perf(codegen): Eliminate size_of_val == 0 for DSTs with Non-zero-sized Prefix via NUW and Assume

[TKanX]

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Summary:

Problem:

size_of_val(p) == 0 fails to optimize away for DST types that have a statically-known non-zero-sized prefix:

pub struct Foo<T: ?Sized>(pub [u32; 3], pub T);

pub fn demo(p: &Foo<dyn std::fmt::Debug>) -> bool {
    std::mem::size_of_val(p) == 0  // always false, but LLVM can't prove it
}

Foo has a 12-byte prefix, so its total size is always ≥ 12. Yet the comparison persists as a runtime computation in LLVM IR. This matters because Box<dyn T> drop emits this exact check to guard the deallocation call — for types with a guaranteed non-zero prefix, the branch should vanish but doesn't.

The slice tail variant Foo<[i32]> already optimized correctly; Foo<dyn Trait> and Foo<[u8]> did not.

Root Cause:

In size_and_align_of_dst (the ADT/Tuple branch), the size computation is:

full_size = (offset + unsized_size + (align-1)) & -align

LLVM cannot prove full_size > 0 because:

1. offset + unsized_size used plain add — no overflow flags, so LLVM cannot conclude the result is ≥ offset.
2. (x + addend) & -align — LLVM has no fold to prove that alignment rounding never reduces the value below x.

Solution:

Two changes:

1. add nuw nsw on offset + unsized_size — the sum is bounded by the rounded size ≤ isize::MAX, so neither signed nor unsigned overflow is possible. Tells LLVM: unrounded_size ≥ offset.

2. assume(full_size ≥ unrounded_size) — round_up(x, a) ≥ x is a mathematical identity for power-of-two a. Tells LLVM: full_size ≥ unrounded_size ≥ offset. If offset > 0, the chain proves full_size > 0.

LLVM IR Comparison:

Foo<dyn Debug> — before (godbolt):

define noundef zeroext i1 @demo(ptr %p.0, ptr %p.1) {
start:
  %0 = getelementptr inbounds nuw i8, ptr %p.1, i64 8
  %1 = load i64, ptr %0, align 8, !range !3, !invariant.load !4
  %2 = getelementptr inbounds nuw i8, ptr %p.1, i64 16
  %3 = load i64, ptr %2, align 8, !range !5, !invariant.load !4
  %4 = tail call i64 @llvm.umax.i64(i64 %3, i64 4)
  %5 = add nuw i64 %1, 11
  %6 = add i64 %5, %4
  %7 = sub i64 0, %4
  %8 = and i64 %6, %7
  %_0 = icmp eq i64 %8, 0
  ret i1 %_0
}

Foo<dyn Debug> — after:

define noundef zeroext i1 @demo(ptr %p.0, ptr %p.1) {
start:
  ret i1 false
}

Foo<[u8]> — before:

define noundef zeroext i1 @demo_lessalignedslice(ptr %p.0, i64 %p.1) {
start:
  %0 = add i64 %p.1, 15
  %_0 = icmp ult i64 %0, 4
  ret i1 %_0
}

Foo<[u8]> — after:

define noundef zeroext i1 @demo_lessalignedslice(ptr %p.0, i64 %p.1) {
start:
  ret i1 false
}

Changes:

• compiler/rustc_codegen_ssa/src/size_of_val.rs: add → unchecked_suadd (NUW+NSW) on offset + unsized_size; add assume(full_size ≥ unrounded_size).
• tests/codegen-llvm/dst-size-of-val-not-zst.rs: new codegen test verifying size_of_val == 0 folds to ret i1 false for Foo<dyn Debug>, Foo<[u8]>, and Foo<[i32]>.

Fixes #152788.

[TKanX]

@rustbot label +A-LLVM +A-codegen +C-optimization +T-compiler

[fmease]

r? codegen

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[TKanX]

@rustbot ready

[scottmcm]

Can you elaborate on which things you tried and why this is the best one? Was it not enough to say that the alignment is a power-of-two? Or...

[scottmcm]

I ask because most of the text in the OP is just useless LLM slop, and the updates to the tests make me suspicious.

[TKanX]

@scottmcm

Can you elaborate on which things you tried and why this is the best one? Was it not enough to say that the alignment is a power-of-two? Or...

Tried nuw-only (unchecked_uadd) first. That gives LLVM unrounded >= offset > 0 but it stops at the rounding — LLVM can't prove (x + a-1) & -a >= x. Also checked whether feeding ctpop(align) == 1 would help, but there's no fold for "round-up is monotonic when alignment is pow2" in InstCombine/ValueTracking. So the assume tells LLVM the conclusion directly.

nsw (making it unchecked_suadd) is because unrounded ≤ rounded ≤ isize::MAX. Same reasoning as your #152867.

