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<h3 style="margin-top: 20px; margin-bottom: 10px;">
Sebastian Graf pushed to branch wip/T20749 at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
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Commits:
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<strong style="font-weight: 600;"><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/ad39e0ed7ec69b0f0257940d9fdb2583b19008bd">ad39e0ed</a></strong>
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<span> by Sebastian Graf </span> <i> at 2024-10-24T08:15:36+02:00 </i>
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<pre class="commit-message" style='white-space: pre-wrap; display: block; font-size: 14px; color: #28272d; position: relative; font-family: "GitLab Mono","JetBrains Mono","Menlo","DejaVu Sans Mono","Liberation Mono","Consolas","Ubuntu Mono","Courier New","andale mono","lucida console",monospace; word-break: break-all; word-wrap: break-word; background-color: #fbfafd; border-radius: 2px; margin: 0; padding: 8px 12px; border: 1px solid #dcdcde;'>Make DataCon workers strict in strict fields (#20749)
This patch tweaks `exprIsConApp_maybe`, `exprIsHNF` and friends, and Demand
Analysis so that they exploit and maintain strictness of DataCon workers. See
`Note [Strict fields in Core]` for details.
Very little needed to change, and it puts field seq insertion done by Tag
Inference into a new perspective: That of *implementing* strict field semantics.
Before Tag Inference, DataCon workers are strict. Afterwards they are
effectively lazy and field seqs happen around use sites. History has shown
that there is no other way to guarantee taggedness and thus the STG Strict Field
Invariant.
Knock-on changes:
* I reworked the whole narrative around "Tag inference".
It's now called "EPT enforcement" and I recycyled the different overview
Notes into `Note [EPT enforcement]`.
* `exprIsHNF` previously used `exprOkForSpeculation` on unlifted arguments
instead of recursing into `exprIsHNF`. That regressed the termination
analysis in CPR analysis (which simply calls out to `exprIsHNF`), so I made
it call `exprOkForSpeculation`, too.
* There's a small regression in Demand Analysis, visible in the changed test
output of T16859: Previously, a field seq on a variable would give that
variable a "used exactly once" demand, now it's "used at least once",
because `dmdTransformDataConSig` accounts for future uses of the field
that actually all go through the case binder (and hence won't re-enter the
potential thunk). The difference should hardly be observable.
* The Simplifier's fast path for data constructors only applies to lazy
data constructors now. I observed regressions involving Data.Binary.Put's
`Pair` data type.
* Unfortunately, T21392 does no longer reproduce after this patch, so I marked
it as "not broken" in order to track whether we regress again in the future.
Fixes #20749, the satisfying conclusion of an annoying saga (cf. the ideas
in #21497 and #22475).
Compiler perf generally improves, sometimes drastically:
Baseline
Test Metric value New value Change
--------------------------------------------------------------------------------
ManyConstructors(normal) ghc/alloc 3,629,760,116 3,711,852,800 +2.3% BAD
MultiLayerModulesTH_OneShot(normal) ghc/alloc 2,502,735,440 2,565,282,888 +2.5% BAD
T12707(normal) ghc/alloc 804,399,798 791,807,320 -1.6% GOOD
T17516(normal) ghc/alloc 964,987,744 1,008,383,520 +4.5%
T18140(normal) ghc/alloc 75,381,152 49,860,560 -33.9% GOOD
T18698b(normal) ghc/alloc 232,614,457 184,262,736 -20.8% GOOD
T18923(normal) ghc/alloc 62,002,368 58,301,408 -6.0% GOOD
T20049(normal) ghc/alloc 75,719,168 70,494,368 -6.9% GOOD
T3294(normal) ghc/alloc 1,237,925,833 1,157,638,992 -6.5% GOOD
T9233(normal) ghc/alloc 686,490,105 635,166,688 -7.5% GOOD
geo. mean -0.7%
minimum -33.9%
maximum +4.5%
I looked at T17516. It seems we do a few more simplifier iterations and end up
with a larger program. It seems that some things inline more, while other things
inline less. I don't see low-hanging fruit.
I also looked at MultiLayerModulesTH_OneShot. It appears we generate a strange
join point in the `getUnique` method of `Uniquable GHC.Unit.Types.Module` that
should better call-site inline, but does not. Perhaps with !11492.
NoFib does not seem affected much either:
+-------------------------------++--+------------+-----------+---------------+-----------+
| || | base/ | std. err. | T20749/ (rel) | std. err. |
+===============================++==+============+===========+===============+===========+
| spectral/last-piece || | 7.263e8 | 0.0% | +0.62% | 0.0% |
+===============================++==+============+===========+===============+===========+
| geom mean || | +0.00% | | | |
+-------------------------------++--+------------+-----------+---------------+-----------+
I had a look at last-piece. Nothing changes in stg-final, but there is a bit
of ... movement around Data.Map.insert's use of GHC.Exts.lazy that is gone in
stg-final.
Co-Authored-By: Jaro Reinders <jaro.reinders@gmail.com>
Metric Decrease:
T12707
T18140
T18698b
T18923
T19695
T20049
T3294
T9233
T21839c
Metric Increase:
ManyConstructors
MultiLayerModulesTH_OneShot
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compiler/GHC/Builtin/Types.hs
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compiler/GHC/Core.hs
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compiler/GHC/Core/DataCon.hs
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