Merge pull request #1548 from PeterWrighten/master

unsafe::asm.md: add some explicit declarations

Author: marioidival

src/unsafe/asm.md

@@ -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;
@@ -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: