Changes to `u128`/`i128` layout in 1.77 and 1.78 | Rust Blog
Rust has long had an inconsistency with C regarding the alignment of 128-bit integers on the x86-32 and x86-64 architectures. This problem has recently been resolved, but the fix comes with some effects that are worth being aware of.
There are also no changes to architectures other than x86-32 and x86-64. If your code makes heavy use of 128-bit integers, you may notice runtime performance increases at a possible cost of additional memory use.
This post documents what the problem was, what changed to fix it, and what to expect with the changes. If you are already familiar with the problem and only looking for a compatibility matrix, jump to the Compatibility section.
Data types have two intrinsic values that relate to how they can be arranged in memory; size and alignment. A type's size is the amount of space it takes up in memory, and its alignment specifies which addresses it is allowed to be placed at.
The size of simple types like primitives is usually unambiguous, being the exact size of the data they represent with no padding (unused space). For example, an i64 always has a size of 64 bits or 8 bytes.
