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  1. 19 Oct, 2019 7 commits
  2. 18 Oct, 2019 21 commits
    • Ben Gamari's avatar
      Don't cleanup until we've stopped the collector · 47d6b1a2
      Ben Gamari authored
      This requires that we break nonmovingExit into two pieces since we need
      to first stop the collector to relinquish any capabilities, then we need
      to shutdown the scheduler, then we need to free the nonmoving
    • Ben Gamari's avatar
    • Ben Gamari's avatar
    • Ben Gamari's avatar
      rts: Implement concurrent collection in the nonmoving collector · d7017446
      Ben Gamari authored
      This extends the non-moving collector to allow concurrent collection.
      The full design of the collector implemented here is described in detail
      in a technical note
          B. Gamari. "A Concurrent Garbage Collector For the Glasgow Haskell
          Compiler" (2018)
      This extension involves the introduction of a capability-local
      remembered set, known as the /update remembered set/, which tracks
      objects which may no longer be visible to the collector due to mutation.
      To maintain this remembered set we introduce a write barrier on
      mutations which is enabled while a concurrent mark is underway.
      The update remembered set representation is similar to that of the
      nonmoving mark queue, being a chunked array of `MarkEntry`s. Each
      `Capability` maintains a single accumulator chunk, which it flushed
      when it (a) is filled, or (b) when the nonmoving collector enters its
      post-mark synchronization phase.
      While the write barrier touches a significant amount of code it is
      conceptually straightforward: the mutator must ensure that the referee
      of any pointer it overwrites is added to the update remembered set.
      However, there are a few details:
       * In the case of objects with a dirty flag (e.g. `MVar`s) we can
         exploit the fact that only the *first* mutation requires a write
       * Weak references, as usual, complicate things. In particular, we must
         ensure that the referee of a weak object is marked if dereferenced by
         the mutator. For this we (unfortunately) must introduce a read
         barrier, as described in Note [Concurrent read barrier on deRefWeak#]
         (in `NonMovingMark.c`).
       * Stable names are also a bit tricky as described in Note [Sweeping
         stable names in the concurrent collector] (`NonMovingSweep.c`).
      We take quite some pains to ensure that the high thread count often seen
      in parallel Haskell applications doesn't affect pause times. To this end
      we allow thread stacks to be marked either by the thread itself (when it
      is executed or stack-underflows) or the concurrent mark thread (if the
      thread owning the stack is never scheduled). There is a non-trivial
      handshake to ensure that this happens without racing which is described
      in Note [StgStack dirtiness flags and concurrent marking].
      Co-Authored-by: Ömer Sinan Ağacan's avatarÖmer Sinan Ağacan <>
    • Ben Gamari's avatar
      rts: Mark binder as const · 26ffa5a2
      Ben Gamari authored
    • Ben Gamari's avatar
      testsuite: Add nonmoving WAY · 49dc09db
      Ben Gamari authored
      This simply runs the compile_and_run tests with `-xn`, enabling the
      nonmoving oldest generation.
    • Ömer Sinan Ağacan's avatar
      rts: Non-concurrent mark and sweep · 2309789a
      Ömer Sinan Ağacan authored
      This implements the core heap structure and a serial mark/sweep
      collector which can be used to manage the oldest-generation heap.
      This is the first step towards a concurrent mark-and-sweep collector
      aimed at low-latency applications.
      The full design of the collector implemented here is described in detail
      in a technical note
          B. Gamari. "A Concurrent Garbage Collector For the Glasgow Haskell
          Compiler" (2018)
      The basic heap structure used in this design is heavily inspired by
          K. Ueno & A. Ohori. "A fully concurrent garbage collector for
          functional programs on multicore processors." /ACM SIGPLAN Notices/
          Vol. 51. No. 9 (presented by ICFP 2016)
      This design is intended to allow both marking and sweeping
      concurrent to execution of a multi-core mutator. Unlike the Ueno design,
      which requires no global synchronization pauses, the collector
      introduced here requires a stop-the-world pause at the beginning and end
      of the mark phase.
      To avoid heap fragmentation, the allocator consists of a number of
      fixed-size /sub-allocators/. Each of these sub-allocators allocators into
      its own set of /segments/, themselves allocated from the block
      allocator. Each segment is broken into a set of fixed-size allocation
      blocks (which back allocations) in addition to a bitmap (used to track
      the liveness of blocks) and some additional metadata (used also used
      to track liveness).
      This heap structure enables collection via mark-and-sweep, which can be
      performed concurrently via a snapshot-at-the-beginning scheme (although
      concurrent collection is not implemented in this patch).
      The mark queue is a fairly straightforward chunked-array structure.
