bug report x86-64 code: je / jbe

[Cecil Ward]

On Friday, 16 June 2023 at 13:12:06 UTC, Iain Buclaw wrote:
> On Wednesday, 14 June 2023 at 12:35:43 UTC, Cecil Ward wrote:
>> I have just noticed a bug in the latest release of GDC that 
>> targets x86-64. For example GDC 12.3 and above versions too, 
>> running on X86-64, targeting self. This was built with:
>>  -O3 -frelease -march=alderlake
>>
>
> What leads you to believe that it is buggy?
>
>> Generated code is:
>>
>>    je L1
>>    jbe L1
>>
>
> What I see is the first instruction is going to relate to this 
> condition.
>
>>     if ( unlikely( p == 1 ) ) return x;
>
> Then the next instruction is the condition in the following 
> for-loop.
>
>>     for ( exp_ui_t i = p; i > 1; i >>= 1 )
>
> Redundant jump? Yes, arguably. Leads to wrong runtime? Doesn't 
> look that way.

Completely agree with Iain, it’s not incorrect code, I wasn’t 
intending to suggest that. I’d just say suboptimal, and not the 
very best code generation possible.