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 ?
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/.*? - -(.*)$/-\1/'
-march=corei7 -mcx16 -msahf -mno-movbe -maes -mno-pclmul
-mpopcnt -mno-abm -mno-lwp -mno-fma -mno-fma4 -mno-xop
-mno-bmi -mno-tbm -mno-avx -msse4.2 -msse4.1 --param
l1-cache-size=32 --param l1-cache-line-size=64 --param
l2-cache-size=4096 -mtune=generic -O3
Hi Danny,
Unfortunately, there's not a good answer to this because
-march=native is inherently impure. Is there any reason you
can't tell users "Find out what -march=native means on your
system and then fill in nixpkgs.config.extra_cflags with the
result?" or some such (Note that config.extra_cflags doesn't
currently exist AFAIK, but could)?
~Shea
On 02/07/2013 07:36 AM, Danny Wilson wrote:
With help of the gentoo wiki (
http://en.gentoo-wiki.com/wiki/Hardware_CFLAGS
)
I found this produces a nice GCC option list:
echo "" | gcc -O3 -march=native -mtune=native -v -E -
2>&1 |grep cc1 |sed -r 's/.*? - -(.*)$/-\1/'
