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<h3>
Sebastian Graf pushed to branch wip/refactor-demand
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
</h3>
<h4>
Commits:
</h4>
<ul>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/d445cf05d47e8c5513c55cb5e7157b33e83c8123">d445cf05</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2020-11-10T10:26:20-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">rts/linker: Fix relocation overflow in PE linker
Previously the overflow check for the IMAGE_REL_AMD64_ADDR32NB
relocation failed to account for the signed nature of the value.
Specifically, the overflow check was:
uint64_t v;
v = S + A;
if (v >> 32) { ... }
However, `v` ultimately needs to fit into 32-bits as a signed value.
Consequently, values `v > 2^31` in fact overflow yet this is not caught
by the existing overflow check.
Here we rewrite the overflow check to rather ensure that
`INT32_MIN <= v <= INT32_MAX`. There is now quite a bit of repetition
between the `IMAGE_REL_AMD64_REL32` and `IMAGE_REL_AMD64_ADDR32` cases
but I am leaving fixing this for future work.
This bug was first noticed by @awson.
Fixes #15808.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/4c407f6e71f096835f8671e2d3ea6bda38074314">4c407f6e</a></strong>
<div>
<span>by Sylvain Henry</span>
<i>at 2020-11-10T10:27:00-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Export SPEC from GHC.Exts (#13681)
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/7814cd5bb0d145c4d83d7566885bdc3992b63d0c">7814cd5b</a></strong>
<div>
<span>by David Eichmann</span>
<i>at 2020-11-10T10:27:35-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">ghc-heap: expose decoding from heap representation
Co-authored-by: Sven Tennie <sven.tennie@gmail.com>
Co-authored-by: Matthew Pickering <matthewtpickering@gmail.com>
Co-authored-by: Ben Gamari <bgamari.foss@gmail.com>
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/fa344d33dba71f31f55269c5fc733daa3830073a">fa344d33</a></strong>
<div>
<span>by Richard Eisenberg</span>
<i>at 2020-11-10T10:28:10-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Add test case for #17186.
This got fixed sometime recently; not worth it trying to
figure out which commit.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/2e63a0fb1bdaecc7916a3cc35dcfd2b2ef37c328">2e63a0fb</a></strong>
<div>
<span>by David Eichmann</span>
<i>at 2020-11-10T10:28:46-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Add code comments for StgInfoTable and StgStack structs
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/fcfda909fd7fcf539ff31717ce01a56292abb92f">fcfda909</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2020-11-11T03:19:59-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">nativeGen: Make makeImportsDoc take an NCGConfig rather than DynFlags
It appears this was an oversight as there is no reason the full DynFlags
is necessary.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/6e23695e7d84aa248e7ca20bdb8d133f9b356548">6e23695e</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2020-11-11T03:19:59-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Move this_module into NCGConfig
In various places in the NCG we need the Module currently being
compiled. Let's move this into the environment instead of chewing threw
another register.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/c6264a2d652517954b7cd076c7bc4487ed17c97d">c6264a2d</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2020-11-11T03:20:00-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">codeGen: Produce local symbols for module-internal functions
It turns out that some important native debugging/profiling tools (e.g.
perf) rely only on symbol tables for function name resolution (as
opposed to using DWARF DIEs). However, previously GHC would emit
temporary symbols (e.g. `.La42b`) to identify module-internal
entities. Such symbols are dropped during linking and therefore not
visible to runtime tools (in addition to having rather un-helpful unique
names). For instance, `perf report` would often end up attributing all
cost to the libc `frame_dummy` symbol since Haskell code was no covered
by any proper symbol (see #17605).
We now rather follow the model of C compilers and emit
descriptively-named local symbols for module internal things. Since this
will increase object file size this behavior can be disabled with the
`-fno-expose-internal-symbols` flag.
With this `perf record` can finally be used against Haskell executables.
