Re: [PATCH 0/2] LoongArch: Add optimized functions.

[Xi Ruoyao <xry111@xry111.site>]

On Mon, 2022-09-19 at 17:16 -0300, Adhemerval Zanella Netto via Libc-
alpha wrote:
> Do you have any breakdown if either loop unrolling or missing string-fzi.h/
> string-fza.h is what is making difference in string routines? 

It looks like there are some difficulties... LoongArch does not have a
dedicated instruction for finding a zero byte among the 8 bytes in a
register (I guess the LoongArch SIMD eXtension will provide such an
instruction, but the full LSX manual is not published yet and some
LoongArch processors may lack LSX).  So the assembly code submitted by
dengjianbo relies on a register to cache the bit pattern
0x0101010101010101.  We can't just rematerialize it (with 3
instructions) in has_zero or has_eq etc. or the performance will be
likely horribly bad.  

> Checking on last iteration [1], it seems that strchr is issuing 2 loads
> on each loop iteration and using bit-manipulation instruction that I am
> not sure compiler could emit with generic code. Maybe we can tune the
> generic implementation to get similar performance, as Richard has done
> for alpha, hppa, sh, and powerpc?
> 
> I am asking because from the brief description of the algorithm, the
> general idea is essentially what my generic code aims to do (mask-off
> initial bytes, use word-aligned load and vectorized compares, extract
> final bytes), and I am hoping that architecture would provide 
> string-fz{i,a}.h to get better code generation instead of pushing
> for more and more hand-write assembly routines.

-- 
Xi Ruoyao <xry111@xry111.site>
School of Aerospace Science and Technology, Xidian University