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Add note about x86 instruction prefixes in asm! to unstable book #90533

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Nov 5, 2021
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Add note about x86 instruction prefixes in asm! to unstable book #90533

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Add note about x86 instruction prefixes in asm!
syvb Nov 3, 2021
773cc4f
Mention possible future rejections
syvb Nov 4, 2021
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2 changes: 2 additions & 0 deletions src/doc/unstable-book/src/library-features/asm.md
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Expand Up @@ -885,5 +885,7 @@ The compiler performs some additional checks on options:
- You are responsible for switching any target-specific state (e.g. thread-local storage, stack bounds).
- The set of memory locations that you may access is the intersection of those allowed by the `asm!` blocks you entered and exited.
- You cannot assume that an `asm!` block will appear exactly once in the output binary. The compiler is allowed to instantiate multiple copies of the `asm!` block, for example when the function containing it is inlined in multiple places.
- On x86, inline assembly must not end with an instruction prefix (such as `LOCK`) that would apply to instructions generated by the compiler.
- The compiler is currently unable to detect this due to the way inline assembly is compiled, but may catch and reject this in the future.

> **Note**: As a general rule, the flags covered by `preserves_flags` are those which are *not* preserved when performing a function call.