-mfpu=neon-fp-armv8 and unrecognized command line option

Jim Wilson jim.wilson at linaro.org
Wed Jul 20 21:33:30 UTC 2016


On Wed, Jul 20, 2016 at 2:14 PM, Jeffrey Walton <noloader at gmail.com> wrote:
> I'm having trouble with ARMv8/Aarch64. One is an early Mustang server

ARMv8 implies 32-bit code (aarch32).  Aaarch64 implies 64-bit code.
These are two different compilers, with two different sets of command
line options.

>     $ g++ -DDEBUG -g3 -O0 -mfpu=neon-fp-armv8 -fPIC -pipe -c cryptlib.cpp
>     g++: error: unrecognized command line option ‘-mfpu=neon-fp-armv8’
>     GNUmakefile:753: recipe for target 'cryptlib.o' failed

-mfpu=neon-fp-armv8 is an arm (32-bit) compiler option.  The aarch64
(64-bit) compiler will not accept it.

Because FP and Neon support is optional in the 32-bit arm
architecture, there are compiler options to enable fp and/or neon
support.  Usually FP support is enabled by default for a linux distro,
but the neon support usually is not, and you can enable neon by using
this -mcpu=neon-fp-armv8 option if running 32-bit code on an ARMv8
architecture part.

Meanwhile, the aarch64 spec requires FP and ASIMD instruction support
in the linux ABI, so there are no options to enable them, they are on
by default.  If you really want to disable them, you can do so by
using a -march= option, e.g. -march=aarch64+fp+simd enables them, and
-march=aarch64+nofp+nosimd disables them.  However, if you disable fp
support, you will break the ABI, and your code may not compile or run,
so don't do that unless perhaps you have an embedded target, and have
your own OS build and your own ABI. or no code that uses FP  You can
also enable/.disable crc (crypto) support this way, but a better way
is to use a -mcpu= option, and let gcc figure out if the target has
crc instructions.

See the aarch64 compiler docs here
    https://gcc.gnu.org/onlinedocs/gcc/AArch64-Options.html#AArch64-Options

Jim


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