The Difference Between HardFP and SoftFP
Raspberry Pi 3 main chip is Broadcom BCM2837, and it's a SoC (System-on-Chip). This SoC includes CPU, GPU and RAM. CPU is ARM Cortex-A53 1.2 GHz processor, capable of doing floating point math in hardware, including VFP extensions (Vector Floating Point), somewhat similar to x86 SIMD called SSE2.
But hardware floating point on ARM architecture at large is optional feature, so there is a default ABI (Application Binary Interface) between applications and C runtime library assuming that there's no floating point support in hardware. All floating point math is emulated in software, so it works slow, but widely complatible among ARM chips and Linux kernels. This ABI is called SoftFP. Most of GNU/Linux distributions for Raspberry Pi, including Debian Jessie, are using this slow SoftFP ABI.
HardFP-enabled GNU/Linux distributions like Raspbian Jessie work much faster when it comes to intensive floating-point computations, like games, but they require special patched glibc and special patched gcc capable of producing binaries tied to HardFP loader (ld-linux-armhf.so.3) instead of default SoftFP loader (ld-linux.so.3).
When it comes to cross-compiling for Raspberry Pi, there's a requirement to build SoftFP toolchain in order to support non-accelerated GNU/Linux distributions without ld-linux-armhf.so.3 and in order to fall back to software emulation if something related to the new HardFP ABI goes wrong.
Between 2012 and 2016 patches for ARM hardfp support got merged from Raspbian into mainline GNU gcc and GNU glibc, so it's no longer necessary to apply them manually for the purpose of building this cross-compiler.
Summary:
| SoftFP ABI | HardFP ABI | |
| CPU supports floating point instructions | Yes | Yes |
| Floating point instructions are emitted by GCC | No | Yes |
| Games and other computationally intensive software is | Slow | Fast |
| Register calling convention for libc and other *.so functions | arm-linux-gnueabi | arm-linux-gnueabihf |
| Dynamic loader | ld-linux.so.3 | ld-linux-armhf.so.3 |
| GCC configuration options | --with-float=soft | --with-float=hard --with-fpu=vfp |
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