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u128 alignment is incorrect on x86_64 #54949

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strega-nil opened this issue Oct 10, 2018 · 2 comments
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u128 alignment is incorrect on x86_64 #54949

strega-nil opened this issue Oct 10, 2018 · 2 comments

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@strega-nil
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strega-nil commented Oct 10, 2018 •
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In C and C++, on x86_64 Linux, alignof(__int128) is equal to 16. However, in Rust, align_of::<u128>() is equal to 8.

C++: https://gcc.godbolt.org/z/YAq1XC
Rust: https://gcc.godbolt.org/z/QvZeqK

This will cause subtle UB if you ever try to use i128s across FFI boundaries; for example:

C++: https://gcc.godbolt.org/z/PrtHlp
Rust: https://gcc.godbolt.org/z/_SdVqD
@strega-nil strega-nil changed the title u128 ABI is incorrect u128 alignment is incorrect on x86_64 Oct 10, 2018
@leonardo-m
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See also: #54341

@nagisa
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nagisa commented Oct 10, 2018

Duplicate of #54341, yes.

@nagisa nagisa closed this as completed Oct 10, 2018
bocharov added a commit to bocharov/entropy-map that referenced this issue Oct 18, 2024
@bocharov
BOTSML-464: Fix rkyv u128 aligment issue for ARM architectures.
1f5a988 
The `aarch64` and `x86_64` architectures have alignment differences for `u128` type:
* `aarch64` has 16 bytes alignment
* `x86_64` has 8 bytes aligment

In order to fix the issue we're going to use `[u8; 16]` instead of `u128`, which will always have alignment of 8 bytes and have backwards compatibility with existing `x86_64` serialized data.

See the following issues for more details:
* rust-lang/rust#54949
* rkyv/rkyv#409
* rust-lang/rust#54341
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@nagisa @strega-nil @leonardo-m