The Error in Bitcoin’s ConstevalHexDigit Function
As a developer working with the Bitcoin project, you’re likely no stranger to its complexities. Recently, I encountered an error related to a specific CMake build process. The issue arises 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 versions, consteval functions are required for certain operations that should always produce constant results.
CMake Build Options
Let’s dive into what happens when you run cmake --build build -j$(sysctl -n hw.ncpu). Here’s a breakdown of the options involved:
-
--build: Specifies the build process to use.
-
-j$(sysctl -n hw.ncpu): Enables concurrent compilation with multiple threads. The$variable is used for dynamic system call replacement, which allows us to dynamically set the number of cores.
The Error
When you compile your code using cmake --build build -j$(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 evaluate to either 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 resolve this issue, we can use constexpr functions in our code. These functions can be declared as consteval, which meets the requirements of the C++ standard 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) {
// Implementation remains the same here...
}
By declaring ConstevalHexDigit as constexpr, we’ve ensured that it meets all the requirements of a non-constant expression, resolving the error.