      The representation is a bit more verbose than a typical mark queue to
      accomodate a combination of two features:
       * a mark FIFO, which improves the locality of marking, reducing one of
         the major overheads seen in mark/sweep allocators (see [1] for
       * the selector optimization and indirection shortcutting, which
         requires that we track where we found each reference to an object
         in case we need to update the reference at a later point (e.g. when
         we find that it is an indirection). See Note [Origin references in
         the nonmoving collector] (in `NonMovingMark.h`) for details.
      Beyond this the mark/sweep is fairly run-of-the-mill.
      [1] R. Garner, S.M. Blackburn, D. Frampton. "Effective Prefetch for
          Mark-Sweep Garbage Collection." ISMM 2007.
      Co-Authored-By: Ben Gamari's avatarBen Gamari <>
    • Ben Gamari's avatar
      rts: Introduce debug flag for non-moving GC · 19fa0e8f
      Ben Gamari authored
    • Ben Gamari's avatar
      rts: Introduce flag to enable the nonmoving old generation · 4a72fb4a
      Ben Gamari authored
      This flag will enable the use of a non-moving oldest generation.
    • Ömer Sinan Ağacan's avatar
      rts/Scav: Expose scavenging functions · a244d137
      Ömer Sinan Ağacan authored
      To keep the non-moving collector nicely separated from the moving
      collector its scavenging phase will live in another file,
      `NonMovingScav.c`. However, it will need to use these functions so
      let's expose them.
    • Ben Gamari's avatar
      rts: Disable aggregate-return warnings from gcc · ac650082
      Ben Gamari authored
      This warning is a bit of a relic; there is little reason to avoid
      aggregate return values in 2019.
    • Ömer Sinan Ağacan's avatar
      rts/StableName: Expose FOR_EACH_STABLE_NAME, freeSnEntry, SNT_size · 4b261381
      Ömer Sinan Ağacan authored
      These will be needed when we implement sweeping in the nonmoving
    • Ben Gamari's avatar
      Merge branches 'wip/gc/sync-without-capability' and... · 1db1f413
      Ben Gamari authored
      Merge branches 'wip/gc/sync-without-capability' and 'wip/gc/aligned-block-allocation' into wip/gc/preparation
    • Ömer Sinan Ağacan's avatar
      rts/BlockAlloc: Allow aligned allocation requests · 921e4e36
      Ömer Sinan Ağacan authored
      This implements support for block group allocations which are aligned to
      an integral number of blocks.
      This will be used by the nonmoving garbage collector, which uses the
      block allocator to allocate the segments which back its heap. These
      segments are a fixed number of blocks in size, with each segment being
      aligned to the segment size boundary. This allows us to easily find the
      segment metadata stored at the beginning of the segment.
    • Ben Gamari's avatar
      rts/Schedule: Allow synchronization without holding a capability · bfcafd39
      Ben Gamari authored
      The concurrent mark-and-sweep will be performed by a GHC task which will
      not hold a capability. This is necessary to avoid a concurrent mark from
      interfering with minor generation collections.
      However, the major collector must synchronize with the mutators at the
      end of marking to flush their update remembered sets. This patch extends
      the `requestSync` mechanism used to synchronize garbage collectors to
      allow synchronization without holding a capability.
      This change is fairly straightforward as the capability was previously
      only required for two reasons:
       1. to ensure that we don't try to re-acquire a capability that we
          the sync requestor already holds.
       2. to provide a way to suspend and later resume the sync request if
          there is already a sync pending.
      When synchronizing without holding a capability we needn't worry about
      consideration (1) at all.
      (2) is slightly trickier and may happen, for instance, when a capability
      requests a minor collection and shortly thereafter the non-moving mark
      thread requests a post-mark synchronization. In this case we need to
      ensure that the non-moving mark thread suspends his request until after
      the minor GC has concluded to avoid dead-locking. For this we introduce
      a condition variable, `sync_finished_cond`, which a
      non-capability-bearing requestor will wait on and which is signalled
      after a synchronization or GC has finished.
    • Ben Gamari's avatar
      rts: Fix macro parenthesisation · 4d674c4e
      Ben Gamari authored
    • Ben Gamari's avatar
      rts/GC: Refactor gcCAFs · 81d4675e
      Ben Gamari authored
    • Ben Gamari's avatar
      rts: Give stack flags proper macros · 206f782a
      Ben Gamari authored
      This were previously quite unclear and will change a bit under the
      non-moving collector so let's clear this up now.
    • Ben Gamari's avatar
      rts/Capability: A few documentation comments · 11395037
      Ben Gamari authored
    • Ben Gamari's avatar
      rts: Add Note explaining applicability of selector optimisation depth limit · 61d2ed42
      Ben Gamari authored
      This was slightly non-obvious so a note seems deserved.
    • Ömer Sinan Ağacan's avatar
      rts/GC: Add an obvious assertion during block initialization · 697be2b6
      Ömer Sinan Ağacan authored
      Namely ensure that block descriptors are initialized with valid
      generation numbers.
