From cebd878cd77dfe5c20cabcf5fd307b59354fd15b Mon Sep 17 00:00:00 2001
From: Andrew Kelley <andrew@ziglang.org>
Date: Wed, 29 Jun 2022 19:01:48 -0700
Subject: [PATCH] LLVM: fix lowering of untagged union types

The LLVM backend was calculating the amount of padding solely based
on the payload size. However, in the case where there is no union
tag, this fails to take into account alignment.

Closes #11857
---
 lib/std/net/test.zig    |  3 ---
 src/codegen/llvm.zig    |  7 +++++--
 test/behavior/union.zig | 19 +++++++++++++++++++
 3 files changed, 24 insertions(+), 5 deletions(-)

diff --git a/lib/std/net/test.zig b/lib/std/net/test.zig
index 710eb91376b3..5d350be7e0eb 100644
--- a/lib/std/net/test.zig
+++ b/lib/std/net/test.zig
@@ -5,7 +5,6 @@ const mem = std.mem;
 const testing = std.testing;
 
 test "parse and render IPv6 addresses" {
-    if (@import("builtin").zig_backend != .stage1) return error.SkipZigTest;
     if (builtin.os.tag == .wasi) return error.SkipZigTest;
 
     var buffer: [100]u8 = undefined;
@@ -68,7 +67,6 @@ test "invalid but parseable IPv6 scope ids" {
 }
 
 test "parse and render IPv4 addresses" {
-    if (@import("builtin").zig_backend != .stage1) return error.SkipZigTest;
     if (builtin.os.tag == .wasi) return error.SkipZigTest;
 
     var buffer: [18]u8 = undefined;
@@ -93,7 +91,6 @@ test "parse and render IPv4 addresses" {
 }
 
 test "parse and render UNIX addresses" {
-    if (@import("builtin").zig_backend != .stage1) return error.SkipZigTest;
     if (builtin.os.tag == .wasi) return error.SkipZigTest;
     if (!net.has_unix_sockets) return error.SkipZigTest;
 
diff --git a/src/codegen/llvm.zig b/src/codegen/llvm.zig
index 2b320923ad17..c62860c88dba 100644
--- a/src/codegen/llvm.zig
+++ b/src/codegen/llvm.zig
@@ -2708,7 +2708,10 @@ pub const DeclGen = struct {
                     if (layout.most_aligned_field_size == layout.payload_size) {
                         break :t llvm_aligned_field_ty;
                     }
-                    const padding_len = @intCast(c_uint, layout.payload_size - layout.most_aligned_field_size);
+                    const padding_len = if (layout.tag_size == 0)
+                        @intCast(c_uint, layout.abi_size - layout.most_aligned_field_size)
+                    else
+                        @intCast(c_uint, layout.payload_size - layout.most_aligned_field_size);
                     const fields: [2]*const llvm.Type = .{
                         llvm_aligned_field_ty,
                         dg.context.intType(8).arrayType(padding_len),
@@ -5756,7 +5759,7 @@ pub const FuncGen = struct {
             // First set the non-null bit.
             const indices: [2]*const llvm.Value = .{
                 index_type.constNull(), // dereference the pointer
-                index_type.constInt(1, .False), // second field is the payload
+                index_type.constInt(1, .False), // second field is the non-null bit
             };
             const non_null_ptr = self.builder.buildInBoundsGEP(operand, &indices, indices.len, "");
             _ = self.builder.buildStore(non_null_bit, non_null_ptr);
diff --git a/test/behavior/union.zig b/test/behavior/union.zig
index 3c8874e3a856..be01b3a048af 100644
--- a/test/behavior/union.zig
+++ b/test/behavior/union.zig
@@ -1194,3 +1194,22 @@ test "union tag is set when initiated as a temporary value at runtime" {
     var b: u32 = 1;
     try (U{ .b = b }).doTheTest();
 }
+
+test "extern union most-aligned field is smaller" {
+    if (builtin.zig_backend == .stage1) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+
+    const U = extern union {
+        in6: extern struct {
+            family: u16,
+            port: u16,
+            flowinfo: u32,
+            addr: [20]u8,
+        },
+        un: [110]u8,
+    };
+    var a: ?U = .{ .un = [_]u8{0} ** 110 };
+    try expect(a != null);
+}