Re: [PATCH 1/3] RISC-V: re-arrange opcode table for consistent alias handling

[Fangrui Song <i@maskray.me>]

On Fri, Sep 16, 2022 at 2:53 AM Nelson Chu <nelson@rivosinc.com> wrote:
>
> On Thu, Sep 15, 2022 at 3:42 PM Jan Beulich <jbeulich@suse.com> wrote:
> >
> > On 15.09.2022 04:30, Nelson Chu wrote:
> > > On Tue, Sep 13, 2022 at 9:02 PM Jan Beulich <jbeulich@suse.com> wrote:
> > >> --- a/opcodes/riscv-opc.c
> > >> +++ b/opcodes/riscv-opc.c
> > >> @@ -290,9 +290,9 @@ const struct riscv_opcode riscv_opcodes[
> > >>  {"jalr",        0, INSN_CLASS_I, "d,s,j",     MATCH_JALR, MASK_JALR, match_opcode, INSN_JSR },
> > >>  {"j",           0, INSN_CLASS_C, "Ca",        MATCH_C_J, MASK_C_J, match_opcode, INSN_ALIAS|INSN_BRANCH },
> > >>  {"j",           0, INSN_CLASS_I, "a",         MATCH_JAL, MASK_JAL|MASK_RD, match_opcode, INSN_ALIAS|INSN_BRANCH },
> > >> +{"jal",         0, INSN_CLASS_I, "a",         MATCH_JAL|(X_RA << OP_SH_RD), MASK_JAL|MASK_RD, match_opcode, INSN_ALIAS|INSN_JSR },
> > >>  {"jal",         0, INSN_CLASS_I, "d,a",       MATCH_JAL, MASK_JAL, match_opcode, INSN_JSR },
> > >>  {"jal",        32, INSN_CLASS_C, "Ca",        MATCH_C_JAL, MASK_C_JAL, match_opcode, INSN_ALIAS|INSN_JSR },
> > >> -{"jal",         0, INSN_CLASS_I, "a",         MATCH_JAL|(X_RA << OP_SH_RD), MASK_JAL|MASK_RD, match_opcode, INSN_ALIAS|INSN_JSR },
> > >>  {"call",        0, INSN_CLASS_I, "d,c",       (X_T1 << OP_SH_RS1), (int) M_CALL, match_never, INSN_MACRO },
> > >>  {"call",        0, INSN_CLASS_I, "c",         (X_RA << OP_SH_RS1)|(X_RA << OP_SH_RD), (int) M_CALL, match_never, INSN_MACRO },
> > >>  {"tail",        0, INSN_CLASS_I, "c",         (X_T1 << OP_SH_RS1), (int) M_CALL, match_never, INSN_MACRO },
> > >> @@ -310,13 +310,13 @@ const struct riscv_opcode riscv_opcodes[
> > >>  {"move",        0, INSN_CLASS_C, "d,CV",      MATCH_C_MV, MASK_C_MV, match_c_add, INSN_ALIAS },
> > >>  {"move",        0, INSN_CLASS_I, "d,s",       MATCH_ADDI, MASK_ADDI|MASK_IMM, match_opcode, INSN_ALIAS },
> > >>  {"zext.b",      0, INSN_CLASS_I, "d,s",       MATCH_ANDI|ENCODE_ITYPE_IMM (255), MASK_ANDI | MASK_IMM, match_opcode, INSN_ALIAS },
> > >> -{"andi",        0, INSN_CLASS_C, "Cs,Cw,Co",  MATCH_C_ANDI, MASK_C_ANDI, match_opcode, INSN_ALIAS },
> > >> -{"andi",        0, INSN_CLASS_I, "d,s,j",     MATCH_ANDI, MASK_ANDI, match_opcode, 0 },
> > >>  {"and",         0, INSN_CLASS_C, "Cs,Cw,Ct",  MATCH_C_AND, MASK_C_AND, match_opcode, INSN_ALIAS },
> > >>  {"and",         0, INSN_CLASS_C, "Cs,Ct,Cw",  MATCH_C_AND, MASK_C_AND, match_opcode, INSN_ALIAS },
> > >>  {"and",         0, INSN_CLASS_C, "Cs,Cw,Co",  MATCH_C_ANDI, MASK_C_ANDI, match_opcode, INSN_ALIAS },
> > >>  {"and",         0, INSN_CLASS_I, "d,s,t",     MATCH_AND, MASK_AND, match_opcode, 0 },
> > >>  {"and",         0, INSN_CLASS_I, "d,s,j",     MATCH_ANDI, MASK_ANDI, match_opcode, INSN_ALIAS },
> > >> +{"andi",        0, INSN_CLASS_C, "Cs,Cw,Co",  MATCH_C_ANDI, MASK_C_ANDI, match_opcode, INSN_ALIAS },
> > >> +{"andi",        0, INSN_CLASS_I, "d,s,j",     MATCH_ANDI, MASK_ANDI, match_opcode, 0 },
> > >
> > > Doesn't ANDI a base instruction?
> >
> > Of course. Like for all aliases, there is a corresponding base
> > instruction. I guess I simply don't understand what you mean to
> > express with the question.
> >
> > >  The operand "d,s,j" of AND is an
> > > alias of ANDI, so the original order seems correct.  Always dump *.i
> > > instructions to the non-i type looks weird, and llvm-dump seems has
> > > the same behavior as current GNU objdump.
> > >
> > > % cat tmp.s
> > > and a0, a1, 0x10
> > > % riscv64-unknown-elf-as tmp.s -o tmp.o
> > > % riscv64-unknown-elf-objdump -d tmp.o
> > >
> > > tmp.o:     file format elf64-littleriscv
> > >
> > >
> > > Disassembly of section .text:
> > >
> > > 0000000000000000 <.text>:
> > >
> > >    0: 0105f513          and a0,a1,16
> >
> > What's weird about that? And if that's weird, would you mind spelling
> > out the conditions under which aliases are to be preferred over base
> > instructions when disassembling? There actually is a "These aliases are
> > for assembly but not disassembly" comment somewhere in the file,
> > clarifying for two of the aliases that they ought to come after their
> > base insns. But for all other aliases which aren't simply a different
> > (but not shorter) name for the same insn (e.g. "bgt" vs "blt") I'd
> > assume the aliases should be preferred, for the reason stated in the
> > patch description. That said - I can see it being a matter of taste
> > for <insn>i vs <insn>, but if so this should be spelled out somewhere.
>
> Yeah, that's what I worried about.  At the beginning, I think dumping
> a base instruction as another base instruction looks weird.  But these
> days I also noticed that - we also dump compressed instructions as
> base i without "c." prefixes, so why I feel weird is just that I'm
> used to it because of historical behavior.  I have no objection to
> this, so please go ahead if there are no objections for a period of
> time.  But if there are any objections, then we probably can mark
> these aliases by something like INSN_ALIAS_CANNOT_DUMP in the opcode
> table, and that's what Kito suggested to me before, but I didn't think
> it was a serious problem at the time.
>
> Thanks
> Nelson

