Calling conventions for C functions for the VFP

Question

If I write a function like this:
float test(float a)
return a*a;
And then I look at the assembly code generated:
_test:
LFB48:
LM1:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, usesanonymousargs = 0
@ link register save eliminated.
LVL0:
LM2:
fmsr s14, r0
LM3:
@ lr needed for prologue
LM4:
fmuls s13, s14, s14
fmrs r0, s13
LVL1:
bx lr
The question is, why are the input register to this function belong to ARM and not to VFP? Surly the input registers should be S0 and not R0. This seems bad to me because the ARM register needs to be converted to a VFP register and then reconverted back to a CPU register after the calculation.
I thought C calling conventions were R0-R3 for ARM and S0-S7 for VFP?
Is there a way to make the compiler follow this convention?

Answer 1

C++ can call on C code as long as C++ is given instructions that the function prototypes are in fact C functions, and not C++ functions.
You have to do this since C and C++-s scheme of handling function names at the object code level are different. By invoking the extern "C" {}, the compiler will generate object code that can link against C code.
You have maybe seen C header files with
#ifdef __cplusplus
extern "C" {
#endif
//... prototypes here
#ifdef __cplusplus
#endif
Last edited by Themaister (2010-05-15 20:19:35)