[Git][ghc/ghc][wip/fendor/ifacetype-deduplication] 5 commits: Add Eq and Ord instance to `IfaceType`

Wed Apr 17 13:04:36 UTC 2024

Hannes Siebenhandl pushed to branch wip/fendor/ifacetype-deduplication at Glasgow Haskell Compiler / GHC

Commits:
8dbbdbbc by Fendor at 2024-04-17T09:31:43+02:00
Add Eq and Ord instance to `IfaceType`

We add an `Ord` instance so that we can store `IfaceType` in a
`Data.Map` container.
This is required to deduplicate `IfaceType` while writing `.hi` files to
disk. Deduplication has many beneficial consequences to both file size
and memory usage, as the deduplication enables implicit sharing of
values.
See issue #24540 for more motivation.

The `Ord` instance would be unnecessary if we used a `TrieMap` instead
of `Data.Map` for the deduplication process. While in theory this is
clerarly the better option, experiments on the agda code base showed
that a `TrieMap` implementation has worse run-time performance
characteristics.

To the change itself, we mostly derive `Eq` and `Ord`. This requires us
to change occurrences of `FastString` with `LexicalFastString`, since
`FastString` has no `Ord` instance.
We change the definition of `IfLclName` to a newtype of
`LexicalFastString`, to make such changes in the future easier.

Bump haddock submodule for IfLclName changes

- - - - -
e927c278 by Fendor at 2024-04-17T10:09:52+02:00
Add deduplication table for `IfaceType`

The type `IfaceType` is a highly redundant, tree-like data structure.
While benchmarking, we realised that the high redundancy of `IfaceType`
causes high memory consumption in GHCi sessions when byte code is
embedded into the `.hi` file via `-fwrite-if-simplified-core` or
`-fbyte-code-and-object-code`.
Loading such `.hi` files from disk introduces many duplicates of
memory expensive values in `IfaceType`, such as `IfaceTyCon`,
`IfaceTyConApp`, `IA_Arg` and many more.

We improve the memory behaviour of GHCi by adding an additional
deduplication table for `IfaceType` to the serialisation of `ModIface`,
similar to how we deduplicate `Name`s and `FastString`s.
When reading the interface file back, the table allows us to automatically
share identical values of `IfaceType`.

To provide some numbers, we evaluated this patch on the agda code base.
We loaded the full library from the `.hi` files, which contained the
embedded core expressions (`-fwrite-if-simplified-core`).

Before this patch:

* Load time: 11.7 s, 2.5 GB maximum residency.

After this patch:

* Load time:  7.3 s, 1.7 GB maximum residency.

This deduplication has the beneficial side effect to additionally reduce
the size of the on-disk interface files tremendously.

For example, on agda, we reduce the size of `.hi` files (with
`-fwrite-if-simplified-core`):

* Before: 101 MB on disk
* Now:     24 MB on disk

This has even a beneficial side effect on the cabal store. We reduce the
size of the store on disk:

* Before: 341 MB on disk
* Now:    310 MB on disk

Note, none of the dependencies have been compiled with
`-fwrite-if-simplified-core`, but `IfaceType` occurs in multiple
locations in a `ModIface`.

We also add IfaceType deduplication table to .hie serialisation and
refactor .hie file serialisation to use the same infrastrucutre as
`putWithTables`.

- - - - -
96c88b75 by Matthew Pickering at 2024-04-17T10:30:02+02:00
Add run-time configurability of .hi file compression

Introduce the flag `-fwrite-if-compression=<n>` which allows to
configure the compression level of writing .hi files.

The motivation is that some deduplication operations are too expensive
for the average use case. Hence, we introduce multiple compression
levels that have a minimal impact on performance, but still reduce the
memory residency and `.hi` file size on disk considerably.

We introduce three compression levels:

* `1`: `Normal` mode. This is the least amount of compression.
    It deduplicates only `Name` and `FastString`s, and is naturally the
    fastest compression mode.
* `2`: `Safe` mode. It has a noticeable impact on .hi file size and is
  marginally slower than `Normal` mode. In general, it should be safe to
  always use `Safe` mode.
* `3`: `Full` deduplication mode. Deduplicate as much as we can,
  resulting in minimal .hi files, but at the cost of additional
  compilation time.

Reading .hi files doesn't need to know the initial compression level,
and can always deserialise a `ModIface`.
This allows users to experiment with different compression levels for
packages, without recompilation of dependencies.

Note, the deduplication also has an additional side effect of reduced
memory consumption to implicit sharing of deduplicated elements.
See https://gitlab.haskell.org/ghc/ghc/-/issues/24540 for example where
that matters.

-------------------------
Metric Decrease:
    T21839c
    T24471
-------------------------

- - - - -
1c5e7698 by Matthew Pickering at 2024-04-17T10:34:55+02:00
Introduce regression tests for `.hi` file sizes

Add regression tests to track how `-fwrite-if-compression` levels affect
the size of `.hi` files.

- - - - -
890e5b5a by Fendor at 2024-04-17T10:39:58+02:00
Implement TrieMap for IfaceType

- - - - -

30 changed files:

- compiler/GHC/Core/Map/Expr.hs
- compiler/GHC/Core/TyCo/Rep.hs
- compiler/GHC/CoreToIface.hs
- compiler/GHC/Data/FastString.hs
- compiler/GHC/Data/TrieMap.hs
- compiler/GHC/Driver/DynFlags.hs
- compiler/GHC/Driver/Main.hs
- compiler/GHC/Driver/Session.hs
- compiler/GHC/Iface/Binary.hs
- compiler/GHC/Iface/Decl.hs
- compiler/GHC/Iface/Env.hs
- compiler/GHC/Iface/Ext/Binary.hs
- compiler/GHC/Iface/Ext/Utils.hs
- compiler/GHC/Iface/Load.hs
- compiler/GHC/Iface/Recomp.hs
- compiler/GHC/Iface/Recomp/Binary.hs
- compiler/GHC/Iface/Syntax.hs
- compiler/GHC/Iface/Type.hs
- + compiler/GHC/Iface/Type/Map.hs
- compiler/GHC/IfaceToCore.hs
- compiler/GHC/Stg/CSE.hs
- compiler/GHC/StgToJS/Object.hs
- compiler/GHC/Types/Basic.hs
- compiler/GHC/Types/Var.hs
- compiler/GHC/Utils/Binary.hs
- compiler/Language/Haskell/Syntax/Type.hs
- compiler/Language/Haskell/Syntax/Type.hs-boot
- compiler/ghc.cabal.in
- docs/users_guide/using-optimisation.rst
- + testsuite/tests/iface/IfaceSharingIfaceType.hs

The diff was not included because it is too large.

View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/481d625b1a5fa5cd1ba9d0f4487b4a128badfa20...890e5b5a7d58793a7232bbf9f2eb44aae5e7f74a

-- 
View it on GitLab: https://gitlab.haskell.org/ghc/ghc/-/compare/481d625b1a5fa5cd1ba9d0f4487b4a128badfa20...890e5b5a7d58793a7232bbf9f2eb44aae5e7f74a
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