Re: FEATURE REQUEST: Specific Processor Optimizations on x86 Architecture

[Nick Piggin <piggin@cyberone.com.au>]

J.A. Magallon wrote:

>On 10.22, Joseph D. Wagner wrote:
>
>>Yes, I know you can select Pentium III, Pentium 4, Athlon, etc, under 
>>processor type when doing a 'make xconfig', but those selections do not 
>>translate into the appropriate -mcpu and -march flags.
>>
>>While the kernel on x86 architecture can be optimized in terms of generic 
>>processor specifications (i.e. i386, i486, i586, i686), the kernel can't be 
>>optimized beyond a i686.
>>
>>If you select Pentium III, the -march flag is set to i686.
>>If you select Pentium 4, the -march flag is set to i686.
>>If you select Athlon 4, the -march flag is set to i686.
>>If you select Athlon XP, the -march flag is set to i686.
>>
>>It should be that...
>>
>>If you select Pentium III, the -march flag is set to pentium3.
>>If you select Pentium 4, the -march flag is set to pentium4.
>>If you select Athlon 4, the -march flag is set to athlon-4.
>>If you select Athlon XP, the -march flag is set to athlon-xp.
>>
>>I don't want to have to hand edit the makefiles just to optimize my kernel.  
>>I think this change is simple enough to do, and would allow kernel 
>>developers the option of processor-specific optimizations in the future.
>>
>>TIA.
>>
>>Joseph D. Wagner
>>
>
>I have sent the attached patches sometimes to the list/Marcelo, and they
>have been rejected to the moment because:
>- gcc can spit some new instructions, reorganize code and other things when
>  you jump from i686 to pentium3, for example.
>- There can be bugs both in gcc and in the kernel that can be triggered by
>  >i686 optimizations/code.
>- This is not safe for a stable kernel, it was done in 2.5, bugs appeared,
>  were corrected, and so on, 'cause this was a development kernel.
>

Not that I think this should go in anyway, but the only way you ever
might get it in is if you measure significant performance improvements.
I don't think you will.