On 22/04/2018 12:22, Gareth's Downstairs Computer wrote:
> For example, due to operator precedence, parentheses might
> prove to be unnecessary, but you'd be justifiably miffed
> if they disappeared from the source code, as source code
> has human understandabilty considerations, and so
> it becomes necessary to
> insert a special form of NOP in the compiled code to
> represent the parentheses; only one NOP for each pair.

No, only the source needs such human readable information, it should not
appear in compiled code, or even in the output of hand written assembler.

> So, the reason for wanting the instruction encoding of the A53
> (now discovered in excelsis) is to seek ambiguous forms of the
> same instruction to let me effectively insert such NOPs but
> without unnecessarily slowing down a program.

I couldn't think of a worse way to encode that information, who will
remember in a years time which ambiguous form mean what?

Incidentally; there is only one guaranteed NOP in ARM, and that is ADD
R0,R0,R0. The 32bit conditional on never instructions (NV) have been
deprecated and  reused for new instructions, and this may well happen to
other "non useful with no side effects" instructions such as ADD
Rx,Rx,Rx where Rx is not 0.

---druck

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