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Re: allowed uses of non-baseline CPU extensions




On 2017-10-05 09:09 +0100, Simon McVittie wrote:

> On Thu, 05 Oct 2017 at 11:01:42 +0800, Paul Wise wrote:
>> Anything that generates different code depending on the instructions
>> supported by the build CPU is going to break reproducible builds. So
>> whatever mechanism is used, it needs to be deterministic. [...]
>> I'd like to see CMAKE_SYSTEM_PROCESSOR
>> and similar be deprecated upstream because of that.
>
> As far as I know, CMAKE_SYSTEM_PROCESSOR is the closest equivalent of the
> CPU part of the GNU host architecture tuple in CMake, so is the only way
> to distinguish between CPU families (i386 vs. ARM, as opposed to armv5
> vs. armv7). I think it's valid to distinguish between CPU *families*,
> and it's something that build systems often want.
>
> Perhaps dh_auto_configure should specify a normalized
> CMAKE_SYSTEM_PROCESSOR by default: -DCMAKE_SYSTEM_PROCESSOR=i686 on
> Debian i386, and so on?

It already does, AFAICS.

> Unfortunately, dpkg's cputable doesn't seem to
> have a column for "what is a normal uname -m on this architecture?",

The closest thing to that is DEB_HOST_GNU_CPU which debhelper uses in
both the cmake and autoconf buildsystems.

> which I think is the form that CMake would want to see. It's also common
> to identify architecture by uname -m in ad-hoc project-specific build
> systems like the one in ioquake3. It seems that newer CPU families
> mostly keep their GNU CPU and Linux uname -m identical, so maybe only
> the historically weird ones (i386, arm*, p(ower)pc, mips(64)el) would
> need special cases?

Using uname -m seems to be wrong, since there are many 32-bit
architectures where the kernel can be 64-bit.

Cheers,
       Sven