Re: [PATCH v5 4/6] x86: control memset() and memcpy() inlining

[Jan Beulich <jbeulich@xxxxxxxx>]

On 05.06.2025 19:34, Teddy Astie wrote:
> Le 05/06/2025 à 12:28, Jan Beulich a écrit :
>> Stop the compiler from inlining non-trivial memset() and memcpy() (for
>> memset() see e.g. map_vcpu_info() or kimage_load_segments() for
>> examples). This way we even keep the compiler from using REP STOSQ /
>> REP MOVSQ when we'd prefer REP STOSB / REP MOVSB (when ERMS is
>> available).
>>
> 
> If the size is known and constant, and the compiler is able to generate 
> a trivial rep movs/stos (usually with a mov $x, %ecx before). I don't 
> see a reason to prevent it or forcing it to make a function call, as I 
> suppose it is very likely that the plain inline rep movs/stos will 
> perform better than a function call (even if it is not the prefered rep 
> movsb/stosb), eventually also being smaller.
> 
> I wonder if it is possible to only generate inline rep movs/stos for 
> "trivial cases" (i.e preceded with a plain mov $x, %ecx), and rely on 
> either inline movs or function call in other cases (non-trivial ones).

Note how the description starts with "Stop the compiler from inlining
non-trivial ...", which indeed remain unaffected according to my
observations (back at the time).

Jan