Setting insn mnemonic partly automagically

[Georg-Johann Lay]

Am 17.06.24 um 21:13 schrieb Stefan Schulze Frielinghaus via Gcc:
> Hi all,
> 
> I'm trying to add an alternative to an existing insn foobar:
> 
> (define_insn "foobar"
>    [(set (match_operand ...)
>          (match_operand ...))]
>    ""
>    "@
>     foo
>     bar
>     #")
> 
> Since the asm output depends on the operands in a non-trivial way which isn't
> easily solved via iterators, I went for a general C function and came up with:
> 
> (define_insn "foobar"
>    [(set (match_operand ...)
>          (match_operand ...))]
>    ""
>    "@
>     foo
>     * return foobar_helper (operands[0], operands[1]);
>     bar
>     #"
>    [(set_attr_alternative "mnemonic" [(const_string "foo")
>                                       (const_string "specialcase")
>                                       (const_string "bar")
>                                       (const_string "unknown")])])
> 
> If there exist a lot of alternatives, then setting the mnemonic attribute like
> this feels repetitive and is error prone.  Furthermore, if there exists no
> other insn with an output template containing foo/bar, then I would have to
> declare foo/bar via
> 
> (define_attr "mnemonic" "...,foo,bar,..." (const_string "unknown"))
> 
> which again is repetitive.  Thus, I'm wondering if there exists a more elegant
> way to achieve this?  Ultimately, I would like to set the mnemonic
> attribute only manually for the alternative which is implemented via C
> code and let the mnemonic attribute for the remaining alternatives be
> set automagically.  Not sure whether this is supported?
> 
> If all fails, I have another idea how to solve this by utilizing PRINT_OPERAND.
> However, now I'm curious whether my current attempt is feasible or not.
> 
> Cheers,
> Stefan

It's a bit unclear to me what you are trying to do, as you are not only
adding an insn alternative, but also are adding insn attribute
"mnemonic", which the original insn did not have.

Also, it's unclear how PRINT_OPERAND would help with setting the attribute.

Johann