From c248af3bdcd17c334e742d53a7ac7bda2422a688 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 710eb91376..5d350be7e0 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 2b320923ad..c62860c88d 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 3c8874e3a8..be01b3a048 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);
+}