在GNU GCC 4.5.3中输入int short to

Question

我想知道，当两个整数相乘并且结果为typecast为short且分配为short时，编译器将如何解析它？下面是代码片段

int a=1,b=2,c; 
short x=3,y=4,z; 

int p; 
short q; 

int main() 
{ 

c = a*b; /* Mul two ints and assign to int 
      [compiler resolves this to __mulsi3()] */ 

z = x*y; /* Mul two short and assign to short 
      [compiler resolves this to __mulhi3()] */ 

p = (x*y); /* Mul two short and assign to int 
       [compiler resolves this to __mulsi3()] */ 

q =(short)(a*b); /* Mul two ints typecast to short and assign to short 
        [compiler resolves this to __mulhi3()] */ 

return 0; 

} 

在这里，在情况下q =(short)(a*b);，应（使用__mulsi3()）来执行第一两个整数乘法，然后将其分配给短。但在这里并非如此，编译器类型会将a和b都设为空格，然后调用__mulhi3()。

我想知道如何更改gcc源代码[哪个文件]，以便我可以达到我的上述要求。

Answer 1

编译器可以分析代码，并看到在立即将结果转换为short时，可以按照short乘法完成乘法操作，而不会影响结果。这和你的例子中的情况二完全一样。

结果是一样的，你不必担心使用哪个乘法函数。