Why Does Compiler Optimization Break This 64-bit Integer Swapping Code?

<br>inline u64 Swap_64(u64 x)<br>{</p>
<div class="code" style="position:relative; padding:0px; margin:0px;"><pre class="brush:php;toolbar:false">u64 tmp;
(*(u32*)&amp;tmp)       = Swap_32(*(((u32*)&amp;x)+1));
(*(((u32*)&amp;tmp)+1)) = Swap_32(*(u32*) &amp;x);

return tmp;

}

Initially interpreted as a coding style issue, the lecturer claimed optimization would render the code ineffective. A reason for this behavior was called into question.

Violation of Strict Aliasing Rules

The cause of the problem lies in the violation of strict aliasing rules. These rules dictate that a memory location can only be accessed through a pointer of a compatible type. In the given code, accesses to a 32-bit word in a 64-bit integer via pointers of different types violate this rule.

Aliasing and Undefined Behavior

The compiler is permitted to optimize based on the strict aliasing rule, assuming no aliasing between pointers of different types. Consequently, the assignments to the temporary variable tmp are eliminated as unnecessary, resulting in no modification to x.

Understanding Strict Aliasing

To address this issue, a deep understanding of strict aliasing is crucial. The C99 standard defines strict aliasing in section 6.5, paragraph 7. This rule ensures that an object's stored value is accessed solely through expressions compatible with its effective type.

Alternative Solutions

To work around this optimization pitfall, several solutions exist. One approach is to use type-punning through a union. This technique allows multiple data types to share the same memory space without violating aliasing rules.

In conclusion, optimization can profoundly impact code behavior. Understanding concepts like strict aliasing is paramount to avoid unintended consequences when optimizations are applied.

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