objc_msgSend is the Objective-C message dispatcher. objc_msgSend_stret is exactly the same, but for methods that return values of struct types. And objc_msgSend_fpret is for methods that return floating-point values on some architectures.

objc_msgSend_stret exists because the parameters are passed in different places for struct-returning functions. That's not the problem that objc_msgSend_fpret solves. Instead, objc_msgSend_fpret exists to handle the return value itself correctly. Specifically, the return value of a message sent to nil on i386 and x86_64.

Messages to nil return zero if possible. On ppc, objc_msgSend with a nil receiver clears registers r3, r4, f1, and f2 before returning. This means any pointer or integer or floating-point return value will be zero, and structs are undefined. On ppc, objc_msgSend_fpret is unnecessary, because clearing f1 and f2 is harmless if the caller is actually expecting a value in r3 or r4.

i386 is different. The floating-point registers there are historically derived from the 8087 FPU for the original 8086 CPU. The x87 unit is an odd beast: it has eight floating-point registers, but the registers themselves are used as if they were a stack. Values are pushed and popped from this stack even though the stack is stored in registers and has a maximum of eight entries.

For return values, the callee pushes the value on the x87 stack and the caller pops it. This is fine for C functions, but not so good when objc_msgSend returns zero. objc_msgSend does not know what return type the caller really wants, and it must not push a zero on the x87 stack unless the caller expects to pop a floating-point value.

The solution is objc_msgSend_fpret. During a message to nil on i386, objc_msgSend_fpret pushes a zero on the x87 stack which the caller will pop, and objc_msgSend does not. The caller knows which return type the caller expects, and uses the matching dispatcher. On ppc, ppc64, and arm, objc_msgSend_fpret is identical to objc_msgSend and is usually unused.

What about x86_64? This architecture is one step forward, two steps back. The good news is that return values of types float and double are returned in the XMM registers. objc_msgSend can handle that itself just like ppc. The bad news is that long double still uses the x87 stack. So objc_msgSend_fpret still exists, but is only used for long double on x86_64. The worse news is that C99's complex long double returns two values on the x87 stack. So now there's objc_msgSend_fp2ret for that case only. Currently no compiler actually uses objc_msgSend_fp2ret, so hopefully nobody is writing code that sends a message to nil on x86_64 and expects a zero return value of type complex long double.

One last thing: Mac OS X 10.4 and earlier did not return zero for floating-point messages to nil on ppc. Be careful if you're writing code for those older systems. All other architectures have always returned floating-point zero, including i386 on 10.4.