Marge Bot pushed to branch master at Glasgow Haskell Compiler / GHC

Commits:

• e8d1a0d6
  by Bernhard M. Wiedemann at 2026-08-10T21:31:23-04:00

  driver: Link object files in a deterministic order
  
  The object files handed to the linker come from the HomePackageTable,
  which is ordered by the order in which modules finished compiling.  With
  -j1 that is the build plan order, with -jN it is whatever the scheduler
  produced, so the same sources can link to different (but equivalent)
  binaries.
  
  The order reaches the output: .text and .rodata contributions are
  concatenated in link order, so e.g. building the hdav executable of the
  DAV package twice, once with -j1 and once with -j4, yields two binaries
  that differ in ~100kB of section contents.
  
  Sort the home modules by module before collecting their linkables,
  guarded under `Opt_ObjectDeterminism` .
  
  Fixes #27612
  
  Signed-off-by: Bernhard M. Wiedemann <bwiedemann@suse.de>
  

2 changed files:

• + changelog.d/link-deterministic-order
• compiler/GHC/Driver/Pipeline.hs

Changes:

+section: compiler

+synopsis: -fobject-determinism sorts the list of object files to be linked for reproducible builds

+issues: #27612

+mrs: !16457

+

         -- know which packages are actually needed at the runtime stage.

         pkg_deps <- map snd . Set.toList <$> hptCollectDependencies hpt

-        -- the linkables to link

-        home_mods <- hptCollectHomeModInfo hpt

+        -- the linkables to link; under -fobject-determinism sorted, because

+        -- the HPT holds modules in the order in which they finished compiling,

+        -- which varies with -j

+        let det_sort

+              | gopt Opt_ObjectDeterminism dflags

+                          = sortWith (mi_module . hm_iface)

+              | otherwise = id

+        home_mods <- det_sort <$> hptCollectHomeModInfo hpt

         let home_modules = map (mi_module . hm_iface) home_mods

         debugTraceMsg logger 3 (text "link: hmi ..." $$ vcat (map ppr home_modules))