There needs an explanation on Inline Assembly page
How to put C3 vars to/from inline assembly
// Gcc/Clang
int add(int a, int b) {
int result;
__asm__ (
"addl %%ebx, %%eax;" // The actual assembly instruction
: "=a" (result) // Output: 'a' refers to the EAX register
: "a" (a), "b" (b) // Inputs: 'a' into EAX, 'b' into EBX
);
return result;
}
int add_optimized(int a, int b) {
int result;
__asm__ (
"addl %2, %0;"
: "=r" (result) // %0: compiler chooses a register for output
: "0" (a), // %1: use the same register as %0 for input 'a'
"r" (b) // %2: compiler chooses a register for input 'b'
);
return result;
}
// Note: Only works on 32-bit MSVC
int add_msvc(int a, int b) {
__asm {
mov eax, a
add eax, b
// Result is left in eax, which is the standard return register
}
}
int add_arm(int a, int b) {
int result;
__asm__ (
"add %w0, %w1, %w2" // Instruction: result = a + b
: "=r" (result) // %0: Output operand
: "r" (a), // %1: Input operand
"r" (b) // %2: Input operand
);
return result;
}
There needs an explanation on Inline Assembly page
How to put C3 vars to/from inline assembly