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    <p>The JIT discards the execution of <code>DoubleCalcTest</code> since it does not have any side-effect (pure calculation) and the result is not used. The loop itself can also be optimized out since there is no effect.</p> <p>Try this with JIT off and it will take around 8000 ms:</p> <pre><code>java -Xint snippet.Snippet </code></pre> <p>At the byteocde level, nothing is optimized out.</p> <pre><code>javap -c snippet.Snippet </code></pre> <p>result:</p> <pre><code>public class snippet.Snippet { public snippet.Snippet(); Code: 0: aload_0 1: invokespecial #8 // Method java/lang/Object."&lt;init&gt;":()V 4: return public static void main(java.lang.String[]); Code: 0: invokestatic #16 // Method java/lang/System.currentTimeMillis:()J 3: lstore_1 4: iconst_0 5: istore_3 6: goto 16 9: invokestatic #22 // Method DoubleCalcTest:()D 12: pop2 13: iinc 3, 1 16: iload_3 17: ldc #26 // int 100000000 19: if_icmplt 9 22: invokestatic #16 // Method java/lang/System.currentTimeMillis:()J 25: lstore_3 26: getstatic #27 // Field java/lang/System.out:Ljava/io/PrintStream; 29: new #31 // class java/lang/StringBuilder 32: dup 33: ldc #33 // String That took 35: invokespecial #35 // Method java/lang/StringBuilder."&lt;init&gt;":(Ljava/lang/String;)V 38: lload_3 39: lload_1 40: lsub 41: invokevirtual #38 // Method java/lang/StringBuilder.append:(J)Ljava/lang/StringBuilder; 44: ldc #42 // String milliseconds 46: invokevirtual #44 // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder; 49: invokevirtual #47 // Method java/lang/StringBuilder.toString:()Ljava/lang/String; 52: invokevirtual #51 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 55: return public static double DoubleCalcTest(); Code: 0: ldc2_w #64 // double 987.654321d 3: dstore_0 4: ldc2_w #66 // double 123.456789d 7: dstore_2 8: dload_0 9: dload_2 10: dadd 11: dstore_0 12: dload_0 13: dload_2 14: dsub 15: dstore_0 16: dload_0 17: dload_2 18: dmul 19: dstore_0 20: dload_0 21: dload_2 22: ddiv 23: dreturn } </code></pre> <p>If you try to use to result of DoubleCalc() by assigning it to a variable and print it afterward. </p> <pre><code>public static void main(String[] args) { long startTime = System.currentTimeMillis(); double res = 0; for (int i = 0; i &lt; 100000000; i++) { res = DoubleCalcTest(); } System.out.println(res); long endTime = System.currentTimeMillis(); System.out.println("That took " + (endTime - startTime) + " milliseconds"); } </code></pre> <p>It will take the same time. Why? The JIT seems to be smart enough to understand that the result does not depend on the number of time the iteration is done.</p> <p>However, if you change this to:</p> <pre><code>public static void main(String[] args) { long startTime = System.currentTimeMillis(); double res = 0; for (int i = 0; i &lt; 100000000; i++) { res += DoubleCalcTest(); } System.out.println(res); long endTime = System.currentTimeMillis(); System.out.println("That took " + (endTime - startTime) + " milliseconds"); } </code></pre> <p>The result depends on the number of iteration and the JIT does not optimize further. In this case, it takes about 100 ms. If I change 100000000 for 200000000, it takes twice as much time.</p> <p>So the conclusion is that the JIT stops there.</p> <p>NOTE:</p> <p>For the give C program:</p> <pre><code>#include &lt;stdio.h&gt; int main(int argc, char** argv) { long x = 0; int i; for(i=0; i&lt;1000000; i++) { x+=i; } printf("%ld", x); } </code></pre> <p>GCC is able to optimized the loop entirely and compute the value of x at compile-time:</p> <pre><code>gcc -O2 -S main.c </code></pre> <p>main.s:</p> <pre><code> .file "main.c" .section .rodata.str1.1,"aMS",@progbits,1 .LC0: .string "%ld" .section .text.startup,"ax",@progbits .p2align 4,,15 .globl main .type main, @function main: .LFB11: .cfi_startproc movabsq $499999500000, %rsi &lt;---- See, this is the pre-computed result movl $.LC0, %edi xorl %eax, %eax jmp printf .cfi_endproc .LFE11: .size main, .-main .ident "GCC: (GNU) 4.7.2 20121109 (Red Hat 4.7.2-8)" .section .note.GNU-stack,"",@progbits </code></pre> <p>Pretty cool, heh?</p>
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    1. COWOW~, thanks for the efforts. Really good answer.
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