Improve the `array::map` codegen #107634

scottmcm · 2023-02-03T11:57:04Z

The map method on arrays is documented as sometimes performing poorly, and after a question on URLO prompted me to take another look at the core try_collect_into_array function, I had some ideas that ended up working better than I'd expected.

There's three main ideas in here, split over three commits:

Don't use array::IntoIter when we can avoid it, since that seems to not get SRoA'd, meaning that every step writes things like loop counters into the stack unnecessarily
Don't return arrays in Results unnecessarily, as that doesn't seem to optimize away even with unwrap_unchecked (perhaps because it needs to get moved into a new LLVM type to account for the discriminant)
Don't distract LLVM with all the Option dances when we know for sure we have enough items (like in map and zip). This one's a larger commit as to do it I ended up adding a new pub(crate) trait, but hopefully those changes are still straight-forward.

(No libs-api changes; everything should be completely implementation-detail-internal.)

It's still not completely fixed -- I think it needs pcwalton's memcpy optimizations still (#103830) to get further -- but this seems to go much better than before. And the remaining memcpys are just transmute-equivalent ([T; N] -> ManuallyDrop<[T; N]> and [MaybeUninit<T>; N] -> [T; N]), so hopefully those will be easier to remove with LLVM16 than the previous subobject copies 🤞

r? @thomcc

As a simple example, this test

pub fn long_integer_map(x: [u32; 64]) -> [u32; 64] {
    x.map(|x| 13 * x + 7)
}

On nightly https://rust.godbolt.org/z/xK7548TGj takes sub rsp, 808

start:
  %array.i.i.i.i = alloca [64 x i32], align 4
  %_3.sroa.5.i.i.i = alloca [65 x i32], align 4
  %_5.i = alloca %"core::iter::adapters::map::Map<core::array::iter::IntoIter<u32, 64>, [closure@/app/example.rs:2:11: 2:14]>", align 8

(and yes, that's a 65-element array alloca despite 64-element input and output)

But with this PR it's only sub rsp, 520

start:
  %array.i.i.i.i.i.i = alloca [64 x i32], align 4
  %array1.i.i.i = alloca %"core::mem::manually_drop::ManuallyDrop<[u32; 64]>", align 4

Similarly, the loop it emits on nightly is scalar-only and horrifying

.LBB0_1:
        mov     esi, 64
        mov     edi, 0
        cmp     rdx, 64
        je      .LBB0_3
        lea     rsi, [rdx + 1]
        mov     qword ptr [rsp + 784], rsi
        mov     r8d, dword ptr [rsp + 4*rdx + 528]
        mov     edi, 1
        lea     edx, [r8 + 2*r8]
        lea     r8d, [r8 + 4*rdx]
        add     r8d, 7
.LBB0_3:
        test    edi, edi
        je      .LBB0_11
        mov     dword ptr [rsp + 4*rcx + 272], r8d
        cmp     rsi, 64
        jne     .LBB0_6
        xor     r8d, r8d
        mov     edx, 64
        test    r8d, r8d
        jne     .LBB0_8
        jmp     .LBB0_11
.LBB0_6:
        lea     rdx, [rsi + 1]
        mov     qword ptr [rsp + 784], rdx
        mov     edi, dword ptr [rsp + 4*rsi + 528]
        mov     r8d, 1
        lea     esi, [rdi + 2*rdi]
        lea     edi, [rdi + 4*rsi]
        add     edi, 7
        test    r8d, r8d
        je      .LBB0_11
.LBB0_8:
        mov     dword ptr [rsp + 4*rcx + 276], edi
        add     rcx, 2
        cmp     rcx, 64
        jne     .LBB0_1

whereas with this PR it's unrolled and vectorized

	vpmulld	ymm1, ymm0, ymmword ptr [rsp + 64]
	vpaddd	ymm1, ymm1, ymm2
	vmovdqu	ymmword ptr [rsp + 328], ymm1
	vpmulld	ymm1, ymm0, ymmword ptr [rsp + 96]
	vpaddd	ymm1, ymm1, ymm2
	vmovdqu	ymmword ptr [rsp + 360], ymm1

(though sadly still stack-to-stack)

the8472 · 2023-02-03T15:30:55Z

library/core/src/iter/traits/unchecked_iterator.rs

+///
+/// It currently requires `TrustedLen` because it's unclear whether it's
+/// reasonably possible to depend on the `size_hint` of anything else.
+pub(crate) trait UncheckedIterator: TrustedLen {


This seems like another take on TrustedRandomAccess, except that the source can drop its elements. I tried something similar in #93243 except by augmenting TRA directly, but it lead to compile time regressions due to the added traits and methods.
Ideally we would only have to maintain one unsafe iteration trait. But this is a lot simpler and sometimes the single-induction-variable that TRA provides isn't necessary, so maybe it's not so bad.

