Implement ptr::{read,write}_unaligned via repr(packed)#158427
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scottmcm
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| // CHECK-NOT: !noundef | ||
| // CHECK-NOT: !range |
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Obviously it would be better to have metadata on these loads, but they're not there in nightly today either (https://rust.godbolt.org/z/753TYoa1Y) so this isn't a regression.
Should be better thanks to mir opts in future, though, without needing additional library changes.
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...or could also be fixed by rust-lang/compiler-team#1007 and using repr(Rust), if that MCP lands.
aapoalas
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Jul 1, 2026
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…d, r=aapoalas
Implement `ptr::{read,write}_unaligned` via `repr(packed)`
We already support doing unaligned reads & writes via `repr(packed)` fields, so this just uses that support from the backend rather than needing to thinks about `memcpy` and `intrinsics::forget` and such.
Turns out in codegen this ends up essentially identical because the packed type read gets `BackendRepr::Memory` and thus the read/write of the packed type *is* an `align 1` memcpy, just like the library code before this PR. But doing this gives much simpler MIR and rust-lang#158291 will make it better than the previous version by not needing to emit the `memcpy` at all for things like `read_unaligned::<u32>`.
First commit are some tests that pass on master; Second commit changes the implementation and shows the corresponding test changes.
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…uwer Rollup of 6 pull requests Successful merges: - #156737 (Implement `DoubleEndedIterator::next_chunk_back`) - #158147 (std: fix stack buffer overflow in Windows junction_point) - #158180 (std: use `OnceLock` for SGX environment variable storage) - #158427 (Implement `ptr::{read,write}_unaligned` via `repr(packed)`) - #158531 (Change `adjust_ident_and_get_scope` arg to `LocalDefId`) - #158364 (rustc_target/asm: add LoongArch LSX/LASX inline asm register support)
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Rollup of 8 pull requests Successful merges: - #156737 (Implement `DoubleEndedIterator::next_chunk_back`) - #158180 (std: use `OnceLock` for SGX environment variable storage) - #158427 (Implement `ptr::{read,write}_unaligned` via `repr(packed)`) - #158531 (Change `adjust_ident_and_get_scope` arg to `LocalDefId`) - #158574 (Clarify ExitStatusExt documentation) - #158334 (rustdoc: Show use-site paths for unevaluated const array lengths) - #158364 (rustc_target/asm: add LoongArch LSX/LASX inline asm register support) - #158667 (rustc_sanitizers: use twox-hash without default features)
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We already support doing unaligned reads & writes via
repr(packed)fields, so this just uses that support from the backend rather than needing to thinks aboutmemcpyandintrinsics::forgetand such.Turns out in codegen this ends up essentially identical because the packed type read gets
BackendRepr::Memoryand thus the read/write of the packed type is analign 1memcpy, just like the library code before this PR. But doing this gives much simpler MIR and #158291 will make it better than the previous version by not needing to emit thememcpyat all for things likeread_unaligned::<u32>.First commit are some tests that pass on master; Second commit changes the implementation and shows the corresponding test changes.
r? libs