stage2: add support for start.zig

This adds a simplified start2.zig that the current stage2 compiler is
able to generate code for.

1 files changed, 58 insertions, 0 deletions

diff --git a/lib/std/start2.zig b/lib/std/start2.zig
new file mode 100644
index 0000000000..22f3578d98
--- /dev/null
+++ b/lib/std/start2.zig
@@ -0,0 +1,58 @@
+const root = @import("root");
+const builtin = @import("builtin");
+
+comptime {
+    if (builtin.output_mode == 0) { // OutputMode.Exe
+        if (builtin.link_libc or builtin.object_format == 5) { // ObjectFormat.c
+            if (!@hasDecl(root, "main")) {
+                @export(otherMain, "main");
+            }
+        } else {
+            if (!@hasDecl(root, "_start")) {
+                @export(otherStart, "_start");
+            }
+        }
+    }
+}
+
+// FIXME: Cannot call this function `main`, because `fully qualified names`
+//        have not been implemented yet.
+fn otherMain() callconv(.C) c_int {
+    root.zigMain();
+    return 0;
+}
+
+// FIXME: Cannot call this function `_start`, because `fully qualified names`
+//        have not been implemented yet.
+fn otherStart() callconv(.Naked) noreturn {
+    root.zigMain();
+    otherExit();
+}
+
+// FIXME: Cannot call this function `exit`, because `fully qualified names`
+//        have not been implemented yet.
+fn otherExit() noreturn {
+    if (builtin.arch == 31) { // x86_64
+        asm volatile ("syscall"
+            :
+            : [number] "{rax}" (231),
+              [arg1] "{rdi}" (0)
+            : "rcx", "r11", "memory"
+        );
+    } else if (builtin.arch == 0) { // arm
+        asm volatile ("svc #0"
+            :
+            : [number] "{r7}" (1),
+              [arg1] "{r0}" (0)
+            : "memory"
+        );
+    } else if (builtin.arch == 2) { // aarch64
+        asm volatile ("svc #0"
+            :
+            : [number] "{x8}" (93),
+              [arg1] "{x0}" (0)
+            : "memory", "cc"
+        );
+    } else @compileError("not yet supported!");
+    unreachable;
+}