5.43 Example of hardware and software support differences for floating-point arithmetic

This example shows how the compiler deals with floating-point arithmetic for different processors supporting either hardware or software floating-point arithmetic.

The following example shows a function implementing floating-point arithmetic in C code.

float foo(float num1, float num2)
{
    float temp, temp2;
    temp = num1 + num2;
    temp2 = num2 * num2;
    return temp2 - temp;
}

When the example C code is compiled with the command-line options --cpu 5TE and --fpu softvfp, the compiler produces machine code with the disassembly shown below. In this case, floating-point arithmetic is performed in software through calls to library routines such as __aeabi_fmul.

||foo|| PROC
    PUSH     {r4-r6, lr}
    MOV      r4, r1
    BL       __aeabi_fadd
    MOV      r5, r0
    MOV      r1, r4
    MOV      r0, r4
    BL       __aeabi_fmul
    MOV      r1, r5
    POP      {r4-r6, lr}
    B        __aeabi_fsub
    ENDP

However, when the example C code is compiled with the command-line option --fpu vfp, the compiler produces machine code with the disassembly shown below. In this case, floating-point arithmetic is performed in hardware through floating-point arithmetic instructions such as VMUL.F32.

||foo|| PROC
    VADD.F32 s2, s0, s1
    VMUL.F32 s0, s1, s1
    VSUB.F32 s0, s0, s2
    BX       lr
    ENDP
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