Inline ASM

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asm("statements" : output_registers : input_registers : clobber_registers)

int mul2(int a, int b) {
    int val;
    __asm__ __volatile__(   "movl  %1, %0\n\t"
                            "imull %2, %0"
                        :   "=r"(val)
                        :   "r"(a), "r"(b)
                        );

    return val;
}

Register Letter Registers That GCC May Use
R General register (EAX, EBX, ECX, EDX, ESI, EDI, EBP, ESP)
q General register for data (EAX, EBX, ECX, EDX)
f Floating-point register
t Top floating-point register
u Second-from-top floating-point register
a EAX register
b EBX register
c ECX register
d EDX register
x SSE register (Streaming SIMD Extension register)
y MMX multimedia registers
A An 8-byte value formed from EAX and EDX
D Destination pointer for string operations (EDI)
S Source pointer for string operations (ESI)```

References and Further Reading