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Merge pull request #1692 from LingMan/patch-1
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Fix two typos in the asm chapter
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marioidival committed Mar 21, 2023
2 parents 2fe3c95 + f0a086d commit cfbfd64
Showing 1 changed file with 2 additions and 2 deletions.
4 changes: 2 additions & 2 deletions src/unsafe/asm.md
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Expand Up @@ -291,9 +291,9 @@ This instruction writes to `eax` with the maximum supported `cpuid` argument and

Even though `eax` is never read we still need to tell the compiler that the register has been modified so that the compiler can save any values that were in these registers before the asm. This is done by declaring it as an output but with `_` instead of a variable name, which indicates that the output value is to be discarded.

This code also works around the limitation that `ebx` is a reserved register by LLVM. That means that LLVM assumes that it has full control over the register and it must be restored to its original state before exiting the asm block, so it cannot be used as an input or output **except** if the compiler uses it to fulfill a general register class (e.g. `in(reg)`). This makes `reg` operands dangerous when using reserved registers as we could unknowingly corrupt out input or output because they share the same register.
This code also works around the limitation that `ebx` is a reserved register by LLVM. That means that LLVM assumes that it has full control over the register and it must be restored to its original state before exiting the asm block, so it cannot be used as an input or output **except** if the compiler uses it to fulfill a general register class (e.g. `in(reg)`). This makes `reg` operands dangerous when using reserved registers as we could unknowingly corrupt our input or output because they share the same register.

To work around this we use `rdi` to store the pointer to the output array, save `ebx` via `push`, read from `ebx` inside the asm block into the array and then restoring `ebx` to its original state via `pop`. The `push` and `pop` use the full 64-bit `rbx` version of the register to ensure that the entire register is saved. On 32 bit targets the code would instead use `ebx` in the `push`/`pop`.
To work around this we use `rdi` to store the pointer to the output array, save `ebx` via `push`, read from `ebx` inside the asm block into the array and then restore `ebx` to its original state via `pop`. The `push` and `pop` use the full 64-bit `rbx` version of the register to ensure that the entire register is saved. On 32 bit targets the code would instead use `ebx` in the `push`/`pop`.

This can also be used with a general register class to obtain a scratch register for use inside the asm code:

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