Why Does My C Loop Run Infinitely When Using Aggressive Compiler Optimizations?

C Compilation Bug Involving Loop Optimization

In a C code snippet, a loop iterates through an array of complex numbers and prints their indices. However, it unexpectedly generates an infinite series instead of the intended output.

Despite the conditional check di < 4 controlling loop termination, this check appears to fail, resulting in continuous execution. The issue arises from an assignment statement delta = mc[di] within the loop, which leads to undefined behavior.

Under certain compiler optimizations, the following assumptions are made:

• Undefined behavior will not occur.
• mc[di] access within the loop is valid.
• The condition di < 4 will always evaluate to true to prevent undefined behavior.

These assumptions result in the elimination of the di < 4 check and its replacement with an unconditional jump. This aggressive loop optimization allows the loop to execute indefinitely.

To avoid this issue, one can either use -fno-aggressive-loop-optimizations to disable such optimizations, or ensure that all array accesses are within bounds. In this specific case, moving the cout statement outside the loop triggers a warning about undefined behavior, helping to identify the problem and prevent undefined behavior.

By understanding the implications of undefined behavior and the potential inconsistencies it can cause in the presence of aggressive compiler optimizations, developers can ensure the correct behavior of their code and avoid unexpected results.

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