Selecting appropriate ARM -mfpu option in gcc based on CPU features - c

Question

My CPU has following CPU features
cat /proc/cpuinfo
Processor : ARMv7 Processor rev 4 (v7l)
processor : 0
BogoMIPS : 1192.96
processor : 1
BogoMIPS : 1197.05
Features : swp half thumb fastmult vfp edsp neon vfpv3 tls vfpv4 idiva idivt
CPU implementer : 0x41
CPU architecture: 7
CPU variant : 0x0
CPU part : 0xc07
CPU revision : 4
Hardware : sun7i
Revision : 0000
And gcc sets
-march=armv7ve -mfloat-abi=hard -mfpu=vfpv3-d16 -meabi=5
options for
cat main.c
#include <stdio.h>
void main()
{
printf("Hello World!\n");
}
compiled with
gcc -march=native -mtune=native -Q -v main.c
Isn't neon-vfpv4, which seems to be supported by CPU features, is superior to vfpv3-d16, which gcc sets?
I got only vague explanation of what vfpv3-d16 is from ARM's documentation and nothing on neon-vfpv4.
I'm using gcc 4.9.1

Answer 1

-march and -mtune (or -mcpu as a shorthand for both) only control the CPU options for instruction selection and scheduling. As an example, with a GCC 4.8-based cross-toolchain, when I do this:
arm-linux-gnueabihf-gcc -mcpu=arm250 -v -c test.c
I get this:
...
COLLECT_GCC_OPTIONS='-mcpu=arm250' '-v' '-c' '-mfloat-abi=hard'
'-mfpu=vfpv3-d16' '-mthumb' '-mtls-dialect=gnu'
...
which is clearly nonsense - the ARM250 predates VFP (and even Thumb) by a long way - because for any unspecified options it's just passing through whatever was configured as the default:
...
Configured with:
... --with-arch=armv7-a --with-tune=cortex-a9 --with-fpu=vfpv3-d16 ...
... --with-mode=thumb --with-float=hard
Your Cortex-A7 indeed supports full VFPv4 and NEON, so passing -mfpu=neon-vfpv4 to override the default would be the right thing to do. Unfortunately there doesn't seem to be an equivalent -mfpu=native option (at least documented - I don't have a native toolchain handy to check).