From: LIU Hao <lh_mouse@126.com>
To: Fangrui Song <i@maskray.me>
Cc: binutils@sourceware.org, GCC Development <gcc@gcc.gnu.org>
Subject: Re: RFC: Formalization of the Intel assembly syntax (PR53929)
Date: Sat, 20 Jan 2024 20:32:17 +0800 [thread overview]
Message-ID: <6d541bb6-4b06-4d2f-9193-53f0ab1241ae@126.com> (raw)
In-Reply-To: <DS7PR12MB5765654642BE3AD4C7F54E05CB702@DS7PR12MB5765.namprd12.prod.outlook.com>
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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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prev parent reply other threads:[~2024-01-20 12:32 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-18 5:34 LIU Hao
2024-01-18 9:02 ` Fangrui Song
2024-01-18 12:54 ` Jan Beulich
2024-01-18 16:40 ` LIU Hao
2024-01-19 9:13 ` Jan Beulich
2024-01-20 12:40 ` LIU Hao
2024-01-22 8:39 ` Jan Beulich
2024-01-23 1:27 ` LIU Hao
2024-01-23 8:38 ` Jan Beulich
2024-01-23 9:00 ` LIU Hao
2024-01-23 9:03 ` Jan Beulich
2024-01-23 9:21 ` LIU Hao
2024-01-23 9:37 ` Jan Beulich
2024-01-30 4:22 ` Hans-Peter Nilsson
2024-01-31 10:11 ` LIU Hao
[not found] ` <DS7PR12MB5765DBF9500DE323DB4A8E29CB712@DS7PR12MB5765.namprd12.prod.outlook.com>
2024-01-19 1:42 ` LIU Hao
2024-01-19 7:41 ` Jan Beulich
2024-01-19 8:19 ` Fangrui Song
[not found] ` <DS7PR12MB5765654642BE3AD4C7F54E05CB702@DS7PR12MB5765.namprd12.prod.outlook.com>
2024-01-20 12:32 ` LIU Hao [this message]
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