Hmm, what's the best resource to read up on TrustedRandomAccessNoCoerce? TBH I've always ignored it 😬 Is https://stdrs.dev/nightly/x86_64-unknown-linux-gnu/std/iter/trait.TrustedRandomAccess.html#safety the go-to place?

I could try changing try_from_trusted_iterator to essentially try_from_fn(Iterator::__iterator_get_unchecked), rather needing next_unchecked at all...

(But next_unchecked has a much simpler safety condition than TRANC.)

s https://stdrs.dev/nightly/x86_64-unknown-linux-gnu/std/iter/trait.TrustedRandomAccess.html#safety the go-to place?

Yes. But nothing actionable for this PR. The trait as it is currently isn't fully applicable for array::IntoIter anyway because it is an owning iterator which needs to drop items when it's not exhausted. So it can only be implemented for T: Copy (as we do for vec::IntoIter) as approximation for needs_drop() == false. To generalize it further would need some changes, as tried in the mentioned PR.

I was just pointing out that we now have two ways for unchecked access. TrustedLen is slightly more general since it can also apply to Chain or Flatten because it supports len > usize::MAX. TRA on the other hand is a bit more powerful when it applies because it allows external iteration with a single induction variable (this really helps Zip) and doesn't need to update internal fields on which the next loop iteration would depend.

(But next_unchecked has a much simpler safety condition than TRANC.)

Agreed, there's some simplicity to it. I have tried using next().unwrap_unchecked() before in conjunction with TrustedLen while it brought some improvements it couldn't match TRA.
I hope that maybe UncheckedIterator could get us closer (if we turn it into a specialization trait, which I thought it was 😅), perhaps even to parity. If we could do away with the TRA machinery that would be awesome because it's chernobyl-levels of unsafe.

But it doesn't cover all the same cases then we'd be left with two unsafe iteration traits, at least one which we might never be able to stabilize and that seems kind of ugly.

I'd be happy to play with it in a future PR :)

What's the best way to make it a specialization trait? Make the method __iterator_next_unchecked instead, like __iterator_get_unchecked, so it's on Iterator instead and just bounded by the specialization trait TrustedLen? (TBH I'm not sure why the unsafe specialization trait keeps things from having methods, but presumably there's a good reason.)

Specialization traits can have methods (TrustedRandomAccessNoCoerce has size()). __iterator_next_unchecked was moved to the iterator trait during the specialization -> min_specialization refactor for reasons I have forgotten (edit, reason was #73565 (comment)).

Slapping #[rustc_specialization_trait] on it might be enough.

Oh, I see -- #[rustc_specialization_trait] can have methods, but TrustedLen is #[rustc_unsafe_specialization_marker], which can't. That's the distinction I'd missed. Thanks!

scottmcm · 2023-02-03T20:34:15Z

Since this doesn't impact for desugaring and doesn't change dyn Iterator it's hopefully perf-ok, but after the8472's message I'll check just in case:

@bors try @rust-timer queue

bors · 2023-02-03T20:34:26Z

⌛ Trying commit 2e20acd169c4fd9c1ee69c089b9c7439ef48a02f with merge 4d8ea29940107409a02030c3f43a0fd2bb7c567c...

the8472 · 2023-02-03T20:41:43Z

Ah, I wasn't thinking that this would have perf impacts. I was more thinking about whether it was a good idea to add more specialization traits for unchecked iteration. And the compiler isn't array-iteration-heavy so it it should affect things much.

the8472 · 2023-02-03T20:48:57Z

Did you run the std benches? There are a few that cover array mapping and collecting arrays for next_chunk.

scottmcm · 2023-02-03T20:51:17Z

I'm not sure whether this is the direction you meant, but I'll note that this PR doesn't actually do any specialization at all.

The methods are internal, so just only accept UncheckedIterator, as all the callers are places where we know for sure that we trust what's going on.

