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

Commits:

f1cb08be

by Hannes Siebenhandl at 2026-10-05T22:05:03-04:00

Prepend in-memory unit database for backpack

The packageDbFlags are stored in reverse order. Thus, prepending will
ensure that the in-memory unit database is the last item on the unit
database stack.

Further, the in-memory unit database needs to be able to shadow units
from earlier unit databases in the unit database stack.

Moreover, a `-clear-package-db` would remove the in-memory database if
it weren't the last item on the stack.

Closes #27835

5 changed files:

changelog.d/unit-index
compiler/GHC/Driver/Backpack.hs
+ testsuite/tests/backpack/should_compile/T27835.bkp
+ testsuite/tests/backpack/should_compile/T27835.stderr
testsuite/tests/backpack/should_compile/all.T

Changes:

changelog.d/unit-index

  section: compiler
  synopsis: Use global ``UnitIndex`` to deduplicate ``UnitInfo``s over multiple home units
 -issues: #27500 #26423 #27748 #27834
 -mrs: !16115 !16598 !16743
 +issues: #27500 #26423 #27748 #27834 #27835
 +mrs: !16115 !16598 !16743 !16748
  description: {
      The ``UnitState`` used to be duplicated for all ``HomeUnitEnv``, not sharing any of the ``UnitInfo``s.

compiler/GHC/Driver/Backpack.hs

@@ -441,9 +441,11 @@ addInMemoryDatabase dflags u = do
+           }
      let uic = hscUIC hsc_env
      liftIO $ cacheExternalUnitDatabase uic newdb
 -    -- added at the end because ordering matters
 +    -- Ordering matters here! We prepend the new unit database, because 'packageDBFlags'
 +    -- works like a stack, where what's on top takes precedence over what's below (same as right-to-left when given at the CLI).
 +    -- The new db needs highest precedence to overwrite any other in-memory units that potentially occur in the dbs below it.
      pure dflags
 -          { packageDBFlags = packageDBFlags dflags ++ [PackageDB (PkgDbPath (unitDatabasePath newdb))]
 +          { packageDBFlags = PackageDB (PkgDbPath (unitDatabasePath newdb)) : packageDBFlags dflags
+           }
  -- | Register a new virtual unit database containing a single unit

testsuite/tests/backpack/should_compile/T27835.bkp

 +unit p where
 +  module P where
 +    p :: Int
 +    p = 42
++
 +unit q where
 +  dependency p
 +  module Q where
 +    import P
 +    q :: Int
 +    q = p + 1

testsuite/tests/backpack/should_compile/T27835.stderr

 +[1 of 2] Processing p
 +  Instantiating p
 +  [1 of 1] Compiling P                ( p/P.hs, T27835.out/p/P.o )
 +[2 of 2] Processing q
 +  Instantiating q
 +  [1 of 1] Including p
 +  [1 of 1] Compiling Q                ( q/Q.hs, T27835.out/q/Q.o )

testsuite/tests/backpack/should_compile/all.T

@@ -61,3 +61,4 @@ test('T13250', normal, backpack_compile, [''])
  test('T13323', normal, backpack_compile, [''])
  test('T20396', normal, backpack_compile, [''])
  test('T23424', [ignore_stdout, ignore_stderr], backpack_compile, ['-ddump-rn-trace -ddump-if-trace -ddump-tc-trace'])
 +test('T27835', normal, backpack_compile, ['-clear-package-db -global-package-db'])