[PATCH 2/4] include: Implement __stosb compiler intrinsic.

[Piotr Caban]

On 9/13/21 7:56 PM, Rémi Bernon wrote:
> On 9/13/21 7:08 PM, Zebediah Figura wrote:
>> On 9/13/21 12:01 PM, Rémi Bernon wrote:
>>> On 9/13/21 7:00 PM, Zebediah Figura wrote:
>>>> On 9/13/21 11:53 AM, Rémi Bernon wrote:
>>>>> On 9/13/21 6:42 PM, Zebediah Figura wrote:
>>>>>> On 9/13/21 10:25 AM, Rémi Bernon wrote:
>>>>>>> On 9/13/21 4:51 PM, Piotr Caban wrote:
>>>>>>>> Hi Rémi,
>>>>>>>>
>>>>>>>> On 9/13/21 2:23 PM, Rémi Bernon wrote:
>>>>>>>>> +static inline void __stosb(unsigned char* dst, unsigned char c,
>>>>>>>>> size_t n)
>>>>>>>>> +{
>>>>>>>>> +    __asm__ __volatile__ ("cld; rep; stosb" : "=D"(dst) : "a"(c),
>>>>>>>>> "D"(dst), "c"(n) : "memory", "cc");
>>>>>>>>> +}
>>>>>>>> I don't know if it's important here but Microsoft's i386 cdecl abi
>>>>>>>> specifies direction flag value on function call. Maybe if 
>>>>>>>> __cdecl is
>>>>>>>> added cld call may be removed.
>>>>>>>>
>>>>>>>
>>>>>>> All the ABIs are apparently requiring it to be cleared before a
>>>>>>> function
>>>>>>> call, or am I missing something? So it looks like it's not needed
>>>>>>> anywhere and I was just over cautious.
>>>>>>>
>>>>>>
>>>>>> Well, ABIs do, but you're not defining that as an asm function; 
>>>>>> you're
>>>>>> using inline assembly. So you can't guarantee anything.
>>>>>>
>>>>>
>>>>> But it's wrapped in a function, which implies what its calling
>>>>> convention ABI implies?
>>>>>
>>>>
>>>> No, not really. The compiler is free to insert whatever assembly it
>>>> wants before and after the __asm__ block, as long as it satisfies the
>>>> constraints.
>>>>
>>>> Not only that, but because it's a static function, the compiler is also
>>>> free not to give it a standard calling convention at all.
>>>>
>>>
>>> Well, anyway MSVC doesn't generate cld with this intrinsic so I think we
>>> should not either.
>>>
>>
>> I don't see why that means anything. At best, that just means MSVC is 
>> checking whether the direction flag was already clear, and not 
>> clearing it again. In theory, GCC could do that too, but I don't see 
>> any clear way to make the value of DF an input constraint.
>>
> 
> I don't think it's doing that, and there's also probably no point.
> 
> These intrinsics are meant to generate assembly instructions, and so you 
> can very well combine them by setting the direction flag before hand, 
> for instance with __writeeflags and effectively reverse later __stosb or 
> __movsb.
> 
> Then although it's what it does it's not documented and maybe isn't very 
> safe.
> 
> Probably I should just add the "cld; rep; stosb" inline instead.
Yes. This will also kind of match with what LLVM has done (they are 
force inlining memset in this case).