Optimizing 0-displacement jumps on i386

[H. Peter Anvin]

On 2025-08-11 23:07, Jan Beulich wrote:
>>
>> One more issue: a zero-offset jump may also be used to sync the prefetch
>> pipe on the CPUs that require that. For example, the Linux boot code
>> contains the following sequence:
>>
>>         /* Self-modify the INT instruction.  Ugly, but works. */
>>         cmpb    %al, 3f
>>         je      1f
>>         movb    %al, 3f
>>         jmp     1f              /* Synchronize pipeline */
>> 1:
> 
> Hence why I think such optimizations may not be turned on by default.
> 

Yes; this really needs an opt-in as otherwise it will break existing,
valid code; preferably as a pseudo-op that can disable it on a localized
basis.

	-hpa