Even more, with `-g3` `perf annotate` provides inline source code.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/584058ddff71460023712a8d816b83b581e6e78f">584058dd</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2020-11-11T03:20:00-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Enable -fexpose-internal-symbols when debug level >=2
This seems like a reasonable default as the object file size increases
by around 5%.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/c34a4b98b1f09ea3096d39a839a86f2d7185c796">c34a4b98</a></strong>
<div>
<span>by Ömer Sinan Ağacan</span>
<i>at 2020-11-11T03:20:35-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Fix and enable object unloading in GHCi
Fixes #16525 by tracking dependencies between object file symbols and
marking symbol liveness during garbage collection
See Note [Object unloading] in CheckUnload.c for details.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/2782487f5f6ad9df4dc8725226a47f07fec77f9f">2782487f</a></strong>
<div>
<span>by Ray Shih</span>
<i>at 2020-11-11T03:20:35-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Add loadNativeObj and unloadNativeObj
(This change is originally written by niteria)
This adds two functions:
* `loadNativeObj`
* `unloadNativeObj`
and implements them for Linux.
They are useful if you want to load a shared object with Haskell code
using the system linker and have GHC call dlclose() after the
code is no longer referenced from the heap.
Using the system linker allows you to load the shared object
above outside the low-mem region. It also loads the DWARF sections
in a way that `perf` understands.
`dl_iterate_phdr` is what makes this implementation Linux specific.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/7a65f9e140906087273ce95f062775f18f6a708d">7a65f9e1</a></strong>
<div>
<span>by GHC GitLab CI</span>
<i>at 2020-11-11T03:20:35-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">rts: Introduce highMemDynamic
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/e9e1b2e75de17be47ab887a26943f5517a8463ac">e9e1b2e7</a></strong>
<div>
<span>by GHC GitLab CI</span>
<i>at 2020-11-11T03:20:35-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Introduce test for dynamic library unloading
This uses the highMemDynamic flag introduced earlier to verify that
dynamic objects are properly unloaded.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/5506f1342e51bad71a7525ddad0650d1ac63afeb">5506f134</a></strong>
<div>
<span>by Krzysztof Gogolewski</span>
<i>at 2020-11-11T03:21:14-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Force argument in setIdMult (#18925)
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/787e93ae141ae0f33bc36895494d48a2a5e49e08">787e93ae</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2020-11-11T23:14:11-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">testsuite: Add testcase for #18733
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/5353fd500b1e92636cd9d45274585fd88a915ff6">5353fd50</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2020-11-12T10:05:30-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">compiler: Fix recompilation checking
In ticket #18733 we noticed a rather serious deficiency in the current
fingerprinting logic for recursive groups. I have described the old
fingerprinting story and its problems in Note [Fingerprinting recursive
groups] and have reworked the story accordingly to avoid these issues.
Fixes #18733.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/63fa399726ff85a3ff4ca42a88f3d8a00921a718">63fa3997</a></strong>
<div>
<span>by Sebastian Graf</span>
<i>at 2020-11-13T14:29:39-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Arity: Rework `ArityType` to fix monotonicity (#18870)
As we found out in #18870, `andArityType` is not monotone, with
potentially severe consequences for termination of fixed-point
iteration. That showed in an abundance of "Exciting arity" DEBUG
messages that are emitted whenever we do more than one step in
fixed-point iteration.
The solution necessitates also recording `OneShotInfo` info for
`ABot` arity type. Thus we get the following definition for `ArityType`:
```
data ArityType = AT [OneShotInfo] Divergence
```
The majority of changes in this patch are the result of refactoring use
sites of `ArityType` to match the new definition.
The regression test `T18870` asserts that we indeed don't emit any DEBUG
output anymore for a function where we previously would have.
Similarly, there's a regression test `T18937` for #18937, which we
expect to be broken for now.
Fixes #18870.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/197d59facbe8f9799df47e86c99f401ced487040">197d59fa</a></strong>
<div>
<span>by Sebastian Graf</span>
<i>at 2020-11-13T14:29:39-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Arity: Emit "Exciting arity" warning only after second iteration (#18937)
See Note [Exciting arity] why we emit the warning at all and why we only
do after the second iteration now.
Fixes #18937.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/de7ec9dd2bd573d5950ae294747d2bdb45051000">de7ec9dd</a></strong>
<div>
<span>by David Eichmann</span>
<i>at 2020-11-13T14:30:16-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Add rts_listThreads and rts_listMiscRoots to RtsAPI.h
These are used to find the current roots of the garbage collector.
