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Matthew Pickering pushed to branch wip/sort-usages at Glasgow Haskell Compiler / GHC

Commits:

8201d1ba

by Ian-Woo Kim at 2026-02-04T17:13:54+00:00

determinism: Sort Usages by fingerprint to ensure consistent ordering

In some situations it has been observed that the ordering of usages can
be non-determinstic in parallel builds. Therefore to be on the safe side
we perform a sort on the usages field before writing them to the
interface.

Fixes #26877

3 changed files:

compiler/GHC/HsToCore/Usage.hs
compiler/GHC/Unit/Module/Deps.hs
testsuite/tests/driver/recomp016/recomp016.stdout

Changes:

compiler/GHC/HsToCore/Usage.hs

@@ -36,7 +36,7 @@ import GHC.Data.Maybe
  import GHC.Data.FastString
  import Data.IORef
 -import Data.List (sortBy)
 +import Data.List (sortBy, sortOn)
  import Data.Map (Map)
  import qualified Data.Map as Map
  import qualified Data.Set as Set
@@ -73,6 +73,8 @@ data UsageConfig = UsageConfig
    { uc_safe_implicit_imps_req :: !Bool -- ^ Are all implicit imports required to be safe for this Safe Haskell mode?
+   }
 +-- | Build the list of 'Usage's that drives recompilation checking.
 +-- The resulting list is deterministically sorted (see 'usageFingerprint').
  mkUsageInfo :: UsageConfig -> Plugins -> FinderCache -> UnitEnv
              -> Module -> ImportedMods -> [ImportUserSpec] -> NameSet
              -> [FilePath] -> [FilePath] -> [(Module, Fingerprint)] -> [Linkable] -> PkgsLoaded
@@ -105,7 +107,10 @@ mkUsageInfo uc plugins fc unit_env
+                                     }
                                 | (mod, hash) <- merged ]
                              ++ object_usages
 -    usages `seqList` return usages
++
 +    -- Sort all the Usages to ensure a deterministic ordering.
 +    let sorted_usages = sortOn usageFingerprint usages
 +    sorted_usages `seqList` return sorted_usages
      -- seq the list of Usages returned: occasionally these
      -- don't get evaluated for a while and we can end up hanging on to
      -- the entire collection of Ifaces.

compiler/GHC/Unit/Module/Deps.hs

@@ -17,6 +17,7 @@ module GHC.Unit.Module.Deps
     , noDependencies
     , pprDeps
     , Usage (..)
 +   , usageFingerprint
     , HomeModImport (..)
     , HomeModImportedAvails (..)
     , ImportAvails (..)
@@ -492,6 +493,17 @@ instance Binary Usage where
            i -> error ("Binary.get(Usage): " ++ show i)
 +-- | Extract the distinguishing fingerprint carried by a particular 'Usage'
 +-- constructor.  Every constructor stores a hash capturing the bit of state
 +-- that drives recompilation decisions, so we can sort on it directly.
 +usageFingerprint :: Usage -> Fingerprint
 +usageFingerprint UsagePackageModule{ usg_mod_hash = fp } = fp
 +usageFingerprint UsageHomeModule{ usg_mod_hash = fp } = fp
 +usageFingerprint UsageFile{ usg_file_hash = fp } = fp
 +usageFingerprint UsageDirectory{ usg_dir_hash = fp } = fp
 +usageFingerprint UsageHomeModuleInterface{ usg_iface_hash = fp } = fp
 +usageFingerprint UsageMergedRequirement{ usg_mod_hash = fp } = fp
++
  -- | Records the imports that we depend on from a home module,
  -- for recompilation checking.
  --

testsuite/tests/driver/recomp016/recomp016.stdout

@@ -9,4 +9,4 @@ second run
  [2 of 5] Compiling B                ( B.hs, B.o ) [Source file changed]
  [3 of 5] Compiling C                ( C.hs, C.o ) [B changed]
  [4 of 5] Compiling D                ( D.hs, D.o ) [C changed]
 -[5 of 5] Compiling E                ( E.hs, E.o ) [B changed]
 +[5 of 5] Compiling E                ( E.hs, E.o ) [D changed]