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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