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17053797
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2013-06-11T21:04:46.353
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2017-10-30T13:25:10.203
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2017-10-30T13:25:10.203
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17052001
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2
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3
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text
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<h2>Bytecode patterns for <code>&&</code> and <code>||</code></h2> <p>Think about what a short-circuiting operator like <code>&&</code> or <code>||</code> actually does. You've got some conditional branching. Let's consider <code>&&</code>. What you're effectively evaluating is:</p> <pre><code>if (left) if (right) <do something> endIf </code></pre> <p>There is no single bytecode instruction that can describe this behavior. You need need some labels and conditional branching instructions:</p> <pre><code>.start <left expression> IFEQ .endIf // if left evaluates to zero (false), skip to end <right expression> IFEQ .endIf // if right evaluates to zero (false), skip to end .ifTrue <body of 'if' block> .endIf </code></pre> <p>The behavior of the <code>||</code> operator is a bit different; in this case, the logic looks something like this:</p> <pre><code> if (left) goto .ifTrue if (!right) goto .endIf .ifTrue <do something> .endIf </code></pre> <p>Note how the check on the right operand is inverted to avoid an additional branch when the right operand evaluates to <code>true</code>. This behavior could be implemented in bytecode like so:</p> <pre><code> <left operand> IFNE .ifTrue // if left evaluates true, skip right, enter 'if' body <right operand> IFEQ .endIf // if right evaluates false, skip 'if' body .ifTrue <do something> .endIf </code></pre> <hr> <h2>When to push your operands</h2> <p>Note that your original question suggested you already have the left and right operands on the stack; that would be bad. You should only evaluate the right operand <em>after</em> the left operand has evaluated to <code>true</code> (nonzero) for <code>&&</code> or <code>false</code> (zero) for <code>||</code>. If the right operand causes side effects, evaluating it prematurely would violate the defined behavior of these operators.</p>
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