The Error in Bitcoin’s ConstevalHexDigit Function
As a developer working with the Bitcoin project, you are probably no stranger to its complexities. Recently, I encountered an error related to a specific CMake build process. The problem occurs when compiling Bitcoin code using the --build option with multiple threads enabled.
The Problem: ConstevalHexDigit Function
The problem lies in the util::ConstevalHexDigit function, which appears to be attempting to call a non-constant expression. In C++11 and later, consteval functions are required for certain operations that must always produce constant results.
CMake Build Options
Let’s dive into what happens when you run cmake --build build -j$(sysctl -n hw.ncpu). Here is a breakdown of the options involved:
--build: Specifies the build process to use.
-j$(sysctl -n hw.ncpu): Enables concurrent compilation with multiple threads. The$variable is used for dynamic replacement of system calls, allowing us to dynamically set the number of cores.
The error
When you compile your code using cmake --build build -j$(sysctl -n hw.ncpu), the compiler encounters an attempt to call a non-constant expression in util::ConstevalHexDigit. This is because according to the C++ standard, a constant expression must be evaluated either as an integer constant (with an explicit type), a pointer to an integer constant, or an arithmetic expression that does not contain any operators.
The solution
To solve this problem, we can use the constexpr functions in our code. These functions can be declared as consteval, which meets the C++ standard requirements for calling non-constant expressions.
Here’s how you can rewrite your code using constexpr:
#include
#include "util/consteval_hex_digit.hpp"
static constexpr uint256_t ConstevalHexDigit(const std::string& input) {
// The implementation remains the same here...
}
By declaring ConstevalHexDigit as constexpr, we have ensured that it meets all the requirements of a non-constant expression, thus resolving the error.