From 3cf71580c460a30bb52da6e2cbc4482a67bd2930 Mon Sep 17 00:00:00 2001
From: Michael Dusan <michael.dusan@gmail.com>
Date: Fri, 1 Sep 2023 10:04:26 -0400
Subject: [PATCH] build stage3: detect system libcxx

Detect system libcxx name with the CMake build system and convey that
information to build.zig via config.h so that the same system libcxx is
used for stage3.

This undoes the hacky search 901457d173467c71b681a8c69f4b77c94d516da7 .

closes #17018
---
 CMakeLists.txt     |  7 +++++++
 build.zig          | 24 ++++++++++++++----------
 stage1/config.h.in |  1 +
 3 files changed, 22 insertions(+), 10 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3b7c1b30b44cc484be95d6c0c633328e26e10f90..2234989f5fd0b77ed79a106e8001fa47836427ca 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -124,6 +124,13 @@ endif()
 
 set(ZIG_PIE off CACHE BOOL "produce a position independent zig executable")
 
+# Detect system libcxx name.
+if ("c++" IN_LIST CMAKE_CXX_IMPLICIT_LINK_LIBRARIES)
+    set(ZIG_SYSTEM_LIBCXX "c++" CACHE STRING "system libcxx name for build.zig")
+else()
+    set(ZIG_SYSTEM_LIBCXX "stdc++" CACHE STRING "system libcxx name for build.zig")
+endif()
+
 find_package(llvm 16)
 find_package(clang 16)
 find_package(lld 16)
diff --git a/build.zig b/build.zig
index 5b5f0004ccfaed5e3224d63e448691158e63c28f..5211a9a70a9bd0ef15efabe1ed0bc04724776f8b 100644
--- a/build.zig
+++ b/build.zig
@@ -632,16 +632,14 @@ fn addCmakeCfgOptionsToExe(
         const lib_suffix = if (static) exe.target.staticLibSuffix()[1..] else exe.target.dynamicLibSuffix()[1..];
         switch (exe.target.getOsTag()) {
             .linux => {
-                // First we try to link against system libstdc++ or libc++.
-                // If that doesn't work, we fall to -lc++ and cross our fingers.
-                const found = for ([_][]const u8{ "stdc++", "c++" }) |name| {
-                    addCxxKnownPath(b, cfg, exe, b.fmt("lib{s}.{s}", .{ name, lib_suffix }), "", need_cpp_includes) catch |err| switch (err) {
-                        error.RequiredLibraryNotFound => continue,
-                        else => |e| return e,
-                    };
-                    break true;
-                } else false;
-                if (!found) exe.linkLibCpp();
+                // First we try to link against the detected libcxx name. If that doesn't work, we fall
+                // back to -lc++ and cross our fingers.
+                addCxxKnownPath(b, cfg, exe, b.fmt("lib{s}.{s}", .{ cfg.system_libcxx, lib_suffix }), "", need_cpp_includes) catch |err| switch (err) {
+                    error.RequiredLibraryNotFound => {
+                        exe.linkLibCpp();
+                    },
+                    else => |e| return e,
+                };
                 exe.linkSystemLibrary("unwind");
             },
             .ios, .macos, .watchos, .tvos => {
@@ -775,6 +773,7 @@ const CMakeConfig = struct {
     llvm_include_dir: []const u8,
     llvm_libraries: []const u8,
     dia_guids_lib: []const u8,
+    system_libcxx: []const u8,
 };
 
 const max_config_h_bytes = 1 * 1024 * 1024;
@@ -840,6 +839,7 @@ fn parseConfigH(b: *std.Build, config_h_text: []const u8) ?CMakeConfig {
         .llvm_include_dir = undefined,
         .llvm_libraries = undefined,
         .dia_guids_lib = undefined,
+        .system_libcxx = undefined,
     };
 
     const mappings = [_]struct { prefix: []const u8, field: []const u8 }{
@@ -891,6 +891,10 @@ fn parseConfigH(b: *std.Build, config_h_text: []const u8) ?CMakeConfig {
             .prefix = "#define ZIG_LLVM_LIB_PATH ",
             .field = "llvm_lib_dir",
         },
+        .{
+            .prefix = "#define ZIG_SYSTEM_LIBCXX",
+            .field = "system_libcxx",
+        },
         // .prefix = ZIG_LLVM_LINK_MODE parsed manually below
     };
 
diff --git a/stage1/config.h.in b/stage1/config.h.in
index 0f1d902ef94752827df07e129478ccbb531a07a2..615967de5094c42c451c5b814c0938e587001bf8 100644
--- a/stage1/config.h.in
+++ b/stage1/config.h.in
@@ -28,5 +28,6 @@
 #define ZIG_LLVM_LIBRARIES "@LLVM_LIBRARIES@"
 #define ZIG_LLVM_LIB_PATH "@LLVM_LIBDIRS@"
 #define ZIG_LLVM_LINK_MODE "@LLVM_LINK_MODE@"
+#define ZIG_SYSTEM_LIBCXX "@ZIG_SYSTEM_LIBCXX@"
 
 #endif
-- 
2.54.0