The blog post explores methods for determining if an expression is constant at compile time in C. It highlights the limitations of sizeof for this purpose, as it can't differentiate between compile-time and run-time constants, and introduces a technique using C11's _Generic keyword. This method leverages the fact that array sizes must be compile-time constants. By attempting to create an array with the expression as its size inside a _Generic selection, the code can distinguish between compile-time constants (which compile successfully) and run-time values (which result in a compilation error). This allows conditional compilation based on the constexpr-ness of an expression, enabling optimized code paths for constant values.
The article "Detecting if an expression is constant in C" explores various techniques to determine at compile time whether a given C expression is a constant. The core problem lies in differentiating between values known during compilation and those calculated at runtime. This distinction is crucial for various optimization strategies and conditional compilation scenarios.
The article begins by introducing the concept of integer constant expressions (ICEs) in C, which are expressions evaluable by the compiler. These are often used in contexts requiring compile-time constants, like array sizes or case labels in switch statements.
Several methods are presented to ascertain if an expression qualifies as an ICE. The simplest approach involves using the sizeof operator. Since sizeof operates at compile time, if it accepts an expression without error, it implies the expression is an ICE. However, this method has limitations, notably with void expressions, where sizeof is valid but might not indicate true constness.
The article then delves into more sophisticated strategies. One such method uses the preprocessor's ability to evaluate constant expressions. By constructing a macro that attempts to take the address of an expression, the compiler can indirectly signal whether the expression is a constant. If the expression is indeed constant, the address-of operator will fail within the preprocessor, triggering a preprocessor error. This error can then be leveraged using conditional compilation directives to conditionally define a macro, effectively indicating the constness of the original expression.
Furthermore, the article explains a variation on this technique involving designated initializers within a compound literal. This method leverages the constraint that designated initializers must use constant expressions. By attempting to initialize a member using the target expression, the compiler will produce an error if the expression isn't constant. This error, like the previous method, can be harnessed with preprocessor directives to identify constant expressions.
The article emphasizes the importance of these detection mechanisms, particularly in generic programming and metaprogramming scenarios, where decisions based on the constness of expressions are essential for code generation and optimization. The ability to differentiate between compile-time and runtime values enables developers to write more efficient and adaptable C code. Finally, the article acknowledges that while these techniques are generally robust, they possess certain limitations, particularly concerning the interaction with non-standard compiler extensions. Despite these limitations, they provide valuable tools for C developers seeking to perform advanced compile-time analysis.
Summary of Comments ( 3 )
https://news.ycombinator.com/item?id=43939029
HN users discuss the nuances and limitations of the presented C++ technique for detecting constant expressions in C. Several point out that
constexpris a C++ feature, not C, and the article's title is misleading. Some discuss alternative approaches in C, like using the preprocessor and#ifdefor build-time evaluation with constant folding. Others highlight the challenges of reliably determining const-ness in C due to factors like linker behavior and external variables. A few commenters delve into the complexities ofconstexpritself within C++, including its interaction with different versions of the standard. The overall sentiment suggests the proposed method is not directly applicable to C and that true compile-time constness detection in C remains tricky.The Hacker News post titled "Detecting if an expression is constant in C" sparked a discussion with several insightful comments.
One commenter highlights the utility of
constexprin C++ for achieving similar goals to the C techniques discussed in the article. They point out thatconstexprallows compile-time evaluation of expressions and provides a more modern and arguably cleaner approach. They also acknowledge that the C++ solution may not be directly applicable in a C context, where the original question originated.Another comment dives into the intricacies of the
__builtin_constant_p()intrinsic function mentioned in the article. They explain its behavior and limitations, particularly emphasizing that it only checks for compile-time constness. They clarify that it doesn't determine if a value is constant at runtime, which is a crucial distinction for understanding the function's purpose and avoiding potential misuse. This commenter also touches on the concept of "integer constant expressions" (ICE) in C and how__builtin_constant_p()relates to them.A further comment elaborates on the difference between compile-time and run-time constants, providing a concrete example using a pointer initialized with the address of a global variable. They illustrate how such a pointer might be considered constant during compilation but could potentially change at runtime (e.g., through dynamic linking or self-modifying code). This clarifies why a function like
__builtin_constant_p()can't definitively determine runtime constness.A separate thread of conversation arises concerning the usefulness of determining expression constness at compile time. One participant suggests that it's most relevant in situations where the compiler can make significant optimizations based on this knowledge. They propose scenarios like replacing calculations with immediate values, eliminating unnecessary branches, or choosing more efficient data structures. This comment provides a practical perspective on the value of the techniques explored in the article.
Another contributor expresses skepticism about the practical applications, arguing that the compiler is already quite capable of performing these optimizations without explicit hints. They suggest that such techniques might have been more valuable in the past with less sophisticated compilers. This counterpoint contributes to a balanced discussion about the true benefits of the approaches being considered.
Finally, some comments briefly discuss alternative methods for achieving similar results in C. These include using macros and conditional compilation, although they don't go into as much detail as the discussions around
__builtin_constant_p(). They serve to broaden the conversation and showcase different perspectives on tackling the problem.