Compiler differences for use of strcmp in a constexpr

Question

The following compiles in GCC but not in Clang:

#include <cstring>

constexpr int test = strcmp("test", "test");

So my question is, how does GCC handle strcmp differently to make this possible? Is strcmp some type of builtin, or does its standard library have a non-standard definition of strcmp that includes constexpr?

Answer 1

The code compiles on gcc because it provides a built-in version of strcmp that is evaluated at compile time, assuming you pass string literals to the function.

gcc will reject the code if you pass the -fno-builtin (or -fno-builtin-strcmp) flag.

Answer 2

Nothing forbids a function from being constexpr. There's just no guarantee that strcmp is constexpr. A good compiler will probably be able to perform string operations on compile-time constant strings efficiently, but that's an implementation detail.