Allow `enum` and `union` literals to also create SSA values by scottmcm · Pull Request #138759 · rust-lang/rust (original) (raw)

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Today, Some(x) always goes through an alloca, even in trivial cases where the niching means the constructor doesn't even change the value.

For example, https://rust.godbolt.org/z/6KG6PqoYz

pub fn demo(r: &i32) -> Option<&i32> { Some(r) }

currently emits the IR

define align 4 ptr @demo(ptr align 4 %r) unnamed_addr { start: %_0 = alloca [8 x i8], align 8 store ptr %r, ptr %_0, align 8 %0 = load ptr, ptr %_0, align 8 ret ptr %0 }

but with this PR it becomes just

define align 4 ptr @demo(ptr align 4 %r) unnamed_addr { start: ret ptr %r }

(Of course the optimizer can clean that up, but it'd be nice if it didn't have to -- especially in debug where it doesn't run. This is like #123886, but that only handled non-simd structs -- this PR generalizes it to all non-simd ADTs.)

Doing this means handing variants other than FIRST_VARIANT, handling the active field for unions, refactoring the discriminant code so the Place and Operand parts can share the calculation, etc.

Other PRs that led up to this one:

r? @Nadrieril

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rustbot added S-waiting-on-review

Status: Awaiting review from the assignee but also interested parties.

T-compiler

Relevant to the compiler team, which will review and decide on the PR/issue.

labels

Mar 20, 2025

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

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These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

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bors added a commit to rust-lang-ci/rust that referenced this pull request

Mar 20, 2025

Allow enum and union literals to also create SSA values

Today, Some(x) always goes through an alloca, even in trivial cases where the niching means the constructor doesn't even change the value.

For example, <https://rust.godbolt.org/z/6KG6PqoYz>

pub fn demo(r: &i32) -> Option<&i32> {
    Some(r)
}

currently emits the IR

define align 4 ptr `@demo(ptr` align 4 %r) unnamed_addr {
start:
  %_0 = alloca [8 x i8], align 8
  store ptr %r, ptr %_0, align 8
  %0 = load ptr, ptr %_0, align 8
  ret ptr %0
}

but with this PR it becomes just

define align 4 ptr `@demo(ptr` align 4 %r) unnamed_addr {
start:
  ret ptr %r
}

(Of course the optimizer can clean that up, but it'd be nice if it didn't have to -- especially in debug where it doesn't run. This is like rust-lang#123886, but that only handled non-simd structs -- this PR generalizes it to all non-simd ADTs.)

There's two commits you can review independently:

The first is simplifying how the aggregate handling works. Past-me wrote something overly complicated, needing arrayvecs and zipping, depending on a careful iteration order of the fields, and fragile enough that even for just structs it needed extra double-checks to make sure it even made the right variant. It's replaced with something far more direct that works just like extract_field: use the offset to put it in exactly the correct immediate in the OperandValue. This doesn't support anything new, just refactors -- including moving some things off FunctionCx that had no reason to be there. (I have no idea why my past self put them there.)
The second extends that work to support more ADTs. That means handing variants other than FIRST_VARIANT, handling the active field for unions, refactoring the discriminant code so the Place and Operand parts can share the calculation, etc.

☀️ Try build successful - checks-actions
Build commit: 875f416 (875f416b9aa583f09153eac5b56d36fcd932b274)

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Finished benchmarking commit (875f416): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

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.

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-perf +perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.0%]	1
Regressions ❌ (secondary)	0.2%	[0.2%, 0.3%]	3
Improvements ✅ (primary)	-0.3%	[-0.5%, -0.2%]	7
Improvements ✅ (secondary)	-0.3%	[-0.4%, -0.3%]	2
All ❌✅ (primary)	-0.3%	[-0.5%, 0.0%]	8

Max RSS (memory usage)

Results (primary -2.3%)

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)	-2.3%	[-2.3%, -2.3%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.3%	[-2.3%, -2.3%]	1

Cycles

Results (secondary -2.1%)

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)	-2.1%	[-2.3%, -1.6%]	8
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.1%, secondary -0.1%)

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)	-0.1%	[-0.2%, -0.0%]	57
Improvements ✅ (secondary)	-0.1%	[-0.3%, -0.0%]	27
All ❌✅ (primary)	-0.1%	[-0.2%, -0.0%]	57

Bootstrap: 775.297s -> 774.424s (-0.11%)
Artifact size: 365.52 MiB -> 365.52 MiB (0.00%)

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

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Ben said I should re-roll this
r? codegen

github-actions bot pushed a commit to rust-lang/rustc-dev-guide that referenced this pull request

Jun 12, 2025

Remove unneeded FunctionCx from some codegen methods

No changes; just removing the self that wasn't needed.

r? workingjubilee cc rust-lang/rust#138759 (comment)

Kobzol added a commit to Kobzol/rust that referenced this pull request

Jun 18, 2025

…kingjubilee

CodeGen: rework Aggregate implemention for rvalue_creates_operand cases

A non-trivial refactor pulled out from rust-lang#138759 r? workingjubilee

The previous implementation I'd written here based on index_by_increasing_offset is complicated to follow and difficult to extend to non-structs.

This changes the implementation, without actually changing any codegen (thus no test changes either), to be more like the existing extract_field (<https://github.com/rust-lang/rust/blob/2b0274c71dba0e24370ebf65593da450e2e91868/compiler/rustc_codegen_ssa/src/mir/operand.rs#L345-L425>) in that it allows setting a particular field directly.

Notably I've found this one much easier to get right, in particular because having the OperandRef<Result<V, Scalar>> gives a really useful thing to include in ICE messages if something did happen to go wrong.

