Can I mess up GCC hash seed of unordered_map?

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Summary

The issue at hand is related to the unordered_map in C++ and how its iteration order can affect the results of floating-point calculations due to floating-point rounding errors. The question revolves around whether it’s possible to manipulate the hash seed used by GCC for unordered_map to reproduce a specific iteration order without modifying the source code, potentially utilizing GCC Tunables.

Root Cause

The root cause of the problem is the non-deterministic iteration order of unordered_map, which can lead to different results when performing floating-point arithmetic. This is due to the following reasons:

  • Hash function: The hash function used by unordered_map can produce different indices for the same key, leading to a different iteration order.
  • Hash seed: The hash seed used by the compiler (in this case, GCC) can also affect the iteration order.
  • Floating-point rounding: The order of operations in floating-point calculations can result in different rounding errors, affecting the final result.

Why This Happens in Real Systems

This issue can occur in real systems when:

  • Using unordered_map to store and calculate floating-point numbers.
  • The iteration order of the map affects the result of the calculations.
  • The system relies on a specific iteration order to produce the correct results.
    Some examples of real-world impacts include:
  • Scientific simulations: Where small differences in calculation order can lead to significant differences in results.
  • Financial calculations: Where precise control over calculation order is crucial for accuracy.

Real-World Impact

The impact of this issue can be significant, leading to:

  • Inconsistent results: Different results when running the same program multiple times.
  • Difficulty in debugging: Hard to reproduce and identify the root cause of the issue.
  • Loss of precision: Inaccurate results due to floating-point rounding errors.

Example or Code

#include 
#include 

int main() {
    std::unordered_map map;
    map[0.016836150909] = 1.0;
    map[0.018234567891] = 2.0;
    map[0.015432109876] = 3.0;

    double result = 0.0;
    for (const auto& pair : map) {
        result += pair.first;
    }

    std::cout << "Result: " << result << std::endl;
    return 0;
}

How Senior Engineers Fix It

Senior engineers can fix this issue by:

  • Using a deterministic iteration order: Such as using a std::map instead of std::unordered_map.
  • Controlling the hash seed: If possible, by using a custom hash function or manipulating the GCC Tunables.
  • Avoiding floating-point arithmetic: If possible, by using alternative data types or calculation methods.
  • Implementing robust testing: To detect and handle potential issues related to floating-point rounding errors.

Why Juniors Miss It

Junior engineers might miss this issue due to:

  • Lack of understanding: Of the non-deterministic iteration order of unordered_map and its implications.
  • Insufficient testing: Failing to test the program thoroughly, leading to undetected issues.
  • Overlooking floating-point rounding errors**: Not considering the potential impact of floating-point arithmetic** on the results.
  • Inadequate knowledge: Of GCC Tunables and their potential use in controlling the hash seed.