Re: ia32 suffix inconsistency

["H.J. Lu" <hjl dot tools at gmail dot com>]

On Thu, Dec 16, 2010 at 12:01 AM, Jan Beulich <JBeulich@novell.com> wrote:
>>>> On 15.12.10 at 17:58, "H.J. Lu" <hjl.tools@gmail.com> wrote:
>> On Wed, Dec 15, 2010 at 8:50 AM, Jan Beulich <JBeulich@novell.com> wrote:
>>> I don't follow: In what way is the "add" case above more
>>> ambiguous than the "mov" one? Both could mean byte, word,
>>> or dword accesses. And for the "add" case, if the default
>>> indeed is meant ot be 32-bits, there's *no* ambiguity wrt
>>> the width of the immediate (as with "addl", one can clearly
>>> expect the smaller to be picked as long as the immediate fits).
>>>
>>
>> I don't see any problems:
>
> How do you not see the gas error for the "mov" case? And how
> do you not see the absence of an error for the "add" case?
>
> Once again, for "mov", the assembler needs to pick one of
>
> - opcode c6
> - opcode c7 with operand size override
> - opcode c7 without operand size override
>
> and it selects the last one.
>
> For "add", the options are
>
> - opcode 0x80
> - opcode 0x81 with operand size override
> - opcode 0x81 without operand size override
> - opcode 0x83 with operand size override
> - opcode 0x83 without operand size override
>
> which it can't handle, despite it being obvious that if in the
> "mov" case it picks the 32-bit variant, after eliminating all
> non-32-bit variants it would have left to select between a
> single 0x81 and a single 0x83 case. This very selection is
> also what it successfully handles if the programmer
> used "addl", but without the suffix it can't - for no good
> reason.
>
> Btw., where is it specified that in AT&T 32-bit operand size
> ought to be the default? Alone the fact that it used to be
> that way in gas (or any other assembler) doesn't mean
> that's correct. Imo, minmally a warning should be issued.

Please file a bug for each issue you found.

Thanks.


H.J.
---
> Jan
>
>> [hjl@gnu-35 tmp]$ cat x.s
>> ?mov ? ? $1, (%eax)
>> [hjl@gnu-35 tmp]$ cat y.s
>> ?add ? ? $1, (%eax)
>> [hjl@gnu-35 tmp]$ cat x.s
>> ?mov ? ? $1, (%eax)
>> [hjl@gnu-35 tmp]$ gcc -c -m32 x.s
>> x.s: Assembler messages:
>> x.s:1: Error: no instruction mnemonic suffix given and no register
>> operands; can't size instruction
>> [hjl@gnu-35 tmp]$ cat y.s
>> ?add ? ? $1, (%eax)
>> [hjl@gnu-35 tmp]$ gcc -c -m32 y.s
>> [hjl@gnu-35 tmp]$ objdump -dw y.o
>>
>> y.o: ? ? file format elf32-i386
>>
>>
>> Disassembly of section .text:
>>
>> 00000000 <.text>:
>> ? ?0: 83 00 01 ? ? ? ? ? ? ? ?addl ? $0x1,(%eax)
>> [hjl@gnu-35 tmp]$
>>
>
>
>
>