Hi, i'm currently trying to port the software FPU emulation of linux-m68k to FreeMiNT, and got some questions about it. As seen in https://github.com/torvalds/linux/blob/ 7eb172143d5508b4da468ed59ee857c6e5e01da6/arch/m68k/kernel/vectors.c#L96 and https://github.com/torvalds/linux/blob/ 7eb172143d5508b4da468ed59ee857c6e5e01da6/arch/m68k/kernel/vectors.c#L119 the FPSP for 040/060 are not installed when the software emulation is used. However, almost all trigonometric functions are not implemented: https:// github.com/torvalds/linux/blob/7eb172143d5508b4da468ed59ee857c6e5e01da6/arch/ m68k/math-emu/fp_trig.c#L21-L28 What sense does it then make to use an emulation, when only basic instructions like fadd/fmul etc. are implemented? Programs are not even aborted when using one of those functions, uprint just prints an error message, and the functions just return with their input arguments as result instead of calculating the expected value. Then i also noticed, that there seem to be some quirks. Eg. the IS_ZERO macro is defined as #define IS_ZERO(a) ((a)->mant.m64 == 0) But that condition is also true for infinities, and there are several places where IS_ZERO is used before checking for infinities. Eg. in the emulation of fmul https://github.com/torvalds/linux/blob/ 7eb172143d5508b4da468ed59ee857c6e5e01da6/arch/m68k/math-emu/fp_arith.c#L157 +inf * +inf will return a NAN instead of +inf. Next problem is the interaction between assembler/C. On entry, a2 is loaded with a pointer to the emulated FPU register set in the thread struct. However, all the arithmetic functions call out to C functions, which in turn callback some assembler functions like fp_long_ext2ext. At that time, a2 may contain any value. Won't that just crash, or did i miss something? Another problem: the fsqrt function seems to be broken when using arguments < 1.0. Eg. fsqrt(0.75) yields 1.7320508075. Looks like the returned exponent is off by one there. So essentially: is that emulation actually used anywhere?
