[Nix-dev] CPU optimized packages

[Shea Levy]

OK, well technically that is possible, with something like this:

extra_cflags = import (pkgs.runCommand "cflags" {} ''
mkdir $out
echo "" | gcc -O3 -march=native -mtune=native -v -E - 2>&1 |grep cc1 
|sed -r 's/.*? - -(.*)$/-\1/' > $out/flags
echo "builtins.readFile ./flags" > $out/default.nix
'');

Basically, what that does is build a derivation which creates a nix 
expression, then imports that nix expression. One problem with this is 
that the result is saved in the nix store, meaning if you share nix 
stores between machines it won't work (because it will run once on one 
machine and the same result will be used on different machines). To get 
around this, you can make the derivation depend on the current time, 
which will force it to calculate the cflags every time you build 
something. Also, it's possible that in the future builds run in chroot 
will be inside namespaces that block how gcc detects the host system, so 
we should make this not run in chroot:

extra_cflags = import (pkgs.runCommand "cflags" {time = 
builtins.currentTime; __noChroot = true;} ''
mkdir $out
echo "" | gcc -O3 -march=native -mtune=native -v -E - 2>&1 |grep cc1 
|sed -r 's/.*? - -(.*)$/-\1/' > $out/flags
echo "builtins.readFile ./flags" > $out/default.nix
'');

I'm not sure of your level of understanding of nix, so please let me 
know if this needs more explanation.

Thanks,
Shea

On 02/07/2013 09:53 AM, Danny Wilson wrote:
> Well in this case I am the user. And I don't want to fill this in for 
> every different server I might have.
>
> My suggestion was to make -march=native pure, by expanding it on the 
> system configuring itself at nix expression evaluation time:
>
> $ echo "" | gcc -O3 -march=native -mtune=native -v -E - 2>&1 |grep cc1 
> |sed -r 's/.*? - -(.*)$/-\1/'
> -march=corei7 -mcx16 -msahf -mno-movbe -maes -mno-pclmul -mpopcnt 
> -mno-abm -mno-lwp -mno-fma -mno-fma4 -mno-xop -mno-bmi -mno-tbm 
> -mno-avx -msse4.2 -msse4.1 --param l1-cache-size=32 --param 
> l1-cache-line-size=64 --param l2-cache-size=4096 -mtune=generic -O3
>
>
>
>> Shea Levy <mailto:shea at shealevy.com>
>> 7 februari 2013 15:33
>> Hi Danny,
>>
>> Unfortunately, there's not a good answer to this because 
>> -march=native is inherently impure. Is there any reason you can't 
>> tell users "Find out what -march=native means on your system and then 
>> fill in nixpkgs.config.extra_cflags with the result?" or some such 
>> (Note that config.extra_cflags doesn't currently exist AFAIK, but could)?
>>
>> ~Shea
>>
>> On 02/07/2013 07:36 AM, Danny Wilson wrote:
>>
>> Danny Wilson <mailto:danny at prime.vc>
>> 7 februari 2013 13:36
>> With help of the gentoo wiki ( 
>> http://en.gentoo-wiki.com/wiki/Hardware_CFLAGS )
>> I found this produces a nice GCC option list:
>>
>> echo "" | gcc -O3 -march=native -mtune=native -v -E - 2>&1 |grep cc1 
>> |sed -r 's/.*? - -(.*)$/-\1/'
>>
>>
>>
>>
>> _______________________________________________
>> nix-dev mailing list
>> nix-dev at lists.science.uu.nl
>> http://lists.science.uu.nl/mailman/listinfo/nix-dev
>> Danny Wilson <mailto:danny at prime.vc>
>> 7 februari 2013 12:37
>> I'd like some packages to be optimized for my server CPU. GCC has
>> -march=native for this, but that would not generate a cpu specific nixos
>> hash.
>>
>> Is there a nix expression available which adds something like this:
>> gcc -march=native -Q --help=target |grep enabled
>>
>> to CFLAGS and correctly hashing it?
>>
>>
>>
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>

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