What's the difference between hard and soft floating point numbers?

Question

When I compile C code with my cross toolchain, the linker prints pages of warnings saying that my executable uses hard floats but my libc uses soft floats. What's the difference?

Answer 1

Hard floats use an on-chip floating point unit. Soft floats emulate one in software. The difference is speed. It's strange to see both used on the same target architecture, since the chip either has an FPU or doesn't. You can enable soft floating point in GCC with -msoft-float. You may want to recompile your libc to use hardware floating point if you use it.

Answer 2

There are three ways to do floating point arithmetic:

• Use float instructions if your CPU has a FPU. (fast)
• Have your compiler translate floating point arithmetic to integer arithmetic. (slow)
• Use float instructions and a CPU with no FPU. Your CPU will generate a exception (Reserved Instruction, Unimplemented Instruction or similar), and if your OS kernel includes a floating point emulator it will emulate those instructions (slowest).