RISC-V: Optimize memset for data sizes less than 16 bytes

From: zhangfei <zhangfei@nj.iscas.ac.cn>

At present, the implementation of the memset function uses byte by byte storage 
when processing tail data or when the initial data size is less than 16 bytes. 
This approach is not efficient. Therefore, I filled head and tail with minimal 
branching. Each conditional ensures that all the subsequently used offsets are 
well-defined and in the dest region. Although this approach may result in 
redundant storage, compared to byte by byte storage, it allows storage instructions 
to be executed in parallel, reduces the number of jumps, and ultimately achieves 
performance improvement.

I used the code linked below for performance testing and commented on the memset 
that calls the arm architecture in the code to ensure it runs properly on the 
risc-v platform.

[1] https://github.com/ARM-software/optimized-routines/blob/m...

The testing platform selected RISC-V SiFive U74.The test data is as follows:

Before optimization
---------------------
Random memset (bytes/ns):
           memset_call 32K:0.45 64K:0.35 128K:0.30 256K:0.28 512K:0.27 1024K:0.25 avg 0.30

Medium memset (bytes/ns):
           memset_call 8B:0.18 16B:0.48 32B:0.91 64B:1.63 128B:2.71 256B:4.40 512B:5.67
Large memset (bytes/ns):
           memset_call 1K:6.62 2K:7.02 4K:7.46 8K:7.70 16K:7.82 32K:7.63 64K:1.40

After optimization
---------------------
Random memset bytes/ns):
           memset_call 32K:0.46 64K:0.35 128K:0.30 256K:0.28 512K:0.27 1024K:0.25 avg 0.31
Medium memset (bytes/ns )
           memset_call 8B:0.27 16B:0.48 32B:0.91 64B:1.64 128B:2.71 256B:4.40 512B:5.67
Large memset (bytes/ns):
           memset_call 1K:6.62 2K:7.02 4K:7.47 8K:7.71 16K:7.83 32K:7.63 64K:1.40

From the results, it can be seen that memset has significantly improved its performance with 
a data volume of around 8B, from 0.18 bytes/ns to 0.27 bytes/ns.

The previous work was as follows:
1. "[PATCH] riscv: Optimize memset"
   6d1cbe2e.3c31d.187eb14d990.Coremail.zhangfei@nj.iscas.ac.cn

Thanks,
Fei Zhang

Andrew Jones (1):
  RISC-V: lib: Improve memset assembler formatting

 arch/riscv/lib/memset.S | 143 ++++++++++++++++++++--------------------
 1 file changed, 72 insertions(+), 71 deletions(-)

zhangfei (1):
  RISC-V: lib: Optimize memset performance

 arch/riscv/lib/memset.S | 40 +++++++++++++++++++++++++++++++++++++---
 1 file changed, 37 insertions(+), 3 deletions(-)