Re: [PATCH 2/2] Loongarch: Add ifunc support and add different versions of strlen

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 03/08/23 10:27, dengjianbo wrote:
> 
> On 2023-08-02 20:59, Adhemerval Zanella Netto wrote:
>>>>>> On 2023-08-02 10:31, Adhemerval Zanella Netto wrote:
>>>>>> +#if IS_IN (libc)
>>>>>> +# define STRLEN __strlen_aligned
>>>>>> +#else
>>>>>> +# define STRLEN strlen
>>>>>> +#endif
>>>>> Is this really an improvement over the generic implementation? It seems to
>>>>> use a quite similar strategy.
>>> Comparing with the code generated by compiler, the assembly code does an 16bytes loop
>>> unrolling, and handles ascii data and non-ascii data separately which could take less
>>> instructions to calculate the length of  ascii data. besides, the assembly code using
>>> fewer instructions to start the loop. I think the performance improvement benefits from
>>> this. Please kindly check bench result also from:
>>> https://github.com/jiadengx/glibc_test/blob/main/strlen/bench-strlen.out
>> From the summarized results [1], it seems that the initial start to mask
>> off unaligned inputs are slight better.  The __strlen_aligned onl seems
>> better to sizes larger than 32 (the 16 lenght results seems strange).
>> Maybe you coult improve shift_find/find_zero_all/index_first on loongarch.
>>
>> Does it improve by explicit instructing compiler to unroll the loop?
> As you know, the assembly versions of strlen uses the same strategy to
> calculate string length, if assembly code only calculate 8 bytes in the
> loop and don't separate ascii and non-ascii data, the code of loop and
> loop end part should be the same as the compiler generated code base on
> generic strlen. Loongarch doesn't provide instructions like alpha
> cmpbge, so there is no much optimizations could be done on
> find_zero_all/index_first/has_zero except we can remove some BIG_ENDIAN
> codes.
>  
> Refer to the latest test results in the chart: The assembly
> implementation vs. generic strlen implementation(compiled by using
> CFLAGS-strlen.c += -funroll-all-loops --param
> max-variable-expandsions-in-unroller=2) the performance
> improvement of the assembly implementation is evident(30% ~ 40%),
> especially in cases when the length is greater than 64 bytes.
> Please kindly see the results via:
> https://github.com/jiadengx/glibc_test/blob/main/strlen2/bench1/generic_strlen_with_loop_unrolling.png

So maybe use the generic implementation plus the compiler flags to loop
unrolling instead of asm optimization?

>>>>> This implementation fails to assembler with binutils 2.40.0.20230525:
>>>>> ../sysdeps/loongarch/lp64/multiarch/strlen-lsx.S: Assembler messages:
>>>>> ../sysdeps/loongarch/lp64/multiarch/strlen-lsx.S:30: Error: no match insn: vld  $vr0,$r4,0
>>>>> ../sysdeps/loongarch/lp64/multiarch/strlen-lsx.S:31: Error: no match insn: vld  $vr1,$r4,16
>>>>>
>>> Sorry, it's my mistake for the wrong version of binutils. Could you please try the latest release
>>> version 2.41?
>> Although it should work, it is unexpected that depending of the assembler used
>> some optimized routines are not enabled. 
> 
> In patch v2, an new configuration variable has been added to control
> whether the LASX/LSX will be compiled according to assembler support
> LASX/LSX or not, so it can be compiled with old versions of binutils.

Yes I am aware and this seems odd, albeit not really wrong.  It means that
you will get less code coverage and optimizations depending of the used 
binutils. 

I would advise to follow what other architecture did to provide arch-specific 
optimization, which is either setup a minimum gcc/binutils version (for 
instance aarch64 libmvec), or encode the instructions in a binutils neutral
mode (as the powerpc implementation I pointed out).