Add `noundef` metadata for fits-in-target-pointer-size array immediate arguments #123425

jieyouxu · 2024-04-03T18:33:44Z

noundef is only added if the small array immediate fits in the target pointer size and if optimizations are enabled.

rustbot · 2024-04-03T18:33:52Z

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tests/codegen/union-abi.rs

clubby789 · 2024-04-03T22:15:38Z

@bors try @rust-timer queue

bors · 2024-04-03T22:16:49Z

⌛ Trying commit 31abdb2 with merge 6be5114...

…<try> Add `noundef` metadata for fits-in-target-pointer-size array immediate arguments `noundef` is only added if the small array immediate fits in the target pointer size and if optimizations are enabled. Closes rust-lang#123183.

bors · 2024-04-03T23:48:02Z

☀️ Try build successful - checks-actions
Build commit: 6be5114 (6be511458bb70e6b7db8ec27a49f5a0067274688)

rust-timer · 2024-04-04T01:18:54Z

Finished benchmarking commit (6be5114): comparison URL.

Overall result: no relevant changes - 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

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	mean	range	count
Regressions ❌ (primary)	0.1%	[0.1%, 0.1%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.4%	[-2.4%, -2.4%]	1
All ❌✅ (primary)	0.1%	[0.1%, 0.1%]	1

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	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.9%	[3.9%, 3.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.4%	[-2.4%, -2.4%]	1
All ❌✅ (primary)	-	-	0

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Bootstrap: 666.914s -> 667.807s (0.13%)
Artifact size: 318.16 MiB -> 318.17 MiB (0.01%)

tests/codegen/array-immediate-param-noundef.rs

DianQK · 2024-04-07T01:45:16Z

Is it working on the following code?

#[repr(transparent)]
pub struct Foo([u8; 4]);

#[repr(transparent)]
pub struct Baz(Foo);

#[no_mangle]
pub fn replace_repr_transparent_struct_short_array_2(r: &mut Baz, v: Baz) -> Baz {
    std::mem::replace(r, v)
}

clubby789 · 2024-04-07T13:14:34Z

define i32 @replace_repr_transparent_struct_short_array_2(ptr noalias nocapture noundef align 1 dereferenceable(4) %r, i32 %0) unnamed_addr #0 {
start:
  %result.sroa.0.0.copyload = load i32, ptr %r, align 1
  store i32 %0, ptr %r, align 1
  ret i32 %result.sroa.0.0.copyload
}

jieyouxu · 2024-04-07T13:18:42Z

aaaaa right, recursive #[repr(transparent)], I have not considered that....

jieyouxu · 2024-04-07T15:02:14Z

@rustbot author

jieyouxu · 2024-04-07T15:18:54Z

I made the is_transparent_array check recursive now, so it should see through nested #[repr(transparent)] cases (added a test case for that).

@rustbot ready

tests/codegen/array-immediate-param-noundef.rs

… immediate arguments

DianQK

LGTM. But I believe this still requires someone with r+ privileges to take a look.

r? compiler

davidtwco · 2024-04-16T10:29:40Z

@bors r=davidtwco,DianQK

bors · 2024-04-16T10:29:44Z

📌 Commit 6bf21cf has been approved by davidtwco,DianQK

It is now in the queue for this repository.

…davidtwco,DianQK Add `noundef` metadata for fits-in-target-pointer-size array immediate arguments `noundef` is only added if the small array immediate fits in the target pointer size and if optimizations are enabled. Closes rust-lang#123183.

bors · 2024-04-16T15:29:42Z

⌛ Testing commit 6bf21cf with merge 57a2597...

RalfJung · 2024-04-16T16:53:08Z

compiler/rustc_ty_utils/src/abi.rs

+ // `#[repr(transparent)] enum` containing array. Note that `#[repr(transparent)]`
+ // can contain other `#[repr(transparent)]` structs or enums, which can eventually
+ // contain an array!
+ if arg.layout.ty.is_array() || is_transparent_array(cx, arg.layout) {


I don't understand why arrays would be special here. What if this is an array of unions, or something like that?

RalfJung · 2024-04-16T16:59:29Z

compiler/rustc_ty_utils/src/abi.rs

+ let mut adt_layout = outermost_layout;
+ // Recursively walk a layout, seeing through all `#[repr(transparent)]` layers.
+ while adt_layout.is_transparent::<LayoutCx<'tcx, TyCtxt<'tcx>>>()
+ && let Some((_, layout)) = adt_layout.non_1zst_field(cx)


Note that unions can be transparent, so wouldn't this do the wrong thing then?

At least for structs, there's not really any need to check for is_transparent. This is an optimization so we can just make use of the fact that a struct is a wrapper (i.e., non_1zst_field returns Some) even without a repr.

