[llvm-dev] Improving the rich disassembly output

[Alain Carlucci via llvm-dev]

Hi,

I'm trying to use the annotation support for the LLVM disassembler, which seems
to be implemented only on ARM and x86.

It was easy to add annotations also on SystemZ, AArch64 and MIPS (at least
<imm:> and <reg:>) and I'm going to send you soon all the patches.

But now I'm facing a problem with the annotations in x86: there are some
instructions that emit registers directly from the AsmString written in the
TableGen file.

For example, llvm/lib/Target/X86/X86InstrInfo.td:1631, this line defines the
instruction with the `al` register hardcoded, which is emitted without markup:
```
def MOV8o32a : Ii32<0xA2, RawFrmMemOffs, (outs), (ins offset32_8:$dst),
                     "mov{b}\t{%al, $dst|$dst, al}", []>, AdSize32;
```

In this case, I tried to change "al" with "$al" (at first by creating a new
RegisterClass "AL8" with only one register (AL) and forwarding it to MOV8o32a)
but I got a crash while trying to emit the instruction. 
And, honestly, creating RegisterClass with only one register doesn't seem to be
the best way to approach this problem.

For the record, the crash was:
```
Unhandled relocation encoding AL8
Unhandled relocation encoding
UNREACHABLE executed at llvm/llvm/utils/TableGen/X86RecognizableInstr.cpp:1212!
```

There are instructions like ADC64i32, ADD64i32 that emit an hardcoded register
in the generated file (something like `O << ", %rax"`) in 
build/lib/Target/X86/X86GenAsmWriter.inc but I can't find them in the any
tablegen file. 

So I'd like to ask you, can you give me some advice about how to enable
annotations for this kind of instructions?

Thanks,
Alain