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POhelp understanding differences between #define, const and enum in C and C++ on assembly level
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copied!<p>recently, i am looking into assembly codes for #define, const and enum:</p> <p>C codes(#define):</p> <pre><code>3 #define pi 3 4 int main(void) 5 { 6 int a,r=1; 7 a=2*pi*r; 8 return 0; 9 } </code></pre> <p>assembly codes(for line 6 and 7 in c codes) generated by GCC:</p> <pre><code>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) </code></pre> <p>C codes(enum):</p> <pre><code>2 int main(void) 3 { 4 int a,r=1; 5 enum{pi=3}; 6 a=2*pi*r; 7 return 0; 8 } </code></pre> <p>assembly codes(for line 4 and 6 in c codes) generated by GCC:</p> <pre><code>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) </code></pre> <p>C codes(const):</p> <pre><code>4 int main(void) 5 { 6 int a,r=1; 7 const int pi=3; 8 a=2*pi*r; 9 return 0; 10 } </code></pre> <p>assembly codes(for line 7 and 8 in c codes) generated by GCC:</p> <pre><code>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) </code></pre> <p>i found that use <code>#define</code> and <code>enum</code>, the assembly codes are the same. The compiler use 3 add instructions to perform multiplication. However, when use <code>const</code>, imul instruction is used. Anyone knows the reason behind that?</p>

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