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This introduces a few events to mark key points in the nonmoving
garbage collection cycle. These include:

 * `EVENT_CONC_MARK_BEGIN`, denoting the beginning of a round of
   marking. This may happen more than once in a single major collection
   since we the major collector iterates until it hits a fixed point.

 * `EVENT_CONC_MARK_END`, denoting the end of a round of marking.

 * `EVENT_CONC_SYNC_BEGIN`, denoting the beginning of the post-mark
   synchronization phase

 * `EVENT_CONC_UPD_REM_SET_FLUSH`, indicating that a capability has
   flushed its update remembered set.

 * `EVENT_CONC_SYNC_END`, denoting that all mutators have flushed their
   update remembered sets.

 * `EVENT_CONC_SWEEP_BEGIN`, denoting the beginning of the sweep portion
   of the major collection.

 * `EVENT_CONC_SWEEP_END`, denoting the end of the sweep portion of the
   major collection.

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
   barrier.

 * 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 <omer@well-typed.com>

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
   details)

 * 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 <ben@well-typed.com>

This flag will enable the use of a non-moving oldest generation.

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.

This were previously quite unclear and will change a bit under the
non-moving collector so let's clear this up now.

Namely ensure that block descriptors are initialized with valid
generation numbers.
Co-Authored-By: Ben Gamari <ben@well-typed.com>

Zeros heap memory after gc freed it.

This commit starts renaming some flip bit related functions for the
generalised heap traversal code and adds provitions for sharing the
per-closure profiling header field currently used exclusively for retainer
profiling with other heap traversal profiling modes.

The `defined(DEBUG_RETAINER) == true` branch doesn't even compile anymore
because 1) retainerSet was renamed to RetainerSet and 2) even if I fix that
the context in Rts.h seems to have changed such that it's not in scope. If
3) I fix that 'flip' is still not in scope :)  At that point I just gave up.

This updates the documentation of the MIN_PAYLOAD_SIZE constant and adds
a new Note [Mark bits in mark-compact collector] explaning why the
mark-compact collector uses two bits per objet and why we need
MIN_PAYLOAD_SIZE.

This patch adds a new eventlog event which indicates the start of
a biographical profiler sample. These are different to normal events as
they also include the timestamp of when the census took place. This is
because the LDV profiler only emits samples at the end of the run.

Now all the different profiling modes emit consumable events to the
eventlog.

Add StgToCmm module hierarchy. Platform modules that are used in several
other places (NCG, LLVM codegen, Cmm transformations) are put into
GHC.Platform.

Effects as I measured them:

RTS Size: +0.1%
Compile times: -0.5%
Runtine nofib: -1.1%

Nofib runtime result seems to mostly come from the `CS` benchmark
which is very sensible to alignment changes so this is likely over
represented.

However the compile time changes are realistic.

This is related to #16961.

These prevent multi-target builds. They were gotten rid of in 3 ways:

1. In the compiler itself, replacing `#if` with runtime `if`. In these
cases, we care about the target platform still, but the target platform
is dynamic so we must delay the elimination to run time.

2. In the compiler itself, replacing `TARGET` with `HOST`. There was
just one bit of this, in some code splitting strings representing lists
of paths. These paths are used by GHC itself, and not by the compiled
binary. (They are compiler lookup paths, rather than RPATHS or something
that does matter to the compiled binary, and thus would legitamentally
be target-sensative.) As such, the path-splitting method only depends on
where GHC runs and not where code it produces runs. This should have
been `HOST` all along.

3. Changing the RTS. The RTS doesn't care about the target platform,
full stop.

4. `includes/stg/HaskellMachRegs.h` This file is also included in the
genapply executable. This is tricky because the RTS's host platform
really is that utility's target platform. so that utility really really
isn't multi-target either. But at least it isn't an installed part of
GHC, but just a one-off tool when building the RTS. Lying with the
`HOST` to a one-off program (genapply) that isn't installed doesn't seem so bad.
It's certainly better than the other way around of lying to the RTS
though not to genapply. The RTS is more important, and it is installed,
*and* this header is installed as part of the RTS.

