safely `transmute<&List<Ty<'tcx>>, &List<GenericArg<'tcx>>>` #93505

lcnr · 2022-01-31T11:27:20Z

This PR has 3 relevant steps which are is split in distinct commits.

The first commit now interns List<Ty<'tcx>> and List<GenericArg<'tcx>> together, potentially reusing memory while allowing free conversions between these two using List<Ty<'tcx>>::as_substs() and SubstsRef<'tcx>::try_as_type_list().

Using this, we then use &'tcx List<Ty<'tcx>> instead of a SubstsRef<'tcx> for tuple fields, simplifying a bunch of code.

Finally, as tuple fields and other generic arguments now use a different TypeFoldable<'tcx> impl, we optimize the impl for List<Ty<'tcx>> improving perf by slightly less than 1% in tuple heavy benchmarks.

rust-highfive · 2022-01-31T11:27:22Z

Some changes occurred in src/tools/clippy.

cc @rust-lang/clippy

rust-highfive · 2022-01-31T11:27:23Z

r? @michaelwoerister

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compiler/rustc_middle/src/ty/context.rs

lcnr · 2022-01-31T11:29:33Z

@bors try @rust-timer queue

rust-timer · 2022-01-31T11:29:34Z

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

bors · 2022-01-31T11:29:41Z

⌛ Trying commit 443bbb12b2f09a19a08527640a9988d13e56203d with merge c462201b7d359b74f466604390e92b54810fe960...

src/tools/clippy/clippy_utils/src/ty.rs

bors · 2022-01-31T12:40:35Z

💔 Test failed - checks-actions

lcnr · 2022-01-31T13:06:49Z

@bors try @rust-timer queue

rust-timer · 2022-01-31T13:06:50Z

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

bors · 2022-01-31T13:06:56Z

⌛ Trying commit e4d2a5f747ca4fa5ec9a1ec042e1ae4d006da63d with merge a70d158d75abc4ec966880549ca103eb120f8cb8...

bors · 2022-01-31T14:35:47Z

☀️ Try build successful - checks-actions
Build commit: a70d158d75abc4ec966880549ca103eb120f8cb8 (a70d158d75abc4ec966880549ca103eb120f8cb8)

rust-timer · 2022-01-31T14:35:49Z

Queued a70d158d75abc4ec966880549ca103eb120f8cb8 with parent 415c9f9, future comparison URL.

danielhenrymantilla · 2022-01-31T16:52:16Z

compiler/rustc_middle/src/ty/context.rs

+// Can't use the macros as we have reuse the `substs` here.
+impl<'a, 'tcx> Lift<'tcx> for &'a List<Ty<'a>> {
+    type Lifted = &'tcx List<Ty<'tcx>>;
+    fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {


Given the transmute inside it, in instances such as this one, I personally like to spell out the aliases explicitly:

Suggested change

fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {

fn lift_to_tcx(self: &'a List<Ty<'a>>, tcx: TyCtxt<'tcx>) -> Option<&'tcx List<Ty<'tcx>>> {

i generally agree, though, considering that all other Lift impls also don't do that i am going to keep the code as is for now.

rust-timer · 2022-01-31T20:03:58Z

Finished benchmarking commit (a70d158d75abc4ec966880549ca103eb120f8cb8): comparison url.

Summary: This benchmark run shows 18 relevant improvements 🎉 but 13 relevant regressions 😿 to instruction counts.

Average relevant regression: 1.4%
Average relevant improvement: -0.9%
Largest improvement in instruction counts: -1.3% on full builds of ucd check
Largest regression in instruction counts: 2.4% on full builds of deeply-nested-async check

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR led to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf +perf-regression

rust-log-analyzer · 2022-02-21T10:05:37Z

A job failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

bors · 2022-02-21T10:06:19Z

💔 Test failed - checks-actions

lcnr · 2022-02-21T10:45:08Z

seems spurious again? 🤔

@bors retry

bors · 2022-02-21T16:03:42Z

⌛ Testing commit 80f56cd with merge 03a8cc7...

bors · 2022-02-21T19:34:20Z

☀️ Test successful - checks-actions
Approved by: michaelwoerister
Pushing 03a8cc7 to master...

rust-timer · 2022-02-21T21:05:02Z

Finished benchmarking commit (03a8cc7): comparison url.

Summary: This benchmark run shows 17 relevant improvements 🎉 but 48 relevant regressions 😿 to instruction counts.

