Can C Compilers Assume a Boolean's Numerical Representation is Only 0 or 1, and Does This Lead to Undefined Behavior?

Compiler Optimization and Undefined Behavior: Does C Allow Certain Assumptions About Bools?

Introduction

This article examines whether the C standard permits compilers to assume certain numerical representations for bools and whether such assumptions can lead to consequences such as program crashes.

The Issue

A programmer encountered a program crash while using an uninitialized bool value in a function that serialized a bool into a string. Surprisingly, the crash occurred only on a specific platform using a specific compiler with optimization enabled.

The problematic code:

void Serialize(bool boolValue) {
    const char* whichString = boolValue ? "true" : "false";
    const size_t len = strlen(whichString);
    memcpy(destBuffer, whichString, len);
}

When the code is executed with Clang 5.0.0 and optimization (-O2), it may crash. This behavior arises due to the optimizer's deduction that the strings "true" and "false" differ only in length by 1. Instead of calculating the actual length, it uses the value of boolValue, assuming it is either 0 or 1.

const size_t len = strlen(whichString); // original code
const size_t len = 5 - boolValue;       // clang optimization

Question: Standard Considerations

The article poses the question: Does the C standard allow a compiler to assume that a bool can only have an internal numerical representation of '0' or '1' and use it in such a way? Or is this a case of implementation-defined behavior where the implementation has assumed all its bools will only ever contain 0 or 1, and any other value is undefined behavior territory?

Answer: Standard Conformity

According to the author, ISO C allows (but doesn't require) implementations to make this choice. ISO C leaves it unspecified what the internal representation of a bool is, allowing implementations to make their own assumptions.

Compiler Optimization Behavior

System V ABI: For platforms using the System V ABI, which is commonly used on x86-64 systems, a bool argument passed to a function is represented by the bit-patterns: 0 = false and 1 = true in the low 8 bits of the register. In memory, bool is a 1-byte type that must have an integer value of 0 or 1.

This ABI decision allows the compiler to take advantage of optimizations, such as assuming 0 or 1 for bool and performing bitwise operations instead of expensive type conversions. In the example provided, the optimizer has exploited this behavior to optimize strlen(whichString) to 5U - boolValue.

Other Implementations and Assumptions:

While the System V ABI is widely used, other implementations could make different assumptions. For example, they could consider 0 = false and any non-zero value = true. In such a scenario, the compiler might not generate code that crashes for uninitialized bool values, but it could still be considered undefined behavior.

The Dangers of Program Crashes

While the C standard allows such optimizations, it's important to note that programs encountering undefined behavior are considered totally undefined for their entire existence. This means that a crash can occur even if the undefined behavior is encountered in a function that is never actually called.

Best Practices and Avoiding Undefined Behavior

Compilers are becoming increasingly aggressive in optimizing code, assuming behaviors based on their internal understanding of the implementation. It's crucial for programmers to avoid relying on implementation assumptions and ensure that their code is valid C without assuming it will behave like a portable assembly language.

To avoid problems, programmers should follow these best practices:

• Use the -Wall compiler flag to enable warnings.
• Fix all warnings generated by your compiler.
• Be aware that assumptions about uninitialized variables can lead to program crashes.
• Consider using tools like Address Sanitizer and Memory Sanitizer to detect usage of uninitialized values and potential undefined behavior.

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