constexpr std::string update

Last month I posted “Just how constexpr is C++20’s std::string?” (2023-09-08), showing some surprising behavior in libc++. As of a week ago, that surprising behavior is gone!

The same day my post hit Hacker News, James Y. Knight filed libstdc++ bug 111351 asking for libstdc++ to match libc++’s more conservative behavior by disabling its SSO at constexpr time. After some pushback there, Knight (quite awesomely, in my view) pivoted to create not only a bug report but an actual patch for libc++ completely implementing constexpr SSO! His pull request was opened 2023-09-16 and merged into trunk on 2023-10-10.

We can even see the change immediately benefiting projects that consume Clang nightlies. For example: Clang can detect unused global static variables only when their initializers are constant-expressions (because dynamic initialization and destruction themselves count as “uses” that disguise the otherwise-unusedness of the variable). So Clang is now able to detect unused static const std::string globals up to 22 characters in length — see Chromium commit 879cb86.

The main point of my post (the “firewall” between constexpr evaluation and runtime execution) is still as valid as ever — perhaps even more so, now that libc++’s special cases are eliminated. But I thought it would be worth revisiting each of that post’s example snippets with libc++ trunk and seeing how they’ve changed.

constexpr std::string s = "William Shakespeare";

This 19-character constinit string is now legal on libc++ (though not on libstdc++ nor Microsoft STL, because their SSO limit is lower).

constexpr std::vector<int> v1 = {1, 2, 3};
constexpr std::vector<int> v2 = {};

v1 remains invalid and v2 remains valid, of course. But now we also have:

constexpr std::string s1 = {'1', '2', '3'};
constexpr std::string s2 = {};

Both s1 and s2 are now valid. Before the patch, libc++ considered them both invalid. (“Both?” Yes; remember that even an empty string requires storage for its trailing null byte. Last month’s libc++ refused to use SSO even for that one byte. This is now fixed.)

libstdc++ rejects the following code (Godbolt) due to its pointer-into-self string representation, while Microsoft and now also libc++ accept.

int main() {
  static constexpr std::string abc = "abc"; // OK
  constexpr std::string def = "def";        // Error!


constinit std::string s = "";

This definition (at global scope) is now accepted by all three vendors.

Posted 2023-10-13