Re: [PATCH v2 2/3] microblaze: Do loop unrolling for optimized memset implementation

[Michal Simek <michal.simek@xilinx.com>]

On 2/25/22 22:50, David Laight wrote:
> From: Michal Simek
>> Sent: 25 February 2022 13:56
>>
>> Align implementation with memcpy and memmove where also remaining bytes are
>> copied via final switch case instead of using simple implementations which
>> loop. But this alignment has much stronger reason and definitely aligning
>> implementation is not the key point here. It is just good to have in mind
>> that the same technique is used already there.
>>
>> In GCC 10, now -ftree-loop-distribute-patterns optimization is on at O2.
>> This optimization causes GCC to convert the while loop in memset.c into a
>> call to memset.
> 
> Gah...
> That is nearly as brain dead as another compiler that would convert
> any byte copy loop (on x86) into 'rep movsb'.
> 
> If I want to call memcpy() I'll call memcpy.
> If I'm copying a few bytes I might write the loop to avoid
> the cost of the call and all the conditional tests for
> buffer length and alignment.
> 
> Don't the compiler writers have better things to do?

Not sure what you want me to say about it. It is current gcc behavior and I 
can't see the way back. I don't think doing loop unrolling here is a big deal 
for me because the same technique is used for years in memcpy and memmove.

Thanks,
Michal