Co-authored-by: Sven Tennie's avatarSven Tennie <sven.tennie@gmail.com>
Co-authored-by: Matthew Pickering's avatarMatthew Pickering <matthewtpickering@gmail.com>
Co-authored-by: default avatarBen Gamari <bgamari.foss@gmail.com>
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/24a86f09da3426cf1006004bc45d312725280dd5">24a86f09</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2020-11-13T14:30:51-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">gitlab-ci: Cache cabal store in linting job
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/0a7e592cb1883824a14639372ba284766849ff3a">0a7e592c</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2020-11-15T03:35:45-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">nativeGen/dwarf: Fix procedure end addresses
Previously the `.debug_aranges` and `.debug_info` (DIE) DWARF
information would claim that procedures (represented with a
`DW_TAG_subprogram` DIE) would only span the range covered by their entry
block. This omitted all of the continuation blocks (represented by
`DW_TAG_lexical_block` DIEs), confusing `perf`. Fix this by introducing
a end-of-procedure label and using this as the `DW_AT_high_pc` of
procedure `DW_TAG_subprogram` DIEs
Fixes #17605.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/1e19183d08a3312ac2331b8284d17bc17170a51e">1e19183d</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2020-11-15T03:35:45-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">nativeGen/dwarf: Only produce DW_AT_source_note DIEs in -g3
Standard debugging tools don't know how to understand these so let's not
produce them unless asked.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/ad73370f6770480f1786aab8f2b7f5fe155152c8">ad73370f</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2020-11-15T03:35:45-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">nativeGen/dwarf: Use DW_AT_linkage instead of DW_AT_MIPS_linkage
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/a2539650cc9c6606c6b50dd5dd96caa0209b408c">a2539650</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2020-11-15T03:35:45-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">gitlab-ci: Add DWARF release jobs for Debian 10, Fedora27
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/d61adb3dace8f52e21f302989182145a0efa103f">d61adb3d</a></strong>
<div>
<span>by Ryan Scott</span>
<i>at 2020-11-15T03:36:21-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Name (tc)SplitForAll- functions more consistently
There is a zoo of `splitForAll-` functions in `GHC.Core.Type` (as well as
`tcSplitForAll-` functions in `GHC.Tc.Utils.TcType`) that all do very similar
things, but vary in the particular form of type variable that they return. To
make things worse, the names of these functions are often quite misleading.
Some particularly egregious examples:
* `splitForAllTys` returns `TyCoVar`s, but `splitSomeForAllTys` returns
`VarBndr`s.
* `splitSomeForAllTys` returns `VarBndr`s, but `tcSplitSomeForAllTys` returns
`TyVar`s.
* `splitForAllTys` returns `TyCoVar`s, but `splitForAllTysInvis` returns
`InvisTVBinder`s. (This in particular arose in the context of #18939, and
this finally motivated me to bite the bullet and improve the status quo
vis-à-vis how we name these functions.)
In an attempt to bring some sanity to how these functions are named, I have
opted to rename most of these functions en masse to use consistent suffixes
that describe the particular form of type variable that each function returns.
In concrete terms, this amounts to:
* Functions that return a `TyVar` now use the suffix `-TyVar`.
This caused the following functions to be renamed:
* `splitTyVarForAllTys` -> `splitForAllTyVars`
* `splitForAllTy_ty_maybe` -> `splitForAllTyVar_maybe`
* `tcSplitForAllTys` -> `tcSplitForAllTyVars`
* `tcSplitSomeForAllTys` -> `tcSplitSomeForAllTyVars`
* Functions that return a `CoVar` now use the suffix `-CoVar`.
This caused the following functions to be renamed:
* `splitForAllTy_co_maybe` -> `splitForAllCoVar_maybe`
* Functions that return a `TyCoVar` now use the suffix `-TyCoVar`.
