[meta-xilinx] [RFC][Question] Enable VFP?

[Bian, Naimeng]

Hi mike

Thanks for your explanation too. ^_^

Thanks
 Bian

> -----Original Message-----
> From: Mike Looijmans [mailto:mike.looijmans at topic.nl]
> Sent: Friday, July 10, 2015 4:17 PM
> To: Bian, Naimeng/卞 乃猛; meta-xilinx at yoctoproject.org
> Subject: Re: [meta-xilinx] [RFC][Question] Enable VFP?
> 
> On 10-07-15 09:09, Bian, Naimeng wrote:
> > Hi all,
> >
> > As I know, VFP is an optional unit for ARMv7, but do all of Zynq Soc
> > support VFP, if so, IMO we should set DEFAULTTUNE to 'cortexa9thf-neon' in
> conf/machine/include/tune-zynq.
> 
> Your suggestion is probably based on the misconception that the "hf" directive
> enables the VFP.
> 
> The "hf" switch only enables the hard-float ABI, which means that the interface
> between libraries etc. will use the VFP registers for parameter transfers.
> Whether gcc outputs VFD code is determined by tuning settings, which are already
> set to "neon", and neon implies VFD, hence gcc will output VFD floating point
> code regardless of whether "hf" is in your tune settings.
> 
> Using cortexa9thf-neon will break ABI compatibility with other ARMv7 code, that
> is why it is rarely used, and OE defaults to soft-float ABI. The difference
> in performance is neglegible, unless you have some weird library interface that
> passes lots of floats on the stack.
> 
> The issue is not specific to Zynq, it is generic for all ARM cpus with hard
> float support.
> 
> cortexa9thf-neon will only work if all code is built from within the same
> environment, or at least, all use the hard-float ABI. For example, using the
> Xilinx SDK to compile a linux binary will then not work correctly (if there
> are any floats in the interface) since the SDK also defaults to the soft-float
> ABI.
> 
> 
> Kind regards,
> 
> Mike Looijmans
> System Expert
> 
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