[Nix-dev] CPU optimized packages

[Danny Wilson]

Great!
Maybe instead of depending on time, it could use `md5sum /proc/cpuinfo`  
or something that identifies the machine?

I get the concepts but I'm very new to the nix language. How would I 
modify an existing package to use this in my ~/.nixpkgs/config.nix ?



> Shea Levy <mailto:shea at shealevy.com>
> 7 februari 2013 16:01
> 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:
>
> Danny Wilson <mailto:danny at prime.vc>
> 7 februari 2013 15:53
> 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/'
>
>
>
>
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> 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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