[fpc-devel] x86_64.inc CompareByte

[Сергей Сергеенко]

On 15 Oct 2017 Florian Klämpfl wrote:
> I had a look and tested it and it worked, I didn't notice the problem below
> either.

Sorry for wrong warning. I cannot provide any example where my suggestions
are true. The reason for it is described on page Vol. 1 3-13 of Intel 64
and IA-32 Architectures Software Developer's Manual:

> When in 64-bit mode, operand size determines the number of valid bits in
> the destination general-purpose register:
>
> [...]
> 
>  32-bit operands generate a 32-bit result, zero-extended to a 64-bit
>  result in the destination general-purpose
>  
> [...]

So, instructions 
>     movzbl  (%rcx),%eax
and
>      movzbl  -1(%rdx),%ecx
and 
>      xorl    %eax,%eax
should put zero into 32 high bits of appropriate registers.

> I think also the final xor should be a xorq %rax,%rax, right?

As I said above xorl %eax, %eax should be enough.

--
With best regards
Sergey