Re: [Discuss-gnuradio] Volk: Am I getting all that's available for ARM?

[Marcus Müller]

On 08/07/2013 09:48 PM, Monahan-Mitchell, Tim wrote:
> Because of the 'abi_softfp' test failing on my x86, I decided I did not need to 
> re-build the ARM tool chain to support soft ABI to try and help Volk. Is that 
> still correct? (I have been able to build and run gnuradio without the soft 
> flag just fine).
I don't think -mfloat_abi=softfp (or even =soft) applies to x86; I think I 
remember the equivalent flag being something along the ways of -msoft-float... 
Or am I misunderstanding you and you talk about cross-compiling on x86 for arm?

However, whenever you use a float library function (that is, with abi_softfp whenever you 
do something with a float), the library needs to "understand" your kind of 
float, so, yes, if you want to use softfp in GNU Radio, I _think_ you'd need to rebuild 
your ARM toolchain to support that. I don't know if that improves performance at all...

But since I'm right now reading my GCC manual[1], I stumbled across one stance:

[1]
""""
http://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html:
-mfpu=name
This specifies what floating-point hardware (or hardware emulation) is 
available on the target. Permissible names are: ‘vfp’, ‘vfpv3’, ‘vfpv3-fp16’, 
‘vfpv3-d16’, ‘vfpv3-d16-fp16’, ‘vfpv3xd’, ‘vfpv3xd-fp16’, ‘neon’, ‘neon-fp16’, 
‘vfpv4’, ‘vfpv4-d16’, ‘fpv4-sp-d16’, ‘neon-vfpv4’, ‘fp-armv8’, ‘neon-fp-armv8’, 
and ‘crypto-neon-fp-armv8’.
If -msoft-float is specified this specifies the format of floating-point values.

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.
"""

So it might make sense to include -funsafe-math-optimizations, if 
vectorize-with-neon-quad does not do that implicitly.