Re: Noob question. Why is the for-pentium4 kernel built with -march=i686 ?

[Simon Strandman <simon.strandman@telia.com>]

Denis Vlasenko skrev:

>On Wednesday 20 July 2005 12:25, Ivan Yosifov wrote:
>  
>
>>>>>Also, I believe that the -march=pentium4 option /was/ actually used up
>>>>>until kernel 2.6.10 where it was dropped because of a risk that some
>>>>>versions of gcc would cause the kernel to use SSE registers for data
>>>>>movement (which is a no-no).
>>>>>
>>>>>          
>>>>>
>>>>You seem right. I fetched a 2.6.9 tarball and it is really built with
>>>>-march=pentium4. Do you know which are versions of gcc in question ?
>>>>
>>>>        
>>>>
>>>No, I'm afraid not. I only know that the advice came from Richard
>>>Henderson who (I think) is one of the core glibc hackers. You can see
>>>the point at which it was introduced by Linus in the ChangeLog (2nd
>>>message from last):
>>>
>>>http://www.kernel.org/pub/linux/kernel/v2.6/ChangeLog-2.6.10
>>>      
>>>
>>Seems to be this one:
>>
>><torvalds@ppc970.osdl.org>
>>	Don't use "-march=pentium3" for gcc tuning.
>>	
>>	rth tells me that some versions of gcc may end up using the
>>	SSE registers for data movement when you do that.
>>	
>>	Use "-march=i686 -mtune=xxxx" instead.
>>	
>>	(We do the same thing for march=pentium2/4 too, just for
>>	consistency).
>>
>>
>>The way it is worded it seems that it is a problem with *some* versions
>>of gcc only on p3, not p4.
>>    
>>
>
>Why do you care? I bet that differences between i686 code and pentium4 code
>are well below noise level.
>--
>vda
>
For x86_64 the flags -mno-sse -mno-mmx -mno-sse2 -mno-3dnow are always 
used for compilation. Why is'nt the same thing done for x86 instead of 
using -march=i686 -mtune=?.

-march=athlon and -march=k6 includes -m3dnow and -mmmx, are those ok for 
the kernel but -msse isn't?

-- 
Simon Strandman <simon.strandman@telia.com>