Re: [Buildroot] generic code with i7 bootlin toolchain?

["Yann E. MORIN" <yann.morin.1998@free.fr>]

Stsp, All,

On 2021-11-03 22:52 +0300, stsp spake thusly:
> I configured buildroot for
> corei7 CPU to use the bootlin
> tool-chain. As expected, nothing
> worked, as I have the (very old)
> AMD CPU.
> Then I did the following:
> BR2_TARGET_OPTIMIZATION="-march=x86-64 -mtune=generic"

Do not use BR2_TARGET_OPTIMIZATION to set your CPU model. Instead,
select the proper entry in the target variant choice. Selecting the
proper variant will pass appropriate -march and -mcpu options.

If a toolchain is not available for the CPU you have (and which you
select in the choice I mentioned above), then that is on-purpose,
because that toolchain is not usable for that CPU; see below...

> make clean
> and full rebuild.
> After that I've got working
> kernel and the user-space
> starts booting, but still many
> user-space apps or libs crash
> with an illegal instruction exception.
> 
> I even checked that the toolchain-wrapper
> generated by buildroot, has these
> strings embedded, so my change
> certainly had some effect. But
> it was not enough.

This is not unsurprising. The toolchain contains the C lirbary, which
has been built with the optimisations for core-i7. So using that
toolchain for a CPU that lacks some features of a core-i7 will not work.

> So it looks like the
> BR2_TARGET_OPTIMIZATION
> is somehow applied not to all
> built packages? Why can that be?
> Is there any other way I can
> utilize bootlin tool-chain for some
> "generic" (non-SSE3/4) CPU?

You will not be able to use that toolchain for the Phenom. You will have
to build your own toolchain.

Note that you can build the toolchain once, and later re-use that as a
pre-built toolchain. See the manual on directions on how to do that:
    https://buildroot.org/downloads/manual/manual.html#_cross_compilation_toolchain

Regards,
Yann E. MORIN.

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