本人在调试一个AVR系统时发现一个百思不得其解的问题，后经仔细分析后才找到答案，原来是GCC在优化时未考虑变量的“非顺序性”改变（如被中断程序修改）,造成了优化后程序执行结果的不同。下面给出一个典型的测试程序：
// 以下程序用AVR Studio v4.12 service Pack4 + WINAVR 20060421 编译
// 其用意是将串口输入的八位数据从 PORT A 口输出。
///////////////////////////////////////
// TEST GCC for AVR optimization
///////////////////////////////////////
#include <avr/io.h>
#include <avr/interrupt.h>
unsigned char Buffer;
void Init( void );
void USART_Init ( unsigned int baud );
unsigned char USART_Receive( void );
int main ( void )
{
Init();
while ( 1 )
{
PORTA = Buffer;
}
}
void Init( void )
{
// set PORTA as output port
DDRA = 0xFF;
// initialize UART
USART_Init( 71 ); // set B.R. 9600 for 11.0592 MHz
// enable interupt
sei();
}
void USART_Init ( unsigned int baud )
{
// set BAUD RATE
UBRRH = (unsigned char) (baud >> 8);
UBRRL = (unsigned char) baud;
// enable receiver and transmitter and RXC interrupt
UCSRB = (1 << RXEN) | (1 << TXEN) | (1 << RXCIE);
// set frame format: 8 data bit, 1 stop bit
UCSRC = (1 << URSEL) | (3 << UCSZ0);
}
unsigned char USART_Receive( void )
{
/* Wait for data to be received */
while ( !(UCSRA & (1 << RXC)) ) ;
/* Get and return received data from buffer */
return UDR;
}
// ISR for UART
ISR ( USART_RXC_vect )
{
Buffer = USART_Receive();
}
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