From 00ebbe6df2249ba8201c0e5472d95022bf73e782 Mon Sep 17 00:00:00 2001
From: Jakub Konka <kubkon@jakubkonka.com>
Date: Fri, 14 May 2021 11:46:50 +0200
Subject: [PATCH] macho: require _main as global export in self-hosted

Clean up type and description flags generation for exports
in self-hosted MachO backend.
---
 src/link/MachO.zig     | 33 +++++++++++++++++++++++++--------
 test/stage2/darwin.zig | 14 +++++++-------
 2 files changed, 32 insertions(+), 15 deletions(-)

diff --git a/src/link/MachO.zig b/src/link/MachO.zig
index d8518c4894..5a0deb67bc 100644
--- a/src/link/MachO.zig
+++ b/src/link/MachO.zig
@@ -1369,15 +1369,32 @@ pub fn updateDeclExports(
                 continue;
             }
         }
-        const n_desc = switch (exp.options.linkage) {
-            .Internal => macho.REFERENCE_FLAG_PRIVATE_DEFINED,
-            .Strong => blk: {
-                if (mem.eql(u8, exp.options.name, "_start")) {
+
+        var n_type: u8 = macho.N_SECT | macho.N_EXT;
+        var n_desc: u16 = 0;
+
+        switch (exp.options.linkage) {
+            .Internal => {
+                // Symbol should be hidden, or in MachO lingo, private extern.
+                // We should also mark the symbol as Weak: n_desc == N_WEAK_DEF.
+                // TODO work out when to add N_WEAK_REF.
+                n_type |= macho.N_PEXT;
+                n_desc |= macho.N_WEAK_DEF;
+            },
+            .Strong => {
+                // Check if the export is _main, and note if os.
+                // Otherwise, don't do anything since we already have all the flags
+                // set that we need for global (strong) linkage.
+                // n_type == N_SECT | N_EXT
+                if (mem.eql(u8, exp.options.name, "_main")) {
                     self.entry_addr = decl_sym.n_value;
                 }
-                break :blk macho.REFERENCE_FLAG_DEFINED;
             },
-            .Weak => macho.N_WEAK_REF,
+            .Weak => {
+                // Weak linkage is specified as part of n_desc field.
+                // Symbol's n_type is like for a symbol with strong linkage.
+                n_desc |= macho.N_WEAK_DEF;
+            },
             .LinkOnce => {
                 try module.failed_exports.ensureCapacity(module.gpa, module.failed_exports.items().len + 1);
                 module.failed_exports.putAssumeCapacityNoClobber(
@@ -1386,8 +1403,8 @@ pub fn updateDeclExports(
                 );
                 continue;
             },
-        };
-        const n_type = decl_sym.n_type | macho.N_EXT;
+        }
+
         if (exp.link.macho.sym_index) |i| {
             const sym = &self.globals.items[i];
             sym.* = .{
diff --git a/test/stage2/darwin.zig b/test/stage2/darwin.zig
index f8d82391cc..4f708729a2 100644
--- a/test/stage2/darwin.zig
+++ b/test/stage2/darwin.zig
@@ -17,7 +17,7 @@ pub fn addCases(ctx: *TestContext) !void {
 
             // Incorrect return type
             case.addError(
-                \\export fn _start() noreturn {
+                \\export fn _main() noreturn {
                 \\}
             , &[_][]const u8{":2:1: error: expected noreturn, found void"});
 
@@ -26,7 +26,7 @@ pub fn addCases(ctx: *TestContext) !void {
                 \\extern "c" fn write(usize, usize, usize) usize;
                 \\extern "c" fn exit(usize) noreturn;
                 \\
-                \\export fn _start() noreturn {
+                \\export fn _main() noreturn {
                 \\    print();
                 \\
                 \\    exit(0);
@@ -46,7 +46,7 @@ pub fn addCases(ctx: *TestContext) !void {
                 \\extern "c" fn write(usize, usize, usize) usize;
                 \\extern "c" fn exit(usize) noreturn;
                 \\
-                \\export fn _start() noreturn {
+                \\export fn _main() noreturn {
                 \\    print();
                 \\    print();
                 \\    print();
@@ -73,7 +73,7 @@ pub fn addCases(ctx: *TestContext) !void {
                 \\extern "c" fn write(usize, usize, usize) usize;
                 \\extern "c" fn exit(usize) noreturn;
                 \\
-                \\export fn _start() noreturn {
+                \\export fn _main() noreturn {
                 \\    print();
                 \\
                 \\    exit(0);
@@ -93,7 +93,7 @@ pub fn addCases(ctx: *TestContext) !void {
                 \\extern "c" fn write(usize, usize, usize) usize;
                 \\extern "c" fn exit(usize) noreturn;
                 \\
-                \\export fn _start() noreturn {
+                \\export fn _main() noreturn {
                 \\    print();
                 \\    print();
                 \\
@@ -119,7 +119,7 @@ pub fn addCases(ctx: *TestContext) !void {
             case.addCompareOutput(
                 \\extern "c" fn exit(usize) noreturn;
                 \\
-                \\export fn _start() noreturn {
+                \\export fn _main() noreturn {
                 \\    exit(0);
                 \\}
             ,
@@ -130,7 +130,7 @@ pub fn addCases(ctx: *TestContext) !void {
                 \\extern "c" fn exit(usize) noreturn;
                 \\extern "c" fn write(usize, usize, usize) usize;
                 \\
-                \\export fn _start() noreturn {
+                \\export fn _main() noreturn {
                 \\    _ = write(1, @ptrToInt("Hey!\n"), 5);
                 \\    exit(0);
                 \\}