请教一下这个函数怎么理解，先一个宏定义，然后定义一个结构体，之后在函数中做判断，这个if.......else if 语句怎么理解，具体代码如下，谢谢各位。
#define LPC_UART LPC_UART0
typedef struct
{
union {
__I uint8_t RBR;
__O uint8_t THR;
__IO uint8_t DLL;
uint32_t RESERVED0;
};
union {
__IO uint8_t DLM;
__IO uint32_t IER;
};
union {
__I uint32_t IIR;
__O uint8_t FCR;
};
__IO uint8_t LCR;
uint8_t RESERVED1[3];
__IO uint8_t MCR;
uint8_t RESERVED2[3];
__I uint8_t LSR;
uint8_t RESERVED3[3];
__I uint8_t MSR;
uint8_t RESERVED4[3];
__IO uint8_t SCR;
uint8_t RESERVED5[3];
__IO uint32_t ACR;
uint32_t RESERVED6;
__IO uint32_t FDR;
uint32_t RESERVED7;
__IO uint8_t TER;
uint8_t RESERVED8[27];
__IO uint8_t RS485CTRL;
uint8_t RESERVED9[3];
__IO uint8_t ADRMATCH;
uint8_t RESERVED10[3];
__IO uint8_t RS485DLY;
uint8_t RESERVED11[3];
__I uint8_t FIFOLVL;
} LPC_UART1_TypeDef;
if (LPC_UART == LPC_UART0)
{}
else if ((LPC_UART1_TypeDef *)LPC_UART == LPC_UART1)
{}
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