This caused the following functions to be renamed:
* `splitForAllTy` -> `splitForAllTyCoVar`
* `splitForAllTys` -> `splitForAllTyCoVars`
* `splitForAllTys'` -> `splitForAllTyCoVars'`
* `splitForAllTy_maybe` -> `splitForAllTyCoVar_maybe`
* Functions that return a `VarBndr` now use the suffix corresponding to the
most relevant type synonym. This caused the following functions to be renamed:
* `splitForAllVarBndrs` -> `splitForAllTyCoVarBinders`
* `splitForAllTysInvis` -> `splitForAllInvisTVBinders`
* `splitForAllTysReq` -> `splitForAllReqTVBinders`
* `splitSomeForAllTys` -> `splitSomeForAllTyCoVarBndrs`
* `tcSplitForAllVarBndrs` -> `tcSplitForAllTyVarBinders`
* `tcSplitForAllTysInvis` -> `tcSplitForAllInvisTVBinders`
* `tcSplitForAllTysReq` -> `tcSplitForAllReqTVBinders`
* `tcSplitForAllTy_maybe` -> `tcSplitForAllTyVarBinder_maybe`
Note that I left the following functions alone:
* Functions that split apart things besides `ForAllTy`s, such as `splitFunTys`
or `splitPiTys`. Thankfully, there are far fewer of these functions than
there are functions that split apart `ForAllTy`s, so there isn't much of a
pressing need to apply the new naming convention elsewhere.
* Functions that split apart `ForAllCo`s in `Coercion`s, such as
`GHC.Core.Coercion.splitForAllCo_maybe`. We could theoretically apply the new
naming convention here, but then we'd have to figure out how to disambiguate
`Type`-splitting functions from `Coercion`-splitting functions. Ultimately,
the `Coercion`-splitting functions aren't used nearly as much as the
`Type`-splitting functions, so I decided to leave the former alone.
This is purely refactoring and should cause no change in behavior.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/645444af9eb185684c750c95e4740d301352b2b9">645444af</a></strong>
<div>
<span>by Ryan Scott</span>
<i>at 2020-11-15T03:36:21-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Use tcSplitForAllInvisTyVars (not tcSplitForAllTyVars) in more places
The use of `tcSplitForAllTyVars` in `tcDataFamInstHeader` was the immediate
cause of #18939, and replacing it with a new `tcSplitForAllInvisTyVars`
function (which behaves like `tcSplitForAllTyVars` but only splits invisible
type variables) fixes the issue. However, this led me to realize that _most_
uses of `tcSplitForAllTyVars` in GHC really ought to be
`tcSplitForAllInvisTyVars` instead. While I was in town, I opted to replace
most uses of `tcSplitForAllTys` with `tcSplitForAllTysInvis` to reduce the
likelihood of such bugs in the future.
I say "most uses" above since there is one notable place where we _do_ want
to use `tcSplitForAllTyVars`: in `GHC.Tc.Validity.forAllTyErr`, which produces
the "`Illegal polymorphic type`" error message if you try to use a higher-rank
`forall` without having `RankNTypes` enabled. Here, we really do want to split
all `forall`s, not just invisible ones, or we run the risk of giving an
inaccurate error message in the newly added `T18939_Fail` test case.
I debated at some length whether I wanted to name the new function
`tcSplitForAllInvisTyVars` or `tcSplitForAllTyVarsInvisible`, but in the end,
I decided that I liked the former better. For consistency's sake, I opted to
rename the existing `splitPiTysInvisible` and `splitPiTysInvisibleN` functions
to `splitInvisPiTys` and `splitPiTysInvisN`, respectively, so that they use the
same naming convention. As a consequence, this ended up requiring a `haddock`
submodule bump.
Fixes #18939.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/8887102fc4ed8ed1089c1aafd19bab424ad706f3">8887102f</a></strong>
<div>
<span>by Moritz Angermann</span>
<i>at 2020-11-15T03:36:56-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">AArch64/arm64 adjustments
This addes the necessary logic to support aarch64 on elf, as well
as aarch64 on mach-o, which Apple calls arm64.
