Re: [SPAM?] Re: [PATCH] mm/pg-r4k.c: Dump the generated code

[Nigel Stephens <nigel@mips.com>]

Maciej W. Rozycki wrote:
> On Tue, 9 Oct 2007, Franck Bui-Huu wrote:
>
>   
>>>  What would be the gain for the kernel from using "-march=4ksd" rather 
>>> than "-march=mips32r2"?
>>>
>>>       
>> It actually results in a kernel image ~30kbytes smaller for the former
>> case. It has been discussed sometimes ago on this list. I'm sorry but
>> I don't know why...
>>     
>
>  Perhaps the pipeline description for the 4KSd CPU is different from the 
> default for the MIPS32r2 ISA.  Barring a study of GCC sources, if that 
> really troubles you, you could build the same version of the kernel with 
> these options:
>
> 1. "-march=mips32r2"
>
> 2. "-march=4ksd"
>
> 3. "-march=mips32r2 -mtune=4ksd"
>
> and compare the results. 



>  I expect the results of #2 and #3 to be the same 
> and it would just back up my suggestion about keeping CPU-specific 
> optimisations separate from the CPU selection.  

Actually the -march=4ksd option will allow gcc to use of the SmartMIPS 
lwxs (indexed load) instruction, which could save a few instructions 
here and there.


> Please also note that our 
> optimisation model is for speed (-O2) rather than size (-Os), so if 
> "-mtune=4ksd" yields smaller code than "-mtune=mips32r2", it just means it 
> is safe for this CPU to shrink code where appropriate without losing 
> performance.  One obvious place for such a choice is the use of the 
> hardware multiplier vs shifts and additions where one multiplicand is a 
> constant.
>
>   

Yes, that's also worth testing.

Nigel