case where some modules require byte-code and some object files, both are +generated by enabling `-fbyte-code-and-object-code`, the test "fat015" tests these +configurations. + +The object files (and interface files if -fwrite-interface is disabled) produced +for Template Haskell are written to temporary files. + +Note that since Template Haskell can run arbitrary IO actions, -fno-code mode +is no more secure than running without it. + +Potential TODOS: +~~~~~ +* Remove -fwrite-interface and have interface files always written in -fno-code + mode +* Both .o and .dyn_o files are generated for template haskell, but we only need + .dyn_o. Fix it. +* In make mode, a message like + Compiling A (A.hs, /tmp/ghc_123.o) + is shown if downsweep enabled object code generation for A. Perhaps we should + show "nothing" or "temporary object file" instead. Note that one + can currently use -keep-tmp-files and inspect the generated file with the + current behaviour. +* Offer a -no-codedir command line option, and write what were temporary + object files there. This would speed up recompilation. +* Use existing object files (if they are up to date) instead of always + generating temporary ones. +-} + -- | Populate the Downsweep cache with the root modules. mkRootMap :: [ModuleNodeInfo] ===================================== compiler/GHC/Driver/Make.hs ===================================== @@ -1246,70 +1246,6 @@ addSptEntries hsc_env mlinkable = , spt <- bc_spt_entries bco ] -{- Note [-fno-code mode] -~~~~~~~~~~~~~~~~~~~~~~~~ -GHC offers the flag -fno-code for the purpose of parsing and typechecking a -program without generating object files. This is intended to be used by tooling -and IDEs to provide quick feedback on any parser or type errors as cheaply as -possible. - -When GHC is invoked with -fno-code no object files or linked output will be -generated. As many errors and warnings as possible will be generated, as if --fno-code had not been passed. The session DynFlags will have -backend == NoBackend. - --fwrite-interface -~~~~~~~~~~~~~~~~ -Whether interface files are generated in -fno-code mode is controlled by the --fwrite-interface flag. The -fwrite-interface flag is a no-op if -fno-code is -not also passed. Recompilation avoidance requires interface files, so passing --fno-code without -fwrite-interface should be avoided. If -fno-code were -re-implemented today, -fwrite-interface would be discarded and it would be -considered always on; this behaviour is as it is for backwards compatibility. - -================================================================ -IN SUMMARY: ALWAYS PASS -fno-code AND -fwrite-interface TOGETHER -================================================================ - -Template Haskell -~~~~~~~~~~~~~~~~ -A module using template haskell may invoke an imported function from inside a -splice. This will cause the type-checker to attempt to execute that code, which -would fail if no object files had been generated. See #8025. To rectify this, -during the downsweep we patch the DynFlags in the ModSummary of any home module -that is imported by a module that uses template haskell, to generate object -code. - -The flavour of the generated code depends on whether `-fprefer-byte-code` is enabled -or not in the module which needs the code generation. If the module requires byte-code then -dependencies will generate byte-code, otherwise they will generate object files. -In the case where some modules require byte-code and some object files, both are -generated by enabling `-fbyte-code-and-object-code`, the test "fat015" tests these -configurations. - -The object files (and interface files if -fwrite-interface is disabled) produced -for template haskell are written to temporary files. - -Note that since template haskell can run arbitrary IO actions, -fno-code mode -is no more secure than running without it. - -Potential TODOS: -~~~~~ -* Remove -fwrite-interface and have interface files always written in -fno-code - mode -* Both .o and .dyn_o files are generated for template haskell, but we only need - .dyn_o. Fix it. -* In make mode, a message like - Compiling A (A.hs, /tmp/ghc_123.o) - is shown if downsweep enabled object code generation for A. Perhaps we should - show "nothing" or "temporary object file" instead. Note that one - can currently use -keep-tmp-files and inspect the generated file with the - current behaviour. -* Offer a -no-codedir command line option, and write what were temporary - object files there. This would speed up recompilation. -* Use existing object files (if they are up to date) instead of always - generating temporary ones. --} -- Note [When source is considered modified] -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/2e2042693ace5f7e857315b40750f02... -- View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/2e2042693ace5f7e857315b40750f02... You're receiving this email because of your account on gitlab.haskell.org.