How Do C Compilers Optimize Function Argument Evaluation Order?

Compiler Optimization of Indeterminate Function Argument Sequencing

C allows implementations to evaluate function arguments in any order, raising questions about whether compilers exploit this indeterminacy for performance benefits.

The standard prescribes that the order of argument evaluation is implementation-dependent. Some compilers may utilize this flexibility in certain scenarios. For example, on an x86 architecture, the Pascal calling convention evaluates arguments left to right, while the C calling convention (__cdecl) evaluates them right to left. This can affect the program's behavior if arguments have side effects, such as modifying global variables.

Platform-specific factors also play a role. Most multi-platform programs consider calling conventions to avoid cross-platform issues. However, it is possible for some compilers to optimize argument evaluation based on specific calling conventions and architecture limitations.

It's important to note that the language standard does not specify the order of argument evaluation, leaving it as "unspecified" rather than "undefined." This means that implementations are free to choose an order that is not guaranteed to be consistent across different environments. As a result, relying on a specific evaluation order is not advisable.

Compilers can take advantage of argument evaluation indeterminacy in limited scenarios, but they typically prioritize compatibility and reliability over performance gains. Programs relying on specific argument evaluation orders should be avoided, as they may lead to unexpected behaviors or platform-specific issues.

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