This implements the correct fix for #11627 by skipping over the slop
(which is zeroed) rather than adding special case logic for LARGE
ARR_WORDS which runs the risk of not performing a correct census by
ignoring any subsequent blocks.

This approach implements similar logic to that in Sanity.c

As discussed in #16744, both the Make and Hadrian build systems
have special code to always pass -eventlog whenever -prof or -debug
are passed. However, there is some similar logic in the RTS itself only
for defining TRACING when the DEBUG macro is defined, but no such logic
is implemented to define TRACING when the PROFILING macro is defined.
This patch adds such a logic and therefore fixes #16744.

This allows a user to observe how long a sampling period lasts so that
the time taken can be removed from the profiling output.

Fixes #16697

This was a bit unclear as we use both one-based and zero-based
tags in GHC.

[skip ci]

- Remove redundant casting in evacuate_static_object
- Remove redundant parens in STATIC_LINK
- Fix a typo in GC.c

This moves all URL references to Trac Wiki to their corresponding
GitLab counterparts.

This substitution is classified as follows:

1. Automated substitution using sed with Ben's mapping rule [1]
    Old: ghc.haskell.org/trac/ghc/wiki/XxxYyy...
    New: gitlab.haskell.org/ghc/ghc/wikis/xxx-yyy...

2. Manual substitution for URLs containing `#` index
    Old: ghc.haskell.org/trac/ghc/wiki/XxxYyy...#Zzz
    New: gitlab.haskell.org/ghc/ghc/wikis/xxx-yyy...#zzz

3. Manual substitution for strings starting with `Commentary`
    Old: Commentary/XxxYyy...
    New: commentary/xxx-yyy...

See also !539

[1]: https://gitlab.haskell.org/bgamari/gitlab-migration/blob/master/wiki-mapping.json

Summary:
This re-applies {D5195} with fixes for i386:
* Fix unused label warnings, see {D5230} or {D5273}
* Fix a silly bug introduced by moving `#if`

{P190}

Add a RTS option -xp to load PIC object anywhere in address space. We do
this by relaxing the requirement of <0x80000000 result of
`mmapForLinker` and implying USE_CONTIGUOUS_MMAP.

We also need to change calls to `ocInit` and `ocGetNames` to avoid
dangling pointers when the address of `oc->image` is changed by
`ocAllocateSymbolExtra`.

Test Plan:
See {D5195}, also test under i386:

```
$ uname -a
Linux watashi-arch32 4.18.5-arch1-1.0-ARCH #1 SMP PREEMPT Tue Aug 28
20:45:30 CEST 2018 i686 GNU/Linux
$ cd testsuite/tests/th/ && make test
...
```

will run `./validate` on stacked diff.

Reviewers: simonmar, bgamari, alpmestan, trommler, hvr, erikd

Reviewed By: simonmar

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5289

This reverts commit 76c8fd67.

- Remove REGISTER_CC and REGISTER_CCS macros, add functions registerCC
  and registerCCS to Profiling.c.

- Reduce scope of symbols: CC_LIST, CCS_LIST, CC_ID, CCS_ID

- Document CC_LIST and CCS_LIST

`EventLogWriter.h` doesn't use anything from `Rts.h`, the include is
redundant. This include is ignored when we include

```
Rts.h -> RtsAPI.h -> rts/EventLogWriter.h -> Rts.h
```

but can can cause problem when we include `RtsApi.h` directly with
errors like

```
In file included from /usr/lib/ghc-8.6.2/include/RtsAPI.h:20:
In file included from
/usr/lib/ghc-8.6.2/include/rts/EventLogWriter.h:14:
In file included from /usr/lib/ghc-8.6.2/include/Rts.h:185:
/usr/lib/ghc-8.6.2/include/rts/storage/GC.h:187:29: error: unknown type
name 'Capability'
StgPtr  allocate          ( Capability *cap, W_ n );
```

Test Plan: ./validate

Reviewers: simonmar, bgamari, afarmer, erikd, alexbiehl

Reviewed By: bgamari, alexbiehl

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5395