RalfJung · 2024-04-16T17:01:11Z

@bors r-
because of missing tests and since at first glance it seems some of these tests may actually fail -- better safe than sorry

RalfJung · 2024-04-16T17:00:11Z

tests/codegen/array-immediate-param-noundef.rs

+
+// # Negative examples
+
+// This inner struct is *not* `#[repr(transparent)]`, so we must not emit `noundef` for the outer


Why "must not"? With the layout we pick this would still be correct.

RalfJung · 2024-04-16T17:00:38Z

tests/codegen/array-immediate-param-noundef.rs

This testcase is missing negative tests with transparent unions, and arrays of unions, to ensure they do not get the attribute.

RalfJung · 2024-04-16T17:08:08Z

How do I get bors to process the next PR?
@bors force

RalfJung · 2024-04-16T17:10:34Z

compiler/rustc_ty_utils/src/abi.rs

@@ -784,6 +784,21 @@ fn fn_abi_adjust_for_abi<'tcx>(
 // an LLVM aggregate type for this leads to bad optimizations,
 // so we pick an appropriately sized integer type instead.
 arg.cast_to(Reg { kind: RegKind::Integer, size });
+
+ // Let's see if we can add a `noundef`. This is only legal for arrays, definitely


I think the elephant in the room here is padding, isn't it? Arrays are fine as they do not have any padding. But the word "padding" does not appear anywhere in the PR diff or description, so I am confused.

bors · 2024-04-16T17:35:55Z

☀️ Try build successful - checks-actions
Build commit: 57a2597 (57a25976fe3c471108640effc63529372c2b61ca)

scottmcm · 2024-04-16T17:45:30Z

I agree that there's nothing particularly special about arrays here -- it might be possible for other aggregates too. I just opened the issue saying arrays because that's the place I noticed it and I was confident about. (Since [i32; 1] definitely can't be undef.)

But for example we don't put noundef on { i32, i32 } returns either right now, it looks like, and we probably could. That's probably better as a "not this PR", though.

jieyouxu · 2024-04-16T18:28:21Z

Thanks for the catch, I'll revisit this PR in a few days when less busy

jieyouxu · 2024-05-04T15:03:15Z

(Still busy, will have more time towards the end of this month)

jieyouxu · 2024-06-09T12:22:14Z

Unfortunately I don't have the bandwidth to work on this, but other people are welcomed to pick this up.

…<try> Annotate eligible small immediate arguments with `noundef` Retake of rust-lang#123425. We try to annotate small (fits within target pointer width) aggregate arguments passed as immediates (specifically casted as "appropriately sized integer type") with `noundef`. Example: ```rs #[no_mangle] pub fn short_array_u64x1(v: [u64; 1]) -> [u64; 1] { v } ``` currently produces ```llvm define i64 `@short_array_u64x1(i64` %0) ... ``` This PR changes that to ```llvm define noundef i64 `@short_array_u64x1(i64` noundef %0) ... ``` The `noundef` attribute is added only if the immediate value has no padding. Specifically, the conservative heuristic we use is to: - Peel away layers of `#[repr(Rust)]` or `#[repr(transparent)]` wrappers if present - Check for innermost simple arrays (whose element type is a primitive type) that can fit within target pointer width Union immediates or otherwise anything that contains unions will not have `noundef` attribute applied. Closes rust-lang#123183. cc `@/RalfJung` who pointed out various problems with the previous take, hopefully I addressed most of them in this take. r? `@ghost` (perf)

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jieyouxu commented Apr 3, 2024

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jieyouxu added 3 commits April 10, 2024 14:24

codegen: add noundef metadata for fits-in-target-pointer-size array…

eb00c05

… immediate arguments

codegen: revert union-abi test

b9f3e3d

codegen: only emit noundef for arrays, not unions or other types

b7b78bb

codegen: update array-immediate-param-noundef test

6bf21cf

rust-cloud-vms bot force-pushed the array-imm-noundef-param branch from 7a67c9d to 6bf21cf Compare April 10, 2024 14:41

DianQK approved these changes Apr 11, 2024

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jieyouxu marked this pull request as draft June 8, 2024 18:25

jieyouxu closed this Jun 9, 2024

jieyouxu deleted the array-imm-noundef-param branch July 1, 2024 11:39

jieyouxu mentioned this pull request Jul 1, 2024

Annotate eligible small immediate arguments with noundef #127210

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Add `noundef` metadata for fits-in-target-pointer-size array immediate arguments #123425

Add `noundef` metadata for fits-in-target-pointer-size array immediate arguments #123425

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		// # Negative examples

		// This inner struct is not `#[repr(transparent)]`, so we must not emit `noundef` for the outer

Add noundef metadata for fits-in-target-pointer-size array immediate arguments #123425

Add noundef metadata for fits-in-target-pointer-size array immediate arguments #123425

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Add `noundef` metadata for fits-in-target-pointer-size array immediate arguments #123425

Add `noundef` metadata for fits-in-target-pointer-size array immediate arguments #123425

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