(I first tried to add next_unchecked to TrustedLen, instead of a new trait, but apparently the specialization attribute makes it a #[marker] trait, so I couldn't do that.)

the8472 · 2023-02-03T21:04:40Z

Ah, I saw TrustedLen and then glossed over the actual use. Then it should affect even fewer things.

bors · 2023-02-03T23:04:42Z

☀️ Try build successful - checks-actions
Build commit: 4d8ea29940107409a02030c3f43a0fd2bb7c567c (4d8ea29940107409a02030c3f43a0fd2bb7c567c)

rust-timer · 2023-02-04T01:04:41Z

Finished benchmarking commit (4d8ea29940107409a02030c3f43a0fd2bb7c567c): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

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 may lead to changes in compiler perf.

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

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.8%	[0.6%, 0.9%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-6.5%, -1.0%]	2
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	2.6%	[0.8%, 3.6%]	7
Regressions ❌ (secondary)	2.7%	[0.7%, 4.6%]	39
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.6%	[0.8%, 3.6%]	7

Cycles

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.1%	[-3.1%, -3.1%]	1
All ❌✅ (primary)	-	-	0

scottmcm · 2023-02-05T00:34:09Z

Benches are essentially unchanged:

Before:

running 1 test
test vec::bench_next_chunk                               ... bench:         342 ns/iter (+/- 14)
running 2 tests
test iter::bench_copied_chunks                                     ... bench:          31 ns/iter (+/- 0)
test iter::bench_trusted_random_access_chunks                      ... bench:          31 ns/iter (+/- 0)

After

running 1 test
test vec::bench_next_chunk                               ... bench:         337 ns/iter (+/- 15)
running 2 tests
test iter::bench_copied_chunks                                     ... bench:          30 ns/iter (+/- 0)
test iter::bench_trusted_random_access_chunks                      ... bench:          30 ns/iter (+/- 0)

(I couldn't find any for array map; let me know if I missed something.)

scottmcm · 2023-02-05T05:07:15Z

library/core/src/array/mod.rs

+        move |_| {
+            // SAFETY: We know that `from_fn` will call this at most N times,
+            // and we checked to ensure that we have at least that many items.
+            unsafe { iter.next_unchecked() }


To your point about unwrap_uncheck not working as well as you'd hoped, @the8472, that's what I saw here too that led me to the new trait. I've lost the intermediate version I'd originally tried, but was able to get back to something similar by swapping this iter.next_unchecked for iter.next().unwrap_unchecked().

That ends up leaving this in the IR for the long_integer_map test:

