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Expand and clarify primitive alignment #1493

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Apr 20, 2024
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Expand and clarify primitive alignment #1493

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11 changes: 7 additions & 4 deletions src/type-layout.md
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Expand Up @@ -44,17 +44,20 @@ The size of most primitives is given in this table.
| `u32` / `i32` | 4 |
| `u64` / `i64` | 8 |
| `u128` / `i128` | 16 |
| `usize` / `isize` | See below |
| `f32` | 4 |
| `f64` | 8 |
| `char` | 4 |

`usize` and `isize` have a size big enough to contain every address on the
target platform. For example, on a 32 bit target, this is 4 bytes and on a 64
target platform. For example, on a 32 bit target, this is 4 bytes, and on a 64
bit target, this is 8 bytes.

Most primitives are generally aligned to their size, although this is
platform-specific behavior. In particular, on many 32-bit platforms `u64`
and `f64` are only aligned to 32 bits.
The alignment of primitives is platform-specific.
In most cases, their alignment is equal to their size, but it may be less.
In particular, `i128` and `u128` are often aligned to 4 or 8 bytes even though
their size is 16, and on many 32-bit platforms, `i64`, `u64`, and `f64` are only
aligned to 4 bytes, not 8.

## Pointers and References Layout

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