[PATCH v2] x86: Add {imm8}/{imm16}/{imm32} pseudo prefixes

[H.J. Lu]

On Tue, Mar 25, 2025, 12:37 AM Jan Beulich <jbeulich@suse.com> wrote:

> On 24.03.2025 19:41, H.J. Lu wrote:
> > On Mon, Mar 24, 2025 at 9:27 AM Jan Beulich <jbeulich@suse.com> wrote:
> >>
> >> On 24.03.2025 17:24, H.J. Lu wrote:
> >>> Instruction templates with only sign-extended 8-bit immediate operand
> >>> also have a second template with full-operand-size immediate operand
> >>> under a different opcode.  Add {imm8}, {imm16} and {imm32} pseudo
> >>> prefixes as a hint to control immediate size.  Don't match the first
> >>> template if {imm16} or {imm32} are used.
> >>>
> >>> gas/
> >>>
> >>>       PR gas/32811
> >>>       * config/tc-i386.c (pseudo_prefixes): Add avoid_imm8s.
> >>>       (operand_size_match): Return false for templates with only sign-
> >>>       extended 8-bit immediate operand if {imm16} or {imm32} are used.
> >>>       (parse_insn): Handle Prefix_Imm8, Prefix_Imm16 and Prefix_Imm32.
> >>>       * doc/c-i386.texi: Document {imm8}, {imm16} and {imm32}.
> >>>       * testsuite/gas/i386/pseudos.s: Add tests for {imm8}, {imm16}
> >>>       and {imm32}.
> >>>       * testsuite/gas/i386/x86-64-pseudos.s: Likewise.
> >>>       * testsuite/gas/i386/pseudos.d: Updated.
> >>>       * testsuite/gas/i386/x86-64-pseudos.d: Likewise.
> >>>
> >>> opcodes/
> >>>
> >>>       PR gas/32811
> >>>       * opcodes/i386-opc.h (Prefix_Imm8): New.
> >>>       (Prefix_Imm16): Likewise.
> >>>       (Prefix_Imm32): Likewise.
> >>>       * i386-opc.tbl: Add {imm8}, {imm16} and {imm32} pseudo prefixes.
> >>>       * i386-mnem.h: Regenerated.
> >>>       * i386-tbl.h: Likewise.
> >>>
> >>> Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
> >>
> >> What's the difference to v1? Importantly {imm16} and {imm32} are still
> there,
> >
> > full-operand-size and avoid_imm8s.
> >
> >> when imo they should be suitably folded.
> >>
> >
> > {imm16} and {imm32} are the same internally.  But for users,
> >
> > {imm16} add $0,%bx
> >  {imm8} add $0,%bx
> >
> > are quite clearer than other alternatives.
>
> And
>
>     {imm16} add $0,%ebx
>
> is, I'm sorry to say so, rubbish. Which however we happily accept to mean
>
>     {imm32} add $0,%ebx
>
> Thinking about it, unlike {disp8} - what exact purpose does {imm8} serve?
> We encode with the smallest possible immediate anyway. {disp8} is useful
> to request a displacement when none would be needed, yet the same situation
> doesn't arise for immediates afaict. Hence the sole use looks to be to
> "override" an earlier {imm16} or {imm32}. I'm not entirely certain that's
> worth it.
>

How about {noimm8s}?


> Jan
>
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