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document how to use memory addresses as operands

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Co-authored-by: Amanieu <amanieu@gmail.com>
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Lzu Tao 2020-09-14 04:59:55 +00:00
parent 8d96cc238e
commit e82be710e4
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src/doc/unstable-book/src/library-features/asm.md
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@ -345,6 +345,25 @@ The `h` modifier will emit the register name for the high byte of that register
If you use a smaller data type (e.g. `u16`) with an operand and forget the use template modifiers, the compiler will emit a warning and suggest the correct modifier to use.
## Memory address operands
Sometimes assembly instructions require operands passed via memory addresses/memory locations.
You have to manually use the memory address syntax specified by the respectively architectures.
For example, in x86/x86_64 and intel assembly syntax, you should wrap inputs/outputs in `[]`
to indicate they are memory operands:
```rust,allow_fail
# #![feature(asm, llvm_asm)]
# fn load_fpu_control_word(control: u16) {
unsafe {
asm!("fldcw [{}]", in(reg) &control, options(nostack));
// Previously this would have been written with the deprecated `llvm_asm!` like this
llvm_asm!("fldcw $0" :: "m" (control) :: "volatile");
}
# }
```
## Options
By default, an inline assembly block is treated the same way as an external FFI function call with a custom calling convention: it may read/write memory, have observable side effects, etc. However in many cases, it is desirable to give the compiler more information about what the assembly code is actually doing so that it can optimize better.