stage2: -lunwind is handled specially

 * `-lc++` now implies `-lc`.
 * `-lunwind` is now pulled out into a separate `link_libunwind` flag in
   the frontend driver code. This allows a project to request zig to
   provide libunwind even if the default situation that causes it to be
   implicitly added, is not active.
 * build.zig: ask for -lunwind when building the self-hosted compiler on
   Linux. Otherwise we get linker errors with unresolved symbols to
   libunwind.

1 files changed, 10 insertions, 7 deletions

diff --git a/src/Compilation.zig b/src/Compilation.zig
index 0f04f02b7d..140e160b69 100644
--- a/src/Compilation.zig
+++ b/src/Compilation.zig
@@ -544,6 +544,7 @@ pub const InitOptions = struct {
     system_libs: []const []const u8 = &[0][]const u8{},
     link_libc: bool = false,
     link_libcpp: bool = false,
+    link_libunwind: bool = false,
     want_pic: ?bool = null,
     /// This means that if the output mode is an executable it will be a
     /// Position Independent Executable. If the output mode is not an
@@ -791,8 +792,13 @@ pub fn create(gpa: *Allocator, options: InitOptions) !*Compilation {
         };
 
         const tsan = options.want_tsan orelse false;
+        // TSAN is implemented in C++ so it requires linking libc++.
+        const link_libcpp = options.link_libcpp or tsan;
+        const link_libc = link_libcpp or options.link_libc or
+            target_util.osRequiresLibC(options.target);
 
-        const link_libc = options.link_libc or target_util.osRequiresLibC(options.target) or tsan;
+        const link_libunwind = options.link_libunwind or
+            (link_libcpp and target_util.libcNeedsLibUnwind(options.target));
 
         const must_dynamic_link = dl: {
             if (target_util.cannotDynamicLink(options.target))
@@ -878,9 +884,6 @@ pub fn create(gpa: *Allocator, options: InitOptions) !*Compilation {
             break :pic explicit;
         } else pie or must_pic;
 
-        // TSAN is implemented in C++ so it requires linking libc++.
-        const link_libcpp = options.link_libcpp or tsan;
-
         // Make a decision on whether to use Clang for translate-c and compiling C files.
         const use_clang = if (options.use_clang) |explicit| explicit else blk: {
             if (build_options.have_llvm) {
@@ -973,6 +976,7 @@ pub fn create(gpa: *Allocator, options: InitOptions) !*Compilation {
         cache.hash.add(strip);
         cache.hash.add(link_libc);
         cache.hash.add(link_libcpp);
+        cache.hash.add(link_libunwind);
         cache.hash.add(options.output_mode);
         cache.hash.add(options.machine_code_model);
         cache.hash.addOptionalEmitLoc(options.emit_bin);
@@ -1157,6 +1161,7 @@ pub fn create(gpa: *Allocator, options: InitOptions) !*Compilation {
             .system_linker_hack = darwin_options.system_linker_hack,
             .link_libc = link_libc,
             .link_libcpp = link_libcpp,
+            .link_libunwind = link_libunwind,
             .objects = options.link_objects,
             .frameworks = options.frameworks,
             .framework_dirs = options.framework_dirs,
@@ -3022,9 +3027,7 @@ fn wantBuildLibUnwindFromSource(comp: *Compilation) bool {
         .Lib => comp.bin_file.options.link_mode == .Dynamic,
         .Exe => true,
     };
-    return comp.bin_file.options.link_libc and is_exe_or_dyn_lib and
-        comp.bin_file.options.link_libcpp and
-        target_util.libcNeedsLibUnwind(comp.getTarget());
+    return is_exe_or_dyn_lib and comp.bin_file.options.link_libunwind;
 }
 
 fn updateBuiltinZigFile(comp: *Compilation, mod: *Module) Allocator.Error!void {