Re: [Qemu-devel] QEMU state of ARM NEON support.

[Dmitry Zhurikhin]

address@hidden wrote:
> Hi,
>
> Thanks for the testing. I fixed all of the issues found by your test programs 
> except:
Great!  This is very good news.  Please look at my comments below regarding 
specific issues.

> - vmull.p8 is not implemented in QEMU.
So seems this is now the only NEON instruction variant left that QEMU doesn't 
handle.

> - vrecpe/vrsqrte returns exact reciprocal/root in QEMU instead of estimate.
A simple algorithm used in hardware is described in the ARMv7 ARM* (see function 
'recip_estimate' on page A2-59) but I don't think this is a big issue as exact 
computations may be even better.

> - Using subnormal floating point input values yields wrong results (this is 
> visible in your vceq/vcgt/vcle tests against zero), I guess this would need to 
> be corrected in the softfloat library which QEMU uses.
This is actually a "Flush-to-zero" feature which is always on for NEON 
instructions (see FZ field description of FPSCR in A2.6.4 chapter of ARMv7 ARM). 
 Before attempting to perform an action on the floating-point data NEON first 
checks if it is close to zero (zero exponent, bits 23-30 for F32, see FPUnpack 
function at page A2-48) and behaves as if it was zero if the check succeeds.

> - vqdmlsl yields different results for some specific input values, however I 
> think QEMU is producing correct results. For example, 
> 1431655765-(2*-32768*-32768)=-715827883 but your reference hardware results say 
> it should be -715827882.
From what I understand hardware gets such result because saturation occurs on 
the (2*-32768*-32768) step.  The result is not 0x80000000 as expected but 
because of overflow is set as the largest 32-bit signed integer 0x7FFFFFFF.  It 
seems that QEMU just doesn't perform saturation (in this particular case at least).

> Anyway, many thanks for the testing you did and especially for posting the test 
> programs as well. All these fixes along with many others are now in the Maemo 
> QEMU repository. We'll try to make a new set of ARM emulation patches against 
> upstream QEMU in the near future.
We'll be waiting for this.  Thanks a lot for your work.

> Regards,
> Juha

* I am referring to the "ARM Architecture Reference Manual, ARMv7-A and ARMv7-R 
edition" version ARM_2009_Q3.


        Regards,
        Dmitry