Fix ICE on offsetted ZST pointer #147576

Mark-Simulacrum · 2025-10-11T01:57:55Z

I'm not sure this is the right fix, but it's simple enough and does roughly what I'd expect. Like with the previous optimization to codegen usize rather than a zero-sized static, there's no guarantee that we continue returning a particular value from the offsetting.

A grep for const_usize.*align found the same code copied to rustc_codegen_gcc and cranelift but a quick skim didn't find other cases of similar 'optimization'. That said, I'm not convinced I caught everything, it's not trivial to search for this.

Closes #147516

rustbot · 2025-10-11T01:57:58Z

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

rustbot · 2025-10-11T01:58:00Z

r? @nnethercote

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tests/ui/consts/zst_no_llvm_alloc.rs

Mark-Simulacrum · 2025-10-12T12:33:01Z

@bors r=nnethercote

bors · 2025-10-12T12:33:04Z

📌 Commit 7a513dd has been approved by nnethercote

It is now in the queue for this repository.

…nnethercote Fix ICE on offsetted ZST pointer I'm not sure this is the *right* fix, but it's simple enough and does roughly what I'd expect. Like with the previous optimization to codegen usize rather than a zero-sized static, there's no guarantee that we continue returning a particular value from the offsetting. A grep for `const_usize.*align` found the same code copied to rustc_codegen_gcc and cranelift but a quick skim didn't find other cases of similar 'optimization'. That said, I'm not convinced I caught everything, it's not trivial to search for this. Closes rust-lang#147516

Rollup of 7 pull requests Successful merges: - #147168 (Don't unconditionally build alloc for `no-std` targets) - #147178 ([DebugInfo] Improve formatting of MSVC enum struct variants) - #147495 (Update wasm-component-ld to 0.5.18) - #147576 (Fix ICE on offsetted ZST pointer) - #147592 (Add tidy to the target of ./x check) - #147597 (Add a regression test for #72207) - #147604 (Some clippy cleanups in compiler) r? `@ghost` `@rustbot` modify labels: rollup

matthiaskrgr · 2025-10-12T17:07:20Z

@bors r-
#147609 (comment)

rustbot · 2025-10-12T17:46:34Z

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

rustbot · 2025-10-12T17:46:36Z

This PR was rebased onto a different master 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.

Mark-Simulacrum · 2025-10-12T17:47:22Z

@bors try jobs=dist-i586-gnu-i586-i686-musl

Fix ICE on offsetted ZST pointer try-job: dist-i586-gnu-i586-i686-musl

compiler/rustc_middle/src/mir/interpret/pointer.rs

compiler/rustc_codegen_llvm/src/common.rs

rust-bors · 2025-10-12T19:31:24Z

☀️ Try build successful (CI)
Build commit: 33c7e18 (33c7e186ed3534529f54d75674a864e6e1617999, parent: ff6dc928c5e33ce8e65c6911a790b9efcb5ef53a)

RalfJung · 2025-10-13T07:02:20Z

tests/ui/consts/zst_no_llvm_alloc.rs


+// This tests for regression of https://github.com/rust-lang/rust/issues/147516
+//
+// THe compiler will codegen `&Zst` without creating a real allocation, just a properly aligned


Suggested change

// THe compiler will codegen `&Zst` without creating a real allocation, just a properly aligned

// The compiler will codegen `&Zst` without creating a real allocation, just a properly aligned

tests/ui/consts/zst_no_llvm_alloc.rs

A grep for `const_usize.*align` found the same code copied to rustc_codegen_gcc but I don't see other cases where we get this wrong.

RalfJung · 2025-10-16T07:37:06Z

@bors r=nnethercote,RalfJung

bors · 2025-10-16T07:37:09Z

📌 Commit a8c79b8 has been approved by nnethercote,RalfJung

It is now in the queue for this repository.

Rollup of 9 pull requests Successful merges: - #144438 (Guard HIR lowered contracts with `contract_checks`) - #147000 (std: Add Motor OS std library port) - #147576 (Fix ICE on offsetted ZST pointer) - #147732 (remove duplicate inline macro) - #147738 (Don't highlight `let` expressions as having type `bool` in let-chain error messages) - #147744 (miri subtree update) - #147751 (Use `bit_set::Word` in a couple more places.) - #147752 (style-guide: fix typo for empty struct advice) - #147773 (`is_ascii` on an empty string or slice returns true) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #147576 - Mark-Simulacrum:fix-offset-zst, r=nnethercote,RalfJung Fix ICE on offsetted ZST pointer I'm not sure this is the *right* fix, but it's simple enough and does roughly what I'd expect. Like with the previous optimization to codegen usize rather than a zero-sized static, there's no guarantee that we continue returning a particular value from the offsetting. A grep for `const_usize.*align` found the same code copied to rustc_codegen_gcc and cranelift but a quick skim didn't find other cases of similar 'optimization'. That said, I'm not convinced I caught everything, it's not trivial to search for this. Closes #147516

bjorn3 · 2025-10-17T11:08:49Z

compiler/rustc_codegen_cranelift/src/constant.rs

-                            assert_eq!(offset, Size::ZERO);
-                            fx.bcx.ins().iconst(fx.pointer_type, alloc.inner().align.bytes() as i64)
+                            let val = alloc.inner().align.bytes().wrapping_add(offset.bytes());
+                            fx.bcx.ins().iconst(
+                                fx.pointer_type,
+                                fx.tcx.truncate_to_target_usize(val) as i64,
+                            )


The proper fix would have been to remove the assert_eq!() here and to do the truncate_to_target_usize at the fx.bcx.ins().iadd_imm(base_addr, i64::try_from(offset.bytes()).unwrap()) below. As is this still ICEs. Will push a fix to the cg_clif repo.
Edit: rust-lang/rustc_codegen_cranelift@ca23203

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Mark-Simulacrum force-pushed the fix-offset-zst branch 4 times, most recently from 060e453 to 7a513dd Compare October 12, 2025 10:55

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matthiaskrgr mentioned this pull request Oct 12, 2025

Rollup of 7 pull requests #147609

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Mark-Simulacrum force-pushed the fix-offset-zst branch from 7a513dd to d58c744 Compare October 12, 2025 17:46

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Auto merge of #147576 - Mark-Simulacrum:fix-offset-zst, r=<try>

33c7e18

Fix ICE on offsetted ZST pointer try-job: dist-i586-gnu-i586-i686-musl

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Mark-Simulacrum force-pushed the fix-offset-zst branch from d58c744 to b27c4ef Compare October 12, 2025 18:46

RalfJung reviewed Oct 13, 2025

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Fix ICE on offsetted ZST pointer

a8c79b8

A grep for `const_usize.*align` found the same code copied to rustc_codegen_gcc but I don't see other cases where we get this wrong.

Mark-Simulacrum force-pushed the fix-offset-zst branch from b27c4ef to a8c79b8 Compare October 16, 2025 00:06

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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 16, 2025

matthiaskrgr mentioned this pull request Oct 16, 2025

Rollup of 9 pull requests #147782

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bors merged commit 5408ec3 into rust-lang:master Oct 16, 2025
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	// THe compiler will codegen `&Zst` without creating a real allocation, just a properly aligned
	// The compiler will codegen `&Zst` without creating a real allocation, just a properly aligned

Fix ICE on offsetted ZST pointer #147576

Fix ICE on offsetted ZST pointer #147576

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