RFC: AArch64 Disassembler: Annotate undefined instructions

[Richard Earnshaw (foss)]

On 29/04/2026 12:03, Nick Clifton wrote:
> Hi Richard,
> 
>>> OK, that makes sense.  So the disassembler should check to see if closest
>>> previous mapping symbol is $d and only proceed with the annotation in this
>>> case, yes ?
> 
>> Do you even need to do that?  Just annotate the disassembly of .word values.
> 
> But the disassembler only knows if an "undefined" instruction is actually
> meant to be data by looking at the mapping symbols.
> 
> This is the test that I am now using in the v2 version of the patch (not
> posted yet as I am waiting to see if there are more comments):
> 
>      /* See if this "undefined instruction" is actually the address of something.  */
>       if (annotate_undefined_insns
>       /* Skip values that have been explicitly tagged as code.  */
>       && last_type == MAP_DATA
>       /* Skip static object files as symbol values have not be resolved yet.  */
>       && info->section != NULL
>       && info->section->owner != NULL
>       && (info->section->owner->flags & (EXEC_P | DYNAMIC)))
>        { ... do the annotation ... }
> 
>> Though I suspect you'll need to look at 64-bit quantities on a 64-bit machine.  
>> 32-bit values aren't addresses.
> 
> Can you get 32-bit undefined AArch64 instructions ?  
I'm not sure I follow.  All aarch64 instructions are 32 bits in size.

> The patch only annotates
> the disassembler's output for undefined instructions, so if it displaying something
> else then nothing new will happen.

Also, .inst does not permit the use of a label - it has no meaning in that context.  So for

        .text
        .inst 0x12345678
        .word 0x12345678
        .word bar  // truncated to 32 bits, but OK
        .inst bar  // Error, not a constant expression
bar:

you get

as test.s
test.s: Assembler messages:
test.s:5: Error: constant expression required

So to even trigger this case you have to contort your sources in ways I really can't see users trying to do.

Is there some user reported case driving this?

R.

> 
> Cheers
>   Nick
> 
>