help understanding differences between #define, const and enum in C and C++ on assembly level.
- by martin
recently, i am looking into assembly codes for #define, const and enum:
C codes(#define):
3 #define pi 3
4 int main(void)
5 {
6 int a,r=1;
7 a=2*pi*r;
8 return 0;
9 }
assembly codes(for line 6 and 7 in c codes) generated by GCC:
6 mov $0x1, -0x4(%ebp)
7 mov -0x4(%ebp), %edx
7 mov %edx, %eax
7 add %eax, %eax
7 add %edx, %eax
7 add %eax, %eax
7 mov %eax, -0x8(%ebp)
C codes(enum):
2 int main(void)
3 {
4 int a,r=1;
5 enum{pi=3};
6 a=2*pi*r;
7 return 0;
8 }
assembly codes(for line 4 and 6 in c codes) generated by GCC:
6 mov $0x1, -0x4(%ebp)
7 mov -0x4(%ebp), %edx
7 mov %edx, %eax
7 add %eax, %eax
7 add %edx, %eax
7 add %eax, %eax
7 mov %eax, -0x8(%ebp)
C codes(const):
4 int main(void)
5 {
6 int a,r=1;
7 const int pi=3;
8 a=2*pi*r;
9 return 0;
10 }
assembly codes(for line 7 and 8 in c codes) generated by GCC:
6 movl $0x3, -0x8(%ebp)
7 movl $0x3, -0x4(%ebp)
8 mov -0x4(%ebp), %eax
8 add %eax, %eax
8 imul -0x8(%ebp), %eax
8 mov %eax, 0xc(%ebp)
i found that use #define and enum, the assembly codes are the same. The compiler use 3 add instructions to perform multiplication. However, when use const, imul instruction is used.
Anyone knows the reason behind that?
