[Buildroot] [PATCH] configs: nitrogen*: enable NEON support

[Thomas Petazzoni <thomas.petazzoni@free-electrons.com>]

Hello,

On Fri, 16 Sep 2016 16:20:42 +0200, Gary Bisson wrote:

> Because the first two configurations are for i.MX6 (ARM Cortex-A9)
> whereas the last one is i.MX7 (ARM Cortex-A7).
> 
> So relying on Config.in.arm, the Cortex-A9 doesn't have VFPv4:
> https://git.buildroot.net/buildroot/tree/arch/Config.in.arm#n125
> https://git.buildroot.net/buildroot/tree/arch/Config.in.arm#n141

OK, then you should be using the VFPv3 instead of the NEON for FPU.
Indeed:

"""
     If the selected floating-point hardware includes the NEON extension
     (e.g.  '-mfpu'='neon'), note that floating-point operations are not
     generated by GCC's auto-vectorization pass unless
     '-funsafe-math-optimizations' is also specified.  This is because
     NEON hardware does not fully implement the IEEE 754 standard for
     floating-point arithmetic (in particular denormal values are
     treated as zero), so the use of NEON instructions may lead to a
     loss of precision.
"""

Thomas
-- 
Thomas Petazzoni, CTO, Free Electrons
Embedded Linux and Kernel engineering
http://free-electrons.com