Re: [PATCH 1/3] RISC-V: prefer SLT{,U} aliases for SLTI{,U}

[Andrew Waterman <andrew@sifive.com>]

On Wed, Jan 25, 2023 at 7:22 AM Maciej W. Rozycki <macro@orcam.me.uk> wrote:
>
> On Wed, 25 Jan 2023, Jan Beulich wrote:
>
> > >  This is however what these instructions have been named in the ISA and
> > > the assembly dialect.  In the case of NOP, MOVE, etc. mnemonics they are
> > > significant assembly idioms (usually mentioned in the ISA manual) and
> > > there are sometimes thousands of alternative encodings that could be used
> > > to effect the same operation, but only the chosen canonical encoding is
> > > disassembled this way.
> >
> > Aren't you changing topics? Being able to use alternative encodings to
> > achieve the same effect isn't what we were talking about.
>
>  No, it just gives you background as to why some encodings are given
> canonical aliases (used for disassembly) and why some are not.
>
> > >  It's rather how the assembly language has been designed (FWIW the RISC-V
> > > ISA and assembly dialect have been largely inspired by the MIPS approach).
> >
> > Well, such a design imo ought to include a clear statement on uses of
> > aliases. Iirc at least the 32-bit Arm ARM is very precise about what
> > aliases exist, and it effectively mandates for at least some of them
> > that they should be use in disassembly.
> >
> > As said before, I'd be happy to see things move in about any direction,
> > just as long as the result is consistent and hence observable behavior
> > is predictable for users of the assembler and disassembler.
>
>  It's been consistent so far AFAICT for the RISC-V assembly dialect (and
> for that matter for the MIPS one as well).  If you disagree, then you're
> welcome to present your view, but I think the context of libopcodes and
> the binutils mailing list is not the correct place to discuss the assembly
> language syntax.  You'd need to take it to the RISC-V ISA maintainters and
> then we can implement whatever they've agreed to.

Precisely specifying the assembly syntax has been a weak spot of the
RISC-V specs, but the general philosophy has been to encourage the use
of aliases in situations that are seemingly obvious (e.g. ret vs. jalr
x0, x1, 0) and to be consistent within a family of instructions (e.g.
since add is an alias for addi, xor had better be an alias for xori).
On the disassembly side, the philosophy has been that more
human-readable aliases should be used when appropriate (e.g. ret, not
jalr x0, x1, 0), unless -Mno-aliases is specified (in which case e.g.
jalr x0, x1, 0 should be printed).

The right place to document this stuff is here:
https://github.com/riscv-non-isa/riscv-asm-manual/blob/master/riscv-asm.md
(I don't own this spec, so I'd recommend reporting problems or feature
requests on the github issue tracker, rather than replying here).

>
>   Maciej