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several fixups

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Alexis Beingessner 2015-07-13 22:36:09 -07:00
parent d4268f9447
commit d96a518316
1 changed files with 25 additions and 15 deletions
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40
other-reprs.md
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@ -26,9 +26,26 @@ the FFI boundary.
* DSTs, tuples, and tagged unions are not a concept in C and as such are never
FFI safe.
* **The drop flag will still be added**
* **The [drop flag][] will still be added**
* This is equivalent to `repr(u32)` for enums (see below)
* This is equivalent to one of `repr(u\*)` (see the next section) for enums. The
chosen size is the default enum size for the target platform's C ABI. Note that
enum representation in C is undefined, and this may be incorrect when the C
code is compiled with certain flags.
# repr(u8), repr(u16), repr(u32), repr(u64)
These specify the size to make a C-like enum. If the discriminant overflows the
integer it has to fit in, it will be an error. You can manually ask Rust to
allow this by setting the overflowing element to explicitly be 0. However Rust
will not allow you to create an enum where two variants have the same discriminant.
On non-C-like enums, this will inhibit certain optimizations like the null-pointer
optimization.
These reprs have no affect on a struct.
@ -40,22 +57,15 @@ byte. This may improve the memory footprint, but will likely have other
negative side-effects.
In particular, most architectures *strongly* prefer values to be aligned. This
may mean the unaligned loads are penalized (x86), or even fault (ARM). In
particular, the compiler may have trouble with references to unaligned fields.
may mean the unaligned loads are penalized (x86), or even fault (some ARM chips).
For simple cases like directly loading or storing a packed field, the compiler
might be able to paper over alignment issues with shifts and masks. However if
you take a reference to a packed field, it's unlikely that the compiler will be
able to emit code to avoid an unaligned load.
`repr(packed)` is not to be used lightly. Unless you have extreme requirements,
this should not be used.
This repr is a modifier on `repr(C)` and `repr(rust)`.
# repr(u8), repr(u16), repr(u32), repr(u64)
These specify the size to make a C-like enum. If the discriminant overflows the
integer it has to fit in, it will be an error. You can manually ask Rust to
allow this by setting the overflowing element to explicitly be 0. However Rust
will not allow you to create an enum where two variants.
These reprs have no affect on a struct or non-C-like enum.
[drop flag]: drop-flags.html