Re: Compiler Optimizations [was Package options dialog]

[Jason Bacon <outpaddling%yahoo.com@localhost>]

On 05/07/18 17:20, Sevan Janiyan wrote:
> On 05/07/18 14:55, Jason Bacon wrote:
> > In theory, I think -march=native alone achieve this with clang and gcc,
> > and this is what I've been looking at.
> Would it just be easier for you maintain a template mk.conf with the
> necessary options set so that those who need to build a package manually
> can uncompress bootstrap kit (or bootstrap), add the mk.conf snippet,
> start building the relevant packages.
I haven't given a lot of thought to how it should be implemented yet.
Could be done globally in mk.conf, in which case I'd probably provide
instructions on a web page and a prompt in auto-pkgsrc-setup, which
automates the bootstrap process for non-technical types.
> > I see that cpuflags adds some additional flags:
> >
> > netbsddev /usr/pkgsrc/wip/plink2 # cpuflags
> > -mfpmath=sse -msse3 -march=native
> >
> > I assume cpuflags is compensating for GCC shortcomings.  I'll do some
> > comparisons on our systems.
> In what regards?
By adding -mfpmath=sse -msse3.  The cpuflags command has so far given me the
same output on every machine I've tried.  I've run a few benchmarks 
using plink and
so far have not seen any measurable difference between -march=native and
-mfpmath=sse -msse3 -march=native.
> > It doesn't appear to directly support clang, although I would guess it
> > will work at least pretty well, given the high level of compatibility
> > with GCC.
> That could just be that it needs some attention rather than intentional.
>
> Just out of curiosity, are you looking to build everything with these
> flags or specific applications which you are certain behave correctly
> with compiler optimisations?
Again, not entirely sure.  Right now I'm focused on just determining the 
simplest
and most fool-proof way for Joe Scientist to get the most out of his CPU.

-march=native is probably pretty safe to set globally, but now that
I said that, it will break something for sure.  ;-)  I suspect problems 
will be rare and
certainly can be fixed once they're exposed.

Thanks,

    JB