Re: [PATCH 3/6] newlib: mem[p]cpy/memmove improve performance for optimized versions

On 28/01/2025 16:11, Corinna Vinschen wrote:
> On Jan 27 10:45, Alexey Lapshin wrote:
>> This change improves performance on memory blocks with sizes in range
>> [4..15]. Performance measurements made for RISCV machine (memset):
>>
>> size  4, CPU cycles change: 50 -> 37
>> size  5, CPU cycles change: 57 -> 40
>> size  6, CPU cycles change: 64 -> 47
>> size  7, CPU cycles change: 71 -> 54
>> size  8, CPU cycles change: 78 -> 44
>> size  9, CPU cycles change: 85 -> 47
>> size 10, CPU cycles change: 92 -> 54
>> size 11, CPU cycles change: 99 -> 61
>> size 12, CPU cycles change: 106 -> 51
>> size 13, CPU cycles change: 113 -> 54
>> size 14, CPU cycles change: 120 -> 61
>> size 15, CPU cycles change: 127 -> 68
> 
> But is that generally true for other architectures as well?
> 

No, it can be very dependent on the microarchitecture.  I know of Arm implementations where it would be better and implementations where it would be (much) worse.  The other variable is that for misaligned copies there's a choice of bringing the source data to alignment or the target data (you really don't want to do a large copy with both misaligned).  That can also vary by micro-architecture.

But we have custom assembler versions for Arm, so it probably doesn't matter for us, except at -Os and there I wouldn't expect us to want large expanded chunks of code for all the cases that misaligned copies might involve.

R.