Can C Compilers Optimize Out Heap Memory Allocations?

Optimizer's Perspective on Heap Memory Allocations

In the realm of C , the question arises: can a compiler optimize out heap memory allocations? Let's delve into this intriguing topic.

Consider the following code snippet:

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

Does the compiler have the liberty to remove the new call? According to our research, g and Visual Studio shun this optimization, while clang embraces it. This disparity begs the question: doesn't new rely on underlying system calls, rendering compiler optimization impractical and impermissible?

The Compiler's Justification

To clarify this conundrum, we must acknowledge the role of N3664, a proposal that allows compilers to optimize around memory allocations. However, this optimization was conceived before N3664, prompting questions about its validity.

To answer this question, we turn to the "as-if" rule, a fundamental aspect of the C standard. This rule permits implementations to deviate from specific requirements as long as they maintain the program's observable behavior.

As new may raise an exception, which would alter the program's return value, the "as-if" rule appears to prohibit its optimization. However, the compiler could argue that exception handling is an implementation detail and, in this case, would not trigger an exception. Therefore, eliminating the new call would not violate the "as-if" rule.

Additionally, the "as-if" rule extends to the non-throwing version of new. However, the presence of an alternative operator new in a separate translation unit could affect observable behavior. Thus, the compiler must ensure such a scenario is not present to perform this optimization safely.

Clang's Aggressive Approach

Previous clang versions optimized even in such cases, but later releases have become more cautious.

In conclusion, compilers have leverage to optimize heap memory allocations, including those made with new. However, this optimization must adhere to the nuances of the "as-if" rule and the intricacies of C 's exception handling mechanisms.

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