We change architecture name to AArch64, which is the official arm
naming scheme.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/fc644b1a643128041cfec25db84e417851e28bab">fc644b1a</a></strong>
<div>
<span>by Ben Gamari</span>
<i>at 2020-11-15T03:37:31-05:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">ghc-bin: Build with eventlogging by default
We now have all sorts of great facilities using the
eventlog which were previously unavailable without
building a custom GHC. Fix this by linking with
`-eventlog` by default.</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/29e792c06b112dbc523d24a1b95574fae7af5292">29e792c0</a></strong>
<div>
<span>by Sebastian Graf</span>
<i>at 2020-11-18T10:27:47+01:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Fix strictness signatures of `prefetchValue*#` primops
Their strictness signatures said the primops are strict in their first
argument, which is wrong: Handing it a thunk will prefetch the pointer
to the thunk, but not evaluate it. Hence not strict.
The regression test `T8256` actually tests for laziness in the first
argument, so GHC apparently never exploited the strictness signature.
See also https://gitlab.haskell.org/ghc/ghc/-/issues/8256#note_310867,
where this came up.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/b19f9da6ea16286d19b4e7bfcc6c80edb67c14b8">b19f9da6</a></strong>
<div>
<span>by Sebastian Graf</span>
<i>at 2020-11-18T11:14:33+01:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Demand: Interleave usage and strictness demands (#18903)
As outlined in #18903, interleaving usage and strictness demands not
only means a more compact demand representation, but also allows us to
express demands that we weren't easily able to express before.
Call demands
============
Call demands are *relative* in the sense that a call demand `Cn(cd)`
on `g` says "`g` is called `n` times. *Whenever `g` is called*, the
result is used according to `cd`". Example from #18903:
```hs
h :: Int -> Int
h m =
let g :: Int -> (Int,Int)
g 1 = (m, 0)
g n = (2 * n, 2 `div` n)
{-# NOINLINE g #-}
in case m of
1 -> 0
2 -> snd (g m)
_ -> uncurry (+) (g m)
```
Without the interleaved representation, we would just get `L` for the
strictness demand on `g`. Now we are able to express that whenever
`g` is called, its second component is used strictly in denoting `g`
by `1C1((1(U),S(U)))`. This would allow Nested CPR to unbox the
division, for example.
Nested strict product demands
=============================
Fixing #18903 gives us enough expressiveness to tackle #18885, where we
have
```hs
f :: Int -> Int
f y =
let x
| expensive y == 1 = (expensive (y+1), expensive (y+2))
| otherwise = (expensive (y+3), expensive (y+4))
in case () of
_ | expensive (y+5) == 42 -> fst x
_ | expensive (y+6) == 41 -> fst x + snd x
_ | otherwise -> 0
```
Here, we used to give `x` demand `1P(1P(U),1P(U))`. The outer `1`
is because `x` is used lazily and the inner `1`s are redundant with
that fact. That leaves some expressiveness on the table. After this
change, we infer `1P(SP(U),1P(U))`, meaning that *whenever we evaluate
`x`*, we evaluate its first component strictly, effectively making
strictness product demands apply *relatively*. Usage product demands
still apply absolutely, though.
More details on how we could exploit the new language in
`Note [Absent sub-demands]`.
Fixes #18903 and fixes #18885.
</pre>
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<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/921b1e5fb177671fe0b79c756252c269d3d0c9e3">921b1e5f</a></strong>
<div>
<span>by Sebastian Graf</span>
<i>at 2020-11-18T11:14:35+01:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Update user's guide entry on demand analysis and worker/wrapper
The demand signature notation has been undocumented for a long time.
The only source to understand it, apart from reading the `Outputable`
instance, has been an outdated wiki page.
Since the previous commits have reworked the demand lattice, I took
it as an opportunity to also write some documentation about notation.
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/8249c98ddfc6c3ecf9863d22632bddf73e447047">8249c98d</a></strong>
<div>
<span>by Sebastian Graf</span>
<i>at 2020-11-18T11:14:35+01:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">fix regression
</pre>
</li>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/fa70a702d12f3833c5600fd00c34b67e2e2b3fba">fa70a702</a></strong>
<div>
<span>by Sebastian Graf</span>
<i>at 2020-11-18T11:14:35+01:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">fix
</pre>
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