[PATCH 2/2] x86/APX: support JMPABS also in assembler

[Jan Beulich]

On 08.10.2024 23:20, H.J. Lu wrote:
> On Wed, Oct 9, 2024 at 5:00 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>> On Tue, Oct 8, 2024 at 6:26 PM Jan Beulich <jbeulich@suse.com> wrote:
>>> Without this APX support isn't really complete.
>>>
>>> Since which operand type to expect in source is ambiguous, permit both
>>> immediate and displacement forms. Make sure though that in Intel syntax
>>> truly memory operand forms (<xyz> ptr and/or square-bracketing) aren't
>>> accepted. Also make sure that no illegal prefixes can be used with the
>>> insn (LOCK is being dealt with more generally elsewhere).

(Note this paragraph of the description.)

>>> --- a/opcodes/i386-opc.tbl
>>> +++ b/opcodes/i386-opc.tbl
>>> @@ -510,6 +510,11 @@ ljmp, 0xea, No64, JumpInterSegment|No_bS
>>>  ljmp, 0xff/5, 0, Amd64|Modrm|JumpAbsolute|No_bSuf|No_sSuf|No_qSuf, { Unspecified|BaseIndex }
>>>  ljmp, 0xff/5, x64, Intel64|Modrm|JumpAbsolute|No_bSuf|No_sSuf, { Unspecified|BaseIndex }
>>>
>>> +// In particular for AT&T syntax permit both immediate and displacement form,
>>> +// to allow people to use what they see as the better fit.
>>
>> Why do we need "jmpabs $xxx" when it is encoded the same
>> as "jmpabs xxx"?  "jmpabs xxx" is sufficient.
> 
> Since disassembler displays "jmpabs $xxx", only "jmpabs $xxx" is
> needed.  It is clear that jmpabs doesn't take a displacement and
> only immediate is supported.

Already by the two replies of yours you demonstrate that both views are
possible. And you're not the first one to be confused / have mixed
feelings.

As indicated in a post-commit-message remark I'm also intending to
relax ljmp's and lcall's requirements. I've always viewed it as a
mistake that only immediates were supported there for the offset operand.
That's simply not consistent.

I'm further inclined to suggest that the disassembler better display the
operand without the leading $ (in AT&T mode), at least when it's not a
purely numeric one (and in particular when it has a relocation associated
with it).

Jan