Kobzol added a commit to Kobzol/rust that referenced this pull request

Jun 18, 2025

…kingjubilee

CodeGen: rework Aggregate implemention for rvalue_creates_operand cases

A non-trivial refactor pulled out from rust-lang#138759 r? workingjubilee

The previous implementation I'd written here based on index_by_increasing_offset is complicated to follow and difficult to extend to non-structs.

Kobzol added a commit to Kobzol/rust that referenced this pull request

Jun 18, 2025

…kingjubilee

CodeGen: rework Aggregate implemention for rvalue_creates_operand cases

A non-trivial refactor pulled out from rust-lang#138759 r? workingjubilee

The previous implementation I'd written here based on index_by_increasing_offset is complicated to follow and difficult to extend to non-structs.

rust-timer added a commit that referenced this pull request

Jun 18, 2025

Rollup merge of #142383 - scottmcm:operandref-builder, r=workingjubilee

CodeGen: rework Aggregate implemention for rvalue_creates_operand cases

A non-trivial refactor pulled out from #138759 r? workingjubilee

The previous implementation I'd written here based on index_by_increasing_offset is complicated to follow and difficult to extend to non-structs.

github-actions bot pushed a commit to rust-lang/rustc-dev-guide that referenced this pull request

Jun 19, 2025

CodeGen: rework Aggregate implemention for rvalue_creates_operand cases

A non-trivial refactor pulled out from rust-lang/rust#138759 r? workingjubilee

The previous implementation I'd written here based on index_by_increasing_offset is complicated to follow and difficult to extend to non-structs.

scottmcm marked this pull request as ready for review

June 19, 2025 06:39

Perf was good three months ago, but having redone this a bunch it's probably worth double-checking:
@bors try @rust-timer queue

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bors added a commit that referenced this pull request

Jun 19, 2025

Allow enum and union literals to also create SSA values

Today, Some(x) always goes through an alloca, even in trivial cases where the niching means the constructor doesn't even change the value.

For example, <https://rust.godbolt.org/z/6KG6PqoYz>

pub fn demo(r: &i32) -> Option<&i32> {
    Some(r)
}

currently emits the IR

define align 4 ptr `@demo(ptr` align 4 %r) unnamed_addr {
start:
  %_0 = alloca [8 x i8], align 8
  store ptr %r, ptr %_0, align 8
  %0 = load ptr, ptr %_0, align 8
  ret ptr %0
}

but with this PR it becomes just

define align 4 ptr `@demo(ptr` align 4 %r) unnamed_addr {
start:
  ret ptr %r
}

Doing this means handing variants other than FIRST_VARIANT, handling the active field for unions, refactoring the discriminant code so the Place and Operand parts can share the calculation, etc.

Other PRs that led up to this one:

This comment has been minimized.

Comment on lines -1046 to -1060

mir::Rvalue::Aggregate(ref kind, _) => {
let allowed_kind = match **kind {
// This always produces a `ty::RawPtr`, so will be Immediate or Pair
mir::AggregateKind::RawPtr(..) => true,
mir::AggregateKind::Array(..) => false,
mir::AggregateKind::Tuple => true,
mir::AggregateKind::Adt(def_id, ..) => {
let adt_def = self.cx.tcx().adt_def(def_id);
adt_def.is_struct() && !adt_def.repr().simd()
}
mir::AggregateKind::Closure(..) => true,
// FIXME: Can we do this for simple coroutines too?
mir::AggregateKind::Coroutine(..) \| mir::AggregateKind::CoroutineClosure(..) => false,
};
allowed_kind && {

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As an update from previous revisions of this PR, note that we no longer need to check the AggregateKind here at all. The layout check is enough to cover all the possibilities.

☀️ Try build successful - checks-actions
Build commit: cf39981 (cf39981b11b1dcc0f8260a7d2ea06f6b942a3daf)

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Finished benchmarking commit (cf39981): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

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

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.3%]	3
Improvements ✅ (secondary)	-0.3%	[-0.3%, -0.3%]	2
All ❌✅ (primary)	-0.4%	[-0.4%, -0.3%]	3

Max RSS (memory usage)

Results (primary -4.6%, secondary -0.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.6%	[3.6%, 3.6%]	1
Improvements ✅ (primary)	-4.6%	[-4.6%, -4.6%]	1
Improvements ✅ (secondary)	-4.6%	[-4.6%, -4.6%]	1
All ❌✅ (primary)	-4.6%	[-4.6%, -4.6%]	1

Cycles

Results (secondary 8.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	8.7%	[8.7%, 8.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.1%, secondary -0.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	41
Improvements ✅ (secondary)	-0.1%	[-0.3%, -0.0%]	18
All ❌✅ (primary)	-0.1%	[-0.2%, -0.0%]	41

Bootstrap: 692.997s -> 691.442s (-0.22%)
Artifact size: 372.00 MiB -> 371.99 MiB (-0.00%)

scottmcm added A-codegen

Area: Code generation

and removed S-waiting-on-author

Status: This is awaiting some action (such as code changes or more information) from the author.

A-LLVM

Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues.

labels

Jun 20, 2025

@workingjubilee I see you emoji'd a comment here; any status updates on reviewing? Anything else you'd like pulled out to a separate PR?

r? saethlin

I'm going to review this on the 3rd

Labels

A-codegen

Area: Code generation

S-waiting-on-review

Status: Awaiting review from the assignee but also interested parties.

T-compiler

Relevant to the compiler team, which will review and decide on the PR/issue.

Allow enum and union literals to also create SSA values by scottmcm · Pull Request #138759 · rust-lang/rust (original) (raw)

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Overall result: ❌✅ regressions and improvements - please read the text below

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Overall result: ✅ improvements - no action needed

Allow `enum` and `union` literals to also create SSA values by scottmcm · Pull Request #138759 · rust-lang/rust (original) (raw)