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Jun 2, 2022
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unsafe::asm.md: add some explicit declarations #1548

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unsafe::asm.md: add some explicit declarations
PeterWrighten Jun 2, 2022
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fix some syntax errors
PeterWrighten Jun 2, 2022
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5 changes: 3 additions & 2 deletions src/unsafe/asm.md
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Expand Up @@ -132,7 +132,7 @@ To achieve this Rust provides a `lateout` specifier. This can be used on any out
written only after all inputs have been consumed.
There is also a `inlateout` variant of this specifier.

Here is an example where `inlateout` *cannot* be used:
Here is an example where `inlateout` *cannot* be used in `release` mode or other optimized cases:

```rust
use std::arch::asm;
Expand All @@ -151,8 +151,9 @@ unsafe {
}
assert_eq!(a, 12);
```
The above could work well in unoptimized cases (`Debug` mode), but if you want optimized performance (`release` mode or other optimized cases), it could not work.

Here the compiler is free to allocate the same register for inputs `b` and `c` since it knows they have the same value. However it must allocate a separate register for `a` since it uses `inout` and not `inlateout`. If `inlateout` was used, then `a` and `c` could be allocated to the same register, in which case the first instruction to overwrite the value of `c` and cause the assembly code to produce the wrong result.
That is because in optimized cases, the compiler is free to allocate the same register for inputs `b` and `c` since it knows they have the same value. However it must allocate a separate register for `a` since it uses `inout` and not `inlateout`. If `inlateout` was used, then `a` and `c` could be allocated to the same register, in which case the first instruction to overwrite the value of `c` and cause the assembly code to produce the wrong result.

However the following example can use `inlateout` since the output is only modified after all input registers have been read:

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