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copied!<p>The difference is that with <code>#define</code> or <code>enum</code> the value 3 doesn't need to exist as an explicit value in the code and so the compiler has decided to use two add instructions rather than allocating space for the constant 3. The <code>add reg,reg</code> instruction is 2 bytes per instruction, so thats 6 bytes of instructions and 0 bytes for constants to multiply by 3, that's smaller code than <code>imul</code> plus a 4 byte constant. Plus the way the add instructions are used, it works out to a pretty literal translation of *2 *3, so this may not be a size optimization, it may be the default compiler output whenever you multiply by 2 or by 3. (add is usually a faster instruction than multiply). </p> <p><code>#define</code> and <code>enum</code> don't declare an instance, they only provide a way to give a symbolic name to the value 3, so the compiler has the option of making smaller code. </p> <pre><code> mov $0x1, -0x4(%ebp) ; r=1 mov -0x4(%ebp), %edx ; edx = r mov %edx, %eax ; eax = edx add %eax, %eax ; *2 add %edx, %eax ; add %eax, %eax ; *3 mov %eax, -0x8(%ebp) ; a = eax </code></pre> <p>But when you declare <code>const int pi = 3</code>, you tell the compiler to allocate space for an integer value and initialize it with 3. That uses 4 bytes, but the constant is now available to use as an operand for the <code>imul</code> instruction. </p> <pre><code> movl $0x3, -0x8(%ebp) ; pi = 3 movl $0x3, -0x4(%ebp) ; r = 3? (typo?) mov -0x4(%ebp), %eax ; eax = r add %eax, %eax ; *2 imul -0x8(%ebp), %eax ; *pi mov %eax, 0xc(%ebp) ; a = eax </code></pre> <p>By the way, this is clearly not optimized code. Because the value <code>a</code> is never used, so if optimization were turned on, the compiler would just execute </p> <pre><code>xor eax, eax ; return 0 </code></pre> <p>In all 3 cases.</p> <h1>Addendum:</h1> <p>I tried this with MSVC and in debug mode I get the same output for all 3 cases, MSVC always uses imul by a literal 6. Even in case 3 when it creates the <code>const int = 3</code> it doesn't actually reference it in the imul. </p> <p>I don't think this test really tells you anything about const vs define vs enum because this is non-optimized code. </p>

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