      Co-Authored-By: Ben Gamari's avatarBen Gamari <>
  3. 17 Oct, 2019 3 commits
    • Ben Gamari's avatar
      testsuite: Assert that testsuite ways are known · c4c9904b
      Ben Gamari authored
      This ensures that all testsuite way names given to `omit_ways`,
      `only_ways`, etc. are known ways.
    • Richard Eisenberg's avatar
    • Ben Gamari's avatar
      testsuite: Ensure that makefile tests get run · b15a7fb8
      Ben Gamari authored
      Previously `makefile_test` and `run_command` tests could easily end up
      in a situation where they wouldn't be run if the user used the
      `only_ways` modifier. The reason is to build the set of a ways to run
      the test in we first start with a candidate set determined by the test
      type (e.g. `makefile_test`, `compile_run`, etc.) and then filter that
      set with the constraints given by the test's modifiers.
      `makefile_test` and `run_command` tests' candidate sets were simply
      `{normal}`, and consequently most uses of `only_ways` would result in
      the test being never run.
      To avoid this we rather use all ways as the candidate sets for these
      test types. This may result in a few more testcases than we would like
      (given that some `run_command` tests are insensitive to way) but this
      can be fixed by adding modifiers and we would much rather run too many
      tests than too few.
      This fixes #16042 and a number of other tests afflicted by the same issue.
      However, there were a few cases that required special attention:
       * `T14028` is currently failing and is therefore marked as broken due
         to #17300
       * `T-signals-child` is fragile in the `threaded1` and `threaded2` ways
         (tracked in #17307)
  4. 16 Oct, 2019 9 commits
    • Brian Wignall's avatar
    • Ryan Scott's avatar
      Make Coverage.TM a newtype · deb96399
      Ryan Scott authored
    • Ben Gamari's avatar
      hadrian: Introduce enableDebugInfo flavour transformer · 11d4fc50
      Ben Gamari authored
      Also refactor things a bit to eliminate repetition.
    • Richard Eisenberg's avatar
      Break up TcRnTypes, among other modules. · 51fad9e6
      Richard Eisenberg authored
      This introduces three new modules:
       - basicTypes/Predicate.hs describes predicates, moving
         this logic out of Type. Predicates don't really exist
         in Core, and so don't belong in Type.
       - typecheck/TcOrigin.hs describes the origin of constraints
         and types. It was easy to remove from other modules and
         can often be imported instead of other, scarier modules.
       - typecheck/Constraint.hs describes constraints as used in
         the solver. It is taken from TcRnTypes.
      No work other than module splitting is in this patch.
      This is the first step toward homogeneous equality, which will
      rely more strongly on predicates. And homogeneous equality is the
      next step toward a dependently typed core language.
    • John Ericson's avatar
      Delete ghctags cabal file · 798037a1
      John Ericson authored
      It came back to life in 381c3ae3 by
    • Sebastian Graf's avatar
      Infer rho-types instead of sigma-types in guard BindStmts and TransStmts · 6ede3554
      Sebastian Graf authored
      In #17343 we saw that we didn't handle the pattern guard `!_ <-
      undefined` correctly: The `undefined` was never evaluated. Indeed,
      elaboration failed to insert the invisible type aruments to `undefined`.
      So `undefined` was trivially a normal-form and in turn never entered.
      The problem is that we used to infer a sigma-type for the RHS of the
      guard, the leading qualifiers of which will never be useful in a pattern
      match situation. Hence we infer a rho-type now.
      Fixes #17343.
    • Takenobu Tani's avatar
      testsuite: Add test for #8305 · 19641957
      Takenobu Tani authored
      This is a test for the current algorithm of GHCi command name resolution.
      I add this test in preparation for updating GHCi command name resolution.
      For the current algorithm, see
    • Adithya Kumar's avatar
    • Andreas Klebinger's avatar
      Add loop level analysis to the NCG backend. · 535a88e1
      Andreas Klebinger authored
      For backends maintaining the CFG during codegen
      we can now find loops and their nesting level.
      This is based on the Cmm CFG and dominator analysis.
      As a result we can estimate edge frequencies a lot better
      for methods, resulting in far better code layout.
      Speedup on nofib: ~1.5%
      Increase in compile times: ~1.9%
      To make this feasible this commit adds:
      * Dominator analysis based on the Lengauer-Tarjan Algorithm.
      * An algorithm estimating global edge frequences from branch
      probabilities - In CFG.hs
      A few static branch prediction heuristics:
      * Expect to take the backedge in loops.
      * Expect to take the branch NOT exiting a loop.
      * Expect integer vs constant comparisons to be false.
      We also treat heap/stack checks special for branch prediction
      to avoid them being treated as loops.