An Application Binary Interface (ABI) defines a calling convention, which defines how parameters are passed from a caller to a called procedure and how a return value is returned. For an ARM CPU the commonly used ABI is called EABI. This ABI allows for two incompatible ways to pass floating point numbers:
| Calling Convention | Description | GCC flag |
|---|---|---|
| EABI soft-float | Floats are passed in normal (integer) registers. | -mfloat-abi=soft or -mfloat-abi=softfp |
| EABI hard-float | Floats are passed in floating point registers (VFP). | -mfloat-abi=hard |
Usually one uses a cross toolchain which is configured for the needed ABI so that one does not need to give the used ABI on the command line. Also tool-chain provided libraries have automatically the correct format.
Two binaries (e.g. executables and C library) compiled using different calling conventions are incompatible with each other. All programs and libraries must be compiled either with softfp/soft or with hard.
If you try to run a program which uses soft calling convention on a system compiled for hard float calling convention you will get a 'No such file or directory' error, despite that the file exists and has executable permissions.
Note that the hard float calling convention uses registers of the floating point unit. So this is inherently incompatible with CPUs which do not have such a unit.
| BSP Version | Calling Convention |
|---|---|
| BSP V1.x | EABI soft-float |
| BSP V2.x and later | EABI hard-float |
ARM CPUs can have a built in floating point unit (FPU) to accelerate operations which use floating point operands. Such an FPU adds additional instructions to the available instruction set.
Related is the NEON instruction set which adds the capability to have a single instruction act on multiple data (SIMD). An implementation of this instruction set is the NEON media processing engine. This coprocessor additionally provides FPU functionality in the form of the VFPv3 instruction set.
See also:
- en.wikipedia.org/wiki/ARM_architecture
- wiki.debian.org/ArmHardFloatPort/VfpComparison
| Module/CPU family | VFP Unit | NEON unit |
|---|---|---|
| Colibri PXA(1) | - | - |
| Colibri/Apalis iMX6 | VFPv3 | Yes |
| Colibri T20 | VFPv3-D16 | - |
| Colibri/Apalis T30 | VFPv3 | Yes |
| Colibri VFxx | VFPv3 | Yes |
1: PXA Modules do not have any hardware floating point unit. However, GCC provides optimized software floating point emulation using Intels Integer SIMD extension (iWMMXt)
gcc.gnu.org/onlinedocs/gcc-4.7.3/gcc/ARM-Options.html#ARM-Options
| BSP Version | GCC flag |
|---|---|
| BSP V1.x | -march=armv7-a -mfloat-abi=softfp -mfpu=vfpv3-d16 |
| BSP V2.x and later | -march=armv7-a -mfloat-abi=hard -mfpu=vfpv3-d16 |
Use a compiler for soft float calling conventions and don't pass an fpu related compiler option to the compiler. The resulting code does not contain instructions which are executed by the FPU but rather depends on library code doing the calculations with the CPU's integer instruction set.
-march=armv5te -mtune=xscale -O3
The resulting code is compatible with a CPU having a NEON coprocessor.
Depending on your calling convention use 'softfp' or 'hard' for the mfloat-abi option.
-march=armv7-a -mfloat-abi=xxx -mfpu=vfpv3-d16 -mtune=cortex-a9 -O3
Depending on your calling convention use 'softfp' or 'hard' for the mfloat-abi option.
-march=armv7-a -mfloat-abi=xxx -mfpu=neon -mtune=cortex-a9 -O3
Depending on your calling convention use 'softfp' or 'hard' for the mfloat-abi option.
The NEON implementation on the Vybrid might even be further optimized with a more specific mfpu option.
-march=armv7-a -mfloat-abi=xxx -mfpu=neon -mtune=cortex-a5 -O3
In our Linux Images all object files use ELF.
en.wikipedia.org/wiki/Executable_and_Linkable_Format
The top level architecture (e.g. i686 or ARM) can be queried with the "file 'afile'" program, information on architecture related properties with the "readelf -A 'afile'" program. (readelf from both native and cross toolchain can be used for this.)
If the calling convention is hard floating point the line "Tag_ABI_VFP_args: VFP registers" is part of the readelf output. If that tag is missing soft floating point convention is used.
Example for a shared object compiled for a Colibri T20 module.
$ file colibri-t20/usr/lib/libcurl.so.5.3.0 colibri-t20/usr/lib/libcurl.so.5.3.0: ELF 32-bit LSB shared object, ARM, version 1 (SYSV), dynamically linked, not stripped $ readelf -A colibri-t20/usr/lib/libcurl.so.5.3.0 Attribute Section: aeabi File Attributes Tag_CPU_name: "7-A" Tag_CPU_arch: v7 Tag_CPU_arch_profile: Application Tag_ARM_ISA_use: Yes Tag_THUMB_ISA_use: Thumb-2 Tag_FP_arch: VFPv3-D16 Tag_ABI_PCS_wchar_t: 4 Tag_ABI_FP_denormal: Needed Tag_ABI_FP_exceptions: Needed Tag_ABI_FP_number_model: IEEE 754 Tag_ABI_align_needed: 8-byte Tag_ABI_align_preserved: 8-byte, except leaf SP Tag_ABI_enum_size: int Tag_ABI_HardFP_use: SP and DP Tag_ABI_VFP_args: VFP registers Tag_CPU_unaligned_access: v6
Example for a shared object compiled for a NXP/Freescale Vybrid.
$ file twr-vf65gs10/usr/lib/libcurl.so.5.3.0 twr-vf65gs10/usr/lib/libcurl.so.5.3.0: ELF 32-bit LSB shared object, ARM, version 1 (SYSV), dynamically linked, not stripped $ readelf -A twr-vf65gs10/usr/lib/libcurl.so.5.3.0 Attribute Section: aeabi File Attributes Tag_CPU_name: "7-A" Tag_CPU_arch: v7 Tag_CPU_arch_profile: Application Tag_ARM_ISA_use: Yes Tag_THUMB_ISA_use: Thumb-2 Tag_FP_arch: VFPv3 Tag_Advanced_SIMD_arch: NEONv1 Tag_ABI_PCS_wchar_t: 4 Tag_ABI_FP_denormal: Needed Tag_ABI_FP_exceptions: Needed Tag_ABI_FP_number_model: IEEE 754 Tag_ABI_align_needed: 8-byte Tag_ABI_align_preserved: 8-byte, except leaf SP Tag_ABI_enum_size: int Tag_ABI_HardFP_use: SP and DP Tag_CPU_unaligned_access: v6
Example for a shared object compiled for a Colibri PXA.
$ file colibri-pxa/usr/lib/libcurl.so.5.3.0 colibri-pxa/usr/lib/libcurl.so.5.3.0: ELF 32-bit LSB shared object, ARM, version 1 (SYSV), dynamically linked, not stripped $ readelf -A colibri-pxa/usr/lib/libcurl.so.5.3.0 Attribute Section: aeabi File Attributes Tag_CPU_name: "5TE" Tag_CPU_arch: v5TE Tag_ARM_ISA_use: Yes Tag_THUMB_ISA_use: Thumb-1 Tag_ABI_PCS_wchar_t: 4 Tag_ABI_FP_denormal: Needed Tag_ABI_FP_exceptions: Needed Tag_ABI_FP_number_model: IEEE 754 Tag_ABI_align8_neededU: Yes Tag_ABI_align8_preserved: Yes, except leaf SP Tag_ABI_enum_size: int
