Re: [RFC] Optimize swab operations on mips_r2 cpu

[Nigel Stephens <nigel@mips.com>]

Franck wrote:

>2006/1/26, Nigel Stephens <nigel@mips.com>:
>  
>
>>1) Using -march=4ksd reduces the cost of a multiply by 1 instruction
>>(from 5 to 4 cycles), so a few more constant multiplications, previously
>>expanded into a sequence of shifts, adds and subs, may now be replaced
>>by a shorter sequence of "li" and "mul" instructions.
>>
>>    
>>
>
>Is it really specific to 4ksd cpu ? Could this behaviour be triggered
>by other options ?
>  
>

Yes, when you use -Os the compiler uses the instruction cost (1) of a 
mul, instead of the cycle cost (4), so it will be even more likely to 
replace the expanded shift/add sequence by a mul.

>>   text    data     bss     dec     hex filename
>>2099642  110784   81956 2292382  22fa9e vmlinux-4ksd
>>2136269  110784   81956 2329009  2389b1 vmlinux-mips32r2
>>1953086  110784   81956 2145826  20be22 vmlinux-4ksd-Os
>>1954489  110784   81956 2147229  20c39d vmlinux-mips32r2-Os
>>
>>I now have to check that your first and second points don't have too
>>much bad impact on the overall speed although I don't know how to
>>measure that...But if so, I could safely use -march=mips32r2 -Os
>>options.
>>    
>>

You could, but why not stick with -march=4ksd if that's your CPU of 
choice? It appears to result in  marginally smaller code even when using 
-Os, and should have (slightly) better performance than a generic 
mips32r2 kernel?

Nigel