Average relevant regression: 1.2%
Average relevant improvement: -0.6%
Largest improvement in instruction counts: -0.8% on incr-full builds of ucd check
Largest regression in instruction counts: 2.7% on full builds of deeply-nested-async check

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression

lcnr · 2022-02-22T08:25:27Z

that perf impact is worse then the previous run, going to look into that a bit xx

michaelwoerister · 2022-02-22T11:02:09Z

compiler/rustc_middle/src/ty/subst.rs

@@ -208,6 +233,17 @@ pub type InternalSubsts<'tcx> = List<GenericArg<'tcx>>;
 pub type SubstsRef<'tcx> = &'tcx InternalSubsts<'tcx>;

 impl<'a, 'tcx> InternalSubsts<'tcx> {
+    /// Checks whether all elements of this list are types, if so, transmute.
+    pub fn try_as_type_list(&'tcx self) -> Option<&'tcx List<Ty<'tcx>>> {
+        if self.iter().all(|arg| matches!(arg.unpack(), GenericArgKind::Type(_))) {


Maybe checking the pointer tag directly would make it possible for LLVM to optimize this? Something like self.iter().all(|arg| arg.ptr.get() & TAG_MASK == TYPE_TAG)?

Adding #[inline] to this method is a good idea in any case.

…ster inline `tuple_fields` more rust-lang#93505 fun, after this i have no idea what might be causing the perf impact.

update `hash_stable` for `List<Ty<'tcx>>` cc rust-lang#93505 (comment) this is the hottest part changed since the pre-merge perf run

…errors Don't transmute `&List<GenericArg>` <-> `&List<Ty>` In rust-lang#93505 we allowed safely transmuting between `&List<GenericArg<'_>>` and `&List<Ty<'_>>`. This was possible because `GenericArg` is a tagged pointer and the tag for types is `0b00`, such that a `GenericArg` with a type inside has the same layout as `Ty`. While this was meant as an optimization, it doesn't look like it was actually any perf or max-rss win (see rust-lang#94799 (comment), rust-lang#94841, rust-lang#110496 (comment)). Additionally the way it was done is quite fragile — `unsafe` code was not properly documented or contained in a module, types were not marked as `repr(C)` (making the transmutes possibly unsound). All of this makes the code maintenance harder and blocks other possible optimizations (as an example I've found out about these `transmutes` when my change caused them to sigsegv compiler). Thus, I think we can safely (pun intended) remove those transmutes, making maintenance easier, optimizations possible, code less cursed, etc. r? `@compiler-errors`

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lcnr force-pushed the substsref-vs-ty-list branch from e4d2a5f to 4a1a0ed Compare February 3, 2022 11:31

bors added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Feb 21, 2022

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bors added the merged-by-bors This PR was explicitly merged by bors. label Feb 21, 2022

bors merged commit 03a8cc7 into rust-lang:master Feb 21, 2022

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lcnr deleted the substsref-vs-ty-list branch February 21, 2022 19:42

bors mentioned this pull request Feb 21, 2022

Fix stack overflows when compiling high-recursion_limit programs #93056

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lcnr mentioned this pull request Mar 10, 2022

update hash_stable for List<Ty<'tcx>> #94799

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lcnr mentioned this pull request Mar 11, 2022

no more transmute<&List<Ty<'tcx>>, &List<GenericArg<'tcx>>> #94841

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lcnr mentioned this pull request Mar 14, 2022

inline tuple_fields #94928

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bors added a commit to rust-lang-ci/rust that referenced this pull request Mar 15, 2022

Auto merge of rust-lang#94928 - lcnr:inline-as_substs, r=michaelwoeri…

be52b4a

…ster inline `tuple_fields` more rust-lang#93505 fun, after this i have no idea what might be causing the perf impact.

compiler-errors mentioned this pull request Apr 18, 2023

Don't transmute &List<GenericArg> <-> &List<Ty> #110496

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safely `transmute<&List<Ty<'tcx>>, &List<GenericArg<'tcx>>>` #93505

safely `transmute<&List<Ty<'tcx>>, &List<GenericArg<'tcx>>>` #93505

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	fn lift_to_tcx(self, tcx: TyCtxt<'tcx>) -> Option<Self::Lifted> {
	fn lift_to_tcx(self: &'a List<Ty<'a>>, tcx: TyCtxt<'tcx>) -> Option<&'tcx List<Ty<'tcx>>> {

safely transmute<&List<Ty<'tcx>>, &List<GenericArg<'tcx>>> #93505

safely transmute<&List<Ty<'tcx>>, &List<GenericArg<'tcx>>> #93505

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safely `transmute<&List<Ty<'tcx>>, &List<GenericArg<'tcx>>>` #93505

safely `transmute<&List<Ty<'tcx>>, &List<GenericArg<'tcx>>>` #93505

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