Can Compilers Optimize Out Heap Memory Allocations Without Violating the As-if Rule?

Compiler Optimization of Heap Memory Allocations

The question of whether a compiler can optimize out heap memory allocations arises from the observation that some compilers perform this optimization, while others do not. In particular, clang 3.0 and later versions optimize out the new call in the following code:

int main()
{
    int* mem = new int[100];
    return 0;
}

while g and Visual Studio do not. This raises the concern that such an optimization might violate the as-if rule, which requires compilers to produce observable behavior as if the standard had been followed.

Clang Optimization and the As-if Rule

The history of clang's optimization sheds light on its validity. Clang's optimization follows N3664, which allows for such optimizations. However, the causality of this decision remains questionable.

Alternatively, the as-if rule could be interpreted to prohibit such an optimization because new can throw an exception, which would affect observable behavior. However, clang could argue that this is an implementation detail and that it has determined that an exception will not be thrown.

Non-Throwing Operator New

The argument for optimizing away the call to the non-throwing version is also valid under the as-if rule. However, if a replacement global operator new is present, optimizations could potentially violate the as-if rule. Clang's previous overaggressive optimization, which even optimized out create() calls, was rectified in later versions.

Conclusion

The compiler's ability to optimize out heap memory allocations remains a topic of debate, with potential implications for the as-if rule and code behavior. While clang's optimization is allowed by N3664, some arguments suggest that it could potentially violate the as-if rule in certain situations. It is crucial for developers to be aware of these subtleties and to consider the observable consequences of such optimizations.

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