I ask because most of the text in the OP is just useless LLM slop, and the updates to the tests make me suspicious.

Sorry about the OP — English isn't my native language, I overwrite when trying to be precise. Will clean it up.

For the tests: CHECK-NOT: icmp broke because assume itself emits an icmp. The !range checks on the first two functions were dropped because the assume keeps the size computation alive, so there's now a size load before the alignment load — FileCheck hits the wrong one. Range metadata is still verified in align_load_from_align_of_val below. RANGE_META → ALIGN_RANGE since it only covers alignment loads now. Range value {1, 0} → {1, 0x20000001} is Align::max_for_target (same change as #152929).

Happy to close this if you'd rather land it as part of #152867.

[scottmcm]

Landing this separately is great -- I opened the issue because this particular bit about what LLVM can prove is different enough from the point of layout_of_val that it's better to have the changes separated. (That's why I pulled out #152929 too 🙂 )

[scottmcm]

Hmm, yeah, I experimented a bit https://llvm.godbolt.org/z/haGYz7aax and even getting lots of annotations on everything and assume it's still not able to understand what's happening properly.

(Also it's so annoying to see add nsw i64 %4, -1 since that used to be sub nuw nsw i64 %4, 1 but LLVM just insists on throwing that information away.)

[dianqk]

r? scottmcm

[scottmcm]

So the problem here is that if this was testing for "not icmp", just removing that check means this test is (potentially) no longer testing what it was trying to test before.

If there's an icmp now, probably what you want instead is something like

    // CHECK-NOT: llvm.umax
    // CHECK-NOT: icmp
    // CHECK-NOT: select
    // CHECK: [[DOES_NOT_SHRINK:%.+]] = icmp ... something here ...
    // CHECK-NEXT: call void @llvm.assume(i1 [[DOES_NOT_SHRINK]])
    // CHECK-NOT: llvm.umax
    // CHECK-NOT: icmp
    // CHECK-NOT: select

so that the test is that the only icmp is the expected one that's used for the assume.

---

Similarly, why remove the !range check? It's not being optimized out, is it? (If it is, that's also interesting.)

[TKanX]

Checked the emitted IR — the assume (and the entire size computation) gets DCE'd in these two functions at -O3, since they only need alignment for the field projection. So there's no extra icmp at all, and the alignment load is still the first one with !range. Restored the original patterns as-is; the file is now unchanged from main.

[scottmcm]

This file is no longer unchanged, so this comment applies again.

@rustbot author

[TKanX]

currently failing on LLVM20, i added min-llvm-version: 21 thinking the ret i1 false fold would work there, but that was just an assumption. on LLVM20 the add nuw nsw + assume(icmp uge ...) survive in IR and can be checked directly. should i rewrite the test to check the emission side instead?

@scottmcm

[TKanX]

I would suggest you survey what testing exists for it and what the intent of the various tests are.

In general, it's fine to limit desirable optimization tests to latest LLVM only, since that's what we ship and if people are using older LLVM then it's at least someone expected that things will optimize less well.

On the other hand, if it's "we're testing what rustc is doing" tests, then those should generally continue to pass on older LLVM because we don't want rustc to break on older LLVMs.

@scottmcm similar to your suggestion: 5eec5e3

[scottmcm]

I would suggest you survey what testing exists for it and what the intent of the various tests are.

In general, it's fine to limit desirable optimization tests to latest LLVM only, since that's what we ship and if people are using older LLVM then it's at least someone expected that things will optimize less well.

On the other hand, if it's "we're testing what rustc is doing" tests, then those should generally continue to pass on older LLVM because we don't want rustc to break on older LLVMs.

---

Another thing you could try would be whether -C opt-level=1 still is sufficient to meet the goals of the tests in question but because they try less they might be more consistent between versions. I don't know.

[scottmcm]

@bors r+ rollup=iffy (conditional-on-llvm-version codegen tests are extra scary)

[rust-bors]

📌 Commit 5eec5e3 has been approved by scottmcm

It is now in the queue for this repository.

[matthiaskrgr]

@bors r-
#154438 (comment)

[rust-bors]

This pull request was unapproved.

This PR was contained in a rollup (#154438), which was unapproved.

[TKanX]

@matthiaskrgr build fail?

[matthiaskrgr]

looks like the test you changed here failed: https://triage.rust-lang.org/gha-logs/rust-lang/rust/68771279640#L2026-03-26T19:26:05.8780382Z

[scottmcm]

An option if it makes your life easier: since this one is "just" an optimization, you could pull it into a separate file with min-llvm-version: 21 and not worry about llvm20