How to (cross-)compile to both ARM hard- and soft-float (softfp) with a single GCC (cross-)compiler?

Question

I'd like to use a single (cross-)compiler to compile code for different ARM calling conventions: since I always want to use floating point and NEON instructions, I just want to select the hard-float calling convention or the soft-float (softfp) calling convention.

My compiler defaults to hard-float, but it supports both architectures that I need:

$ arm-linux-gnueabihf-gcc -print-multi-lib
.;
arm-linux-gnueabi;@marm@march=armv4t@mfloat-abi=soft
$

When I compile with the default parameters:

$ arm-linux-gnueabihf-g++ -Wall -o hello_world_armhf hello_world.cpp

It succeeds without any errors.

If I compile with the parameters returned by -print-multi-lib:

$ arm-linux-gnueabihf-g++ -marm -march=armv4t -mfloat-abi=soft -Wall -o hello_world hello_world.cpp

It again compiles without error (By the way, how can I test that the resultant code is hard- or soft-float?)

Unfortunately, if I try this:

$ arm-linux-gnueabihf-g++ -march=armv7-a -mthumb-interwork -mfloat-abi=softfp -mfpu=neon -Wall -o hello_world hello_world.cpp
[...]/gcc/bin/../lib/gcc/arm-linux-gnueabihf/4.7.3/../../../../arm-linux-gnueabihf/bin/ld: error: hello_world uses VFP register arguments, /tmp/ccwvfDJo.o does not
[...]/gcc/bin/../lib/gcc/arm-linux-gnueabihf/4.7.3/../../../../arm-linux-gnueabihf/bin/ld: failed to merge target specific data of file /tmp/ccwvfDJo.o
collect2: error: ld returned 1 exit status
$

I've tested some other permutations of the parameters, but it seems that anything other than the combination shown by -print-multi-lib results in an error.

I've read ARM compilation error, VFP registered used by executable, not object file but the problem there was that some parts of the binary were soft- and some were hard-float. I have a single C++ file to compile...

What parameter(s) I miss to be able to compile with -march=armv7-a -mthumb-interwork -mfloat-abi=softfp -mfpu=neon?

How is it possible that the error is about VFP register arguments while I explicitly have -mfloat-abi=softfp in the command line which prohibits VFP register arguments?

Thanks!

For the records, hello_world.cpp contains the following:

#include <iostream>

int main()
{
  std::cout << "Hello, world!" << std::endl;
  return 0;
}

Answer 1

You need another compiler with corresponding multilib support. You can check multilib support with next command.

arm-none-eabi-gcc -print-multi-lib
.;
thumb;@mthumb
fpu;@mfloat-abi=hard
armv6-m;@mthumb@march=armv6s-m
armv7-m;@mthumb@march=armv7-m
armv7e-m;@mthumb@march=armv7e-m
armv7-ar/thumb;@mthumb@march=armv7
cortex-m7;@mthumb@mcpu=cortex-m7
armv7e-m/softfp;@mthumb@march=armv7e-m@mfloat-abi=softfp@mfpu=fpv4-sp-d16
armv7e-m/fpu;@mthumb@march=armv7e-m@mfloat-abi=hard@mfpu=fpv4-sp-d16
armv7-ar/thumb/softfp;@mthumb@march=armv7@mfloat-abi=softfp@mfpu=vfpv3-d16
armv7-ar/thumb/fpu;@mthumb@march=armv7@mfloat-abi=hard@mfpu=vfpv3-d16
cortex-m7/softfp/fpv5-sp-d16;@mthumb@mcpu=cortex-m7@mfloat-abi=softfp@mfpu=fpv5-sp-d16
cortex-m7/softfp/fpv5-d16;@mthumb@mcpu=cortex-m7@mfloat-abi=softfp@mfpu=fpv5-d16
cortex-m7/fpu/fpv5-sp-d16;@mthumb@mcpu=cortex-m7@mfloat-abi=hard@mfpu=fpv5-sp-d16
cortex-m7/fpu/fpv5-d16;@mthumb@mcpu=cortex-m7@mfloat-abi=hard@mfpu=fpv5-d16
https://stackoverflow.com/questions/37418986/how-to-interpret-the-output-of-gcc-print-multi-lib

How to interpret the output of gcc -print-multi-lib With this configuration gcc -mfloat-abi=hard not only will build your files using FPU instructions but also link them with corresponding libs, avoiding "X uses VFP register arguments, Y does not" error.

The above-mentioned -print-multi-lib output produced by gcc with this patch and --with-multilib-list=armv6-m,armv7,armv7-m,armv7e-m,armv7-r,armv7-a,cortex-m7 configuration option. If you are interested in building your own gcc with Cortex-A series multilib support, just use --with-multilib-list=aprofile configuration option for any arm*-*-* target without any patches (at list with gcc-6.2.0).