"_ZN93_$LT$core..array..drain..Drain$LT$T$GT$$u20$as$u20$core..iter..traits..iterator..Iterator$GT$4next17h51881ffc2935de71E.exit.i.i.i.i.i.i.i.i.i.1": ; preds = %bb5.i.i.i.i.i.i.i %generator.sroa.4.07.i.i.i.i.ptr.i.i.i.1 = getelementptr inbounds i32, ptr %array1.i.i.i, i64 %generator.sroa.4.1.i.i.i.i.idx.i.i.i %generator.sroa.4.07.i.i.i.i.add.i.i.i.1 = add nsw i64 %generator.sroa.4.1.i.i.i.i.idx.i.i.i, 1 %tmp.0.copyload.i.i.i.i.i.i.i.i.i.i.1 = load i32, ptr %generator.sroa.4.07.i.i.i.i.ptr.i.i.i.1, align 4, !noalias !31 %7 = insertvalue { i32, i32 } { i32 1, i32 undef }, i32 %tmp.0.copyload.i.i.i.i.i.i.i.i.i.i.1, 1 %_3.i.i.i.i.i.i.i.i.i.i.i.i.i.1 = mul i32 %tmp.0.copyload.i.i.i.i.i.i.i.i.i.i.1, 13 %8 = add i32 %_3.i.i.i.i.i.i.i.i.i.i.i.i.i.1, 7 br label %bb5.i.i.i.i.i.i.i.1 bb5.i.i.i.i.i.i.i.1: ; preds = %"_ZN93_$LT$core..array..drain..Drain$LT$T$GT$$u20$as$u20$core..iter..traits..iterator..Iterator$GT$4next17h51881ffc2935de71E.exit.i.i.i.i.i.i.i.i.i.1", %bb5.i.i.i.i.i.i.i %generator.sroa.4.1.i.i.i.i.idx.i.i.i.1 = phi i64 [ 64, %bb5.i.i.i.i.i.i.i ], [ %generator.sroa.4.07.i.i.i.i.add.i.i.i.1, %"_ZN93_$LT$core..array..drain..Drain$LT$T$GT$$u20$as$u20$core..iter..traits..iterator..Iterator$GT$4next17h51881ffc2935de71E.exit.i.i.i.i.i.i.i.i.i.1" ] %9 = phi { i32, i32 } [ { i32 0, i32 undef }, %bb5.i.i.i.i.i.i.i ], [ %7, %"_ZN93_$LT$core..array..drain..Drain$LT$T$GT$$u20$as$u20$core..iter..traits..iterator..Iterator$GT$4next17h51881ffc2935de71E.exit.i.i.i.i.i.i.i.i.i.1" ] %10 = phi i32 [ undef, %bb5.i.i.i.i.i.i.i ], [ %8, %"_ZN93_$LT$core..array..drain..Drain$LT$T$GT$$u20$as$u20$core..iter..traits..iterator..Iterator$GT$4next17h51881ffc2935de71E.exit.i.i.i.i.i.i.i.i.i.1" ] %_3.0.i.i.i.i.i.i.i.i.1 = extractvalue { i32, i32 } %9, 0 %switch.i.i.i.i.i.i.i.i.i.1 = icmp ne i32 %_3.0.i.i.i.i.i.i.i.i.1, 0 tail call void @llvm.assume(i1 %switch.i.i.i.i.i.i.i.i.i.1) %11 = getelementptr inbounds i32, ptr %array.i.i.i.i.i.i, i64 %6 store i32 %10, ptr %11, align 4, !alias.scope !28, !noalias !38 %12 = or i64 %guard.sroa.7.08.i.i.i.i.i.i.i, 2 %_10.i.i.i.i.i.i.i.i.i.i.i.2 = icmp eq i64 %generator.sroa.4.1.i.i.i.i.idx.i.i.i.1, 64 br i1 %_10.i.i.i.i.i.i.i.i.i.i.i.2, label %bb5.i.i.i.i.i.i.i.2, label %"_ZN93_$LT$core..array..drain..Drain$LT$T$GT$$u20$as$u20$core..iter..traits..iterator..Iterator$GT$4next17h51881ffc2935de71E.exit.i.i.i.i.i.i.i.i.i.2"

Those variable names are a horrible mess, but from the instructions you can see that the Options just don't optimize away: it does insertvalue { i32, i32 } to make an option, gets phi'd to the next block, then there's the extractvalue { i32, i32 } to pull things out again, and assumes the discriminant.

Arguably the next_unchecked shouldn't be doing something different, but somehow it makes a huge difference.

There's way too much code here to file a bug, but I'll see whether I can at least find something simpler that I can file on the rust side to see if an expert can tell why LLVM can't eliminate stuff here.

EDIT: Oh, it was easier to find a simple example than I thought, #107681

emilHof · 2023-02-08T16:07:20Z

library/core/src/array/mod.rs

-{
-    let mut map = iter.map(NeverShortCircuit);
-    try_collect_into_array(&mut map).map(|NeverShortCircuit(arr)| arr)
+pub(crate) fn iter_next_chunk<T, const N: usize>(


I am curious about what motivated your reasoning to change the type signature here?

@emilHof It's essentially the same; I didn't have to change it.

This is just a bit more consistent with how I did the other things in this file. (Like how try_from_fn_erased takes impl FnMut, not F … where F: FnMut.) I tend to think of APIT like this as "generic elision" -- for things like iterators and closures, I tend not to actually care about the specific type, so having a used-once generic for it where I have to read the bounds to follow the signature is kinda meh.

I like thinking of it as "this function takes a mutable reference to an iterator of Ts, and returns an array of Ts". Like how you might write a function taking Iterator<T> in Java, or an IEnumerable<T> in C#, just without the dyn indirection that that results in those languages. Thinking of it as "This takes an I, and returns an array of the item type of I, and BTW I needs to be an iterator" is just less direct in my brain. The item type is the important one, not the iterator's type, for me.

TL/DR: Entirely preference.

@scottmcm That is a really explanation!

