RFC: Formalization of the Intel assembly syntax (PR53929)

[LIU Hao]

在 2024-01-19 16:19, Fangrui Song 写道:
> It is also unfortunate that whether the displacement is an immediate
> or not change the behavior of brackets.
> 
> mov eax, DWORD PTR 0          # mov    $0x0,%eax
> mov eax, DWORD PTR [0]        # mov    0x0,%eax
> mov eax, DWORD PTR sym        # mov    0x0,%eax with relocation
> mov eax, DWORD PTR [sym]      # mov    0x0,%eax with relocation
> 
> The above reveals yet another inconsistency. For a memory reference,
> it seems that we should use [] but [sym] could be ambiguous if sym
> matches a register name or operator name.

This is sort of tautology, as `DWORD PTR` and `[]` means the same thing. It's unfortunate that 
neither could replace the other; not to mention the `DWORD BCST` thing in AVX512.

That is to say, these two

    mov eax, DWORD PTR 0
    mov eax, DWORD PTR [0]

should denote the same operation. This is more useful with a segment override, to access 
thread-local data, as in

   mov rax, QWORD PTR 0         #    48 8B0425 00000000
   mov rax, QWORD PTR gs:0      # 65 48 8B0425 00000000


> Does the proposal change the placement of the displacement depending
> on whether it is an immediate?
> This is inconsistent, but perhaps there is not much we can improve...

The proposal is about elimination of ambiguity. Roughly speaking, an identifier that follows `PTR`, 
`BCST` or `OFFSET` is to be interpreted as a symbol, and an identifier that appears with in a pair 
of brackets is to be interpreted as a register.

As a numeric displacement does not cause ambiguity, it can be accepted either way.



-- 
Best regards,
LIU Hao

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