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.)

The commits:

1. 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.)
2. 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.
3. Apparently I can't just remove a dep without updating the lock file, oops.
4. I got annoyed at tag_field being usize, so changed it to FieldIdx

No need to build ArrayVecs; just put everything exactly where it goes.

r? @Nadrieril

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rustbot added S-waiting-on-review

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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:

1. 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.)
2. 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

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Max RSS (memory usage)

Results (primary -2.3%)

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Results (secondary -2.1%)

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Binary size

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

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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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It was already available as a generic parameter anyway, and it's not like we'll ever put a tag in the 5-billionth field.

Ben said I should re-roll this
r? codegen

ah, this is a "tomorrow" PR. poke me if I do not get to it then.