How Does Compiler Optimization Affect Indeterminate Function Argument Evaluation in C ?

Indeterminate Function Argument Evaluation: Implementation Strategies

In C , the order of evaluation for function arguments is unspecified, allowing implementations to optimize as they see fit. However, do compilers leverage this flexibility to impact program execution?

Impact of Argument Evaluation Order

Consider the following classic example:

int i = 0;
foo(i++, i++);

The indeterminate evaluation order of i allows the compiler to sequence it differently, potentially affecting the program's logic.

Compiler Strategies

Compilers handle indeterminate evaluation in various ways, depending on factors like argument type, calling convention, architecture, and specific compiler implementation.

x86 Calling Conventions

• Pascal Calling Convention: Arguments are evaluated in a left-to-right order.
• C Calling Convention (__cdecl): Arguments are evaluated in a right-to-left order.

Cross-platform programs typically consider calling conventions to avoid unexpected behavior.

Compiler-Specific Optimizations

Some compilers may employ optimizations based on the argument evaluation order. For example, if the compiler detects that the arguments are not used in the function body, it may decide not to perform any evaluation at all.

Uncertainties and Portability

It's important to note that, while some compilers may optimize based on evaluation order, the standard does not mandate any specific behavior. This means that code relying on a particular evaluation order may exhibit unexpected behavior on different platforms or with different compilers.

Therefore, it is better to write code that does not depend on specific argument evaluation order and to always consider platform and compiler compatibility when working with such scenarios.

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