Your reasoning definitely makes sense. In a way, the T is maybe more relevant to the purpose of these methods, so having the T and not some "elusive" I in the signature definitely seems very sensible! At least, this is how I am interpreting what you are saying.

Thank you for the great response! :)

Yup, that's a good phrasing of it!

library/core/src/array/mod.rs

scottmcm · 2023-02-11T23:05:53Z

@thomcc There's been lots of conversation here, so in case it wasn't clear I think this is still ready to go as-is.

(I'll experiment with the various other future possibilities later, since I don't want to mix those in with this improvement.)

thomcc · 2023-02-13T02:22:13Z

@thomcc There's been lots of conversation here, so in case it wasn't clear I think this is still ready to go as-is.

Yep, it looks good to me (it did before, but I hadn't read through all the conversations to determine that nobody else wanted anything). Thanks a ton, excited to see this.

@bors r+

bors · 2023-02-13T02:22:15Z

📌 Commit bb77860 has been approved by thomcc

It is now in the queue for this repository.

bors · 2023-02-13T10:18:52Z

⌛ Testing commit bb77860 with merge 2d91939...

bors · 2023-02-13T13:37:15Z

☀️ Test successful - checks-actions
Approved by: thomcc
Pushing 2d91939 to master...

rust-timer · 2023-02-13T14:54:21Z

Finished benchmarking commit (2d91939): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

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
cc @rust-lang/wg-compiler-performance

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.3%	[0.3%, 0.4%]	6
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.2%]	4
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	1.1%	[1.1%, 1.1%]	1
Regressions ❌ (secondary)	1.6%	[1.6%, 1.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.4%	[-2.4%, -2.4%]	1
All ❌✅ (primary)	1.1%	[1.1%, 1.1%]	1

Cycles

This benchmark run did not return any relevant results for this metric.

rylev · 2023-02-14T16:29:32Z

Perf is a wash - going to mark as triaged.

@rustbot label: perf-regression-triaged

rustbot assigned thomcc Feb 3, 2023

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scottmcm added 4 commits February 4, 2023 16:41

Stop using into_iter in array::map

5a7342c

Stop forcing array::map through an unnecessary Result

52df055

Allow canonicalizing the array::map loop in trusted cases

5bc328f

Add another autovectorization codegen test using array zip-map

bb77860

scottmcm force-pushed the array-drain branch from c516903 to bb77860 Compare February 5, 2023 00:45

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bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Feb 13, 2023

bors added the merged-by-bors This PR was explicitly merged by bors. label Feb 13, 2023

bors merged commit 2d91939 into rust-lang:master Feb 13, 2023

rustbot added this to the 1.69.0 milestone Feb 13, 2023

rustbot added the perf-regression Performance regression. label Feb 13, 2023

scottmcm deleted the array-drain branch February 14, 2023 03:03

rustbot added the perf-regression-triaged The performance regression has been triaged. label Feb 14, 2023

gmorenz mentioned this pull request Mar 10, 2023

array::from_fn should be fully inlined #108765

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scottmcm mentioned this pull request May 19, 2023

Medium Array of Vecs cloned causes stack overflow but doesn't provide any details #111733

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scottmcm mentioned this pull request Apr 23, 2024

Add Box<[T; N]>: IntoIterator without any method dispatch hacks #124108

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scottmcm mentioned this pull request Jun 6, 2024

Reword the caveats on array::map #126100

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scottmcm mentioned this pull request Jun 17, 2024

Increase MSRV to 1.63 rust-itertools/itertools#959

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scottmcm commented Feb 3, 2023

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bors commented Feb 3, 2023

the8472 commented Feb 3, 2023

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rust-timer commented Feb 4, 2023

Overall result: ❌✅ regressions and improvements - no action needed

scottmcm commented Feb 5, 2023

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This comment was marked as resolved.

scottmcm commented Feb 11, 2023

thomcc commented Feb 13, 2023

bors commented Feb 13, 2023

bors commented Feb 13, 2023

bors commented Feb 13, 2023

rust-timer commented Feb 13, 2023

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

rylev commented Feb 14, 2023

Improve the `array::map` codegen #107634

Improve the `array::map` codegen #107634

scottmcm commented Feb 3, 2023 •

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the8472 Feb 3, 2023 •

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the8472 Feb 5, 2023 •

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scottmcm Feb 5, 2023 •

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