Re: [gentoo-dev] Moving CPU flags into USE_EXPAND

[Christopher Head]

On Wed, 14 Jan 2015 11:01:16 -0800
Zac Medico <zmedico@g.o> wrote:

> Why should we have to foresee the future? We can easily add support
> for new flags in CPU_FLAGS_* variables at any time.

Ah, what I meant was that whoever maintains this flag list only needs
to forsee the present—when AMD or Intel adds a new instruction set
extension, you add a flag for it to the variable immediately, whether
or not any package actually uses it yet. Why? Here’s why:

Case 1: flags are added only as packages need them. This is pretty much
what we have today, only without the USE-expand feature. Every time a
package adds support for a new CPU feature, it gets a new USE flag, and
I see it in my emerge output. Now I have to go and look up what the
feature is, what it would be called in cpuinfo, and whether I have it.
Maybe I already looked up the same CPU feature two or three times, many
months ago, and forgot about it, for some other package. Lots of wasted
work. But I can’t just ignore it, because maybe this is the first time
that flag showed up, because no package ever used that feature before,
but my CPU *does* have it, so I really want to turn it on!

Case 2: flags are all added immediately as the CPU features are
announced. When I install Gentoo, all the possible flags for my CPU are
already listed in the variable. I immediately turn all those I have on
and all those I don’t have off. Done. Now I can completely stop paying
attention to the flags. As packages gain support, they gain new flags,
and I can ignore them, secure in the knowledge that the flags for those
features I have will all be turned on, while those I don’t have will be
turned off, with no further input from me. Nice and easy.

I’m not saying add flags for feature sets that haven’t been announced
yet. I’m just saying, add flags for feature sets that *are* announced
but that no package actually uses yet.
-- 
Christopher Head