I apologize as I haven't read all prior discussions. For many
instructions, the "i" form is written in the ISA manual and prevalent.
I wonder whether we can give these add/and/xor/etc without "i" lower
priority so that objdump -d will not show them, even without using -M
no-aliases.

% cat b.s
add a0,a1,13
and a2,a3,4
xor a2,a3,4
or a2,a3,4
sll a2,a3,4
% riscv64-linux-gnu-gcc -c b.s
% ~/Dev/binutils-gdb/out/riscv64/binutils/objdump -d b.o

b.o:     file format elf64-littleriscv


Disassembly of section .text:

0000000000000000 <.text>:
   0:   00d58513                add     a0,a1,13
   4:   0046f613                and     a2,a3,4
   8:   0046c613                xor     a2,a3,4
   c:   0046e613                or      a2,a3,4
  10:   00469613                sll     a2,a3,0x4


When LLVM integrated assembler added these aliases
(https://reviews.llvm.org/D50046), these instructions are assigned a
low priority "let EmitPriority = 0" so llvm-objdump -d will never show
them.

> > > % llvm-objdump -d tmp.o
> > >
> > > tmp.o: file format elf64-littleriscv
> > >
> > > Disassembly of section .text:
> > >
> > > 0000000000000000 <$x>:
> > >        0: 13 f5 05 01  andi a0, a1, 16
> >
> > I don't think I view llvm as a "canonical reference". Perhaps they merely
> > followed the GNU behavior?
> >
> > Jan