Commit f356324325dc3637b93859d7b53f98f4e66009f5 | zig

author	Pavel Verigo <paul.verigo@gmail.com> 2026-10-20 22:03:13+02:00
committer	Alex Rønne Petersen <alex@alexrp.com> 2026-10-21 00:47:14+02:00
log	f356324325dc3637b93859d7b53f98f4e66009f5
tree	b108392beeb8578f0075c9bad76f81ee88aabfb5
parent	427ddef0e84384c075dd01e8f1b512922c814a1c

codegen: fix union padding

3 files changed, 29 insertions(+), 11 deletions(-)

src
- codegen.zig
- codegen
  - llvm.zig
test
- behavior
  - union.zig

src/codegen.zig+4-4

 @@ -617,6 +617,7 @@ pub fn generateSymbol(
             else => unreachable,
         },
         .un => |un| {
+            const union_begin = w.end;
             const layout = ty.unionGetLayout(zcu);
             if (layout.payload_size == 0) {
 @@ -648,11 +649,10 @@ pub fn generateSymbol(
             if (layout.tag_size > 0 and layout.tag_align.compare(.lt, layout.payload_align)) {
                 try generateSymbol(bin_file, pt, Value.fromInterned(un.tag), w, reloc_parent);
-                if (layout.padding > 0) {
-                    try w.splatByteAll(0, layout.padding);
-                }
             }
+            const padding: usize = @intCast(union_begin + layout.abi_size - w.end);
+            if (padding > 0) try w.splatByteAll(0, padding);
         },
         .bitpack => |bitpack| try generateSymbol(bin_file, pt, .fromInterned(bitpack.backing_int_val), w, reloc_parent),
         .memoized_call => unreachable,

src/codegen/llvm.zig+9-7

 @@ -2982,13 +2982,13 @@ pub const Object = struct {
                     } else .normal;
                     const payload_ty = ty: {
-                        if (layout.most_aligned_field_size == layout.payload_size) {
-                            break :ty aligned_field_llvm_ty;
-                        }
                         const padding_len = if (layout.tag_size == 0)
                             layout.abi_size - layout.most_aligned_field_size
                         else
                             layout.payload_size - layout.most_aligned_field_size;
+                        if (padding_len == 0) {
+                            break :ty aligned_field_llvm_ty;
+                        }
                         break :ty try o.builder.structType(.@"packed", &.{
                             aligned_field_llvm_ty,
                             try o.builder.arrayType(padding_len, .i8),
 @@ -3582,6 +3582,11 @@ pub const Object = struct {
                 const payload = if (un.tag != .none) p: {
                     const field_index = zcu.unionTagFieldIndex(union_obj, Value.fromInterned(un.tag)).?;
                     const field_ty = Type.fromInterned(union_obj.field_types.get(ip)[field_index]);
+                    const field_size = field_ty.abiSize(zcu);
+                    const padding_len = if (layout.tag_size == 0)
+                        layout.abi_size - field_size
+                    else
+                        layout.payload_size - field_size;
                     // Sometimes we must make an unnamed struct because LLVM does
                     // not support bitcasting our payload struct to the true union payload type.
 @@ -3590,7 +3595,6 @@ pub const Object = struct {
                     need_unnamed = layout.most_aligned_field != field_index;
                     if (!field_ty.hasRuntimeBits(zcu)) {
-                        const padding_len = layout.payload_size;
                         break :p try o.builder.undefConst(try o.builder.arrayType(padding_len, .i8));
                     }
                     const payload = try o.lowerValue(un.val, repr);
 @@ -3598,9 +3602,7 @@ pub const Object = struct {
                     if (payload_ty != union_ty.structFields(&o.builder)[
                         @intFromBool(layout.tag_size > 0 and layout.tag_align.compare(.gte, layout.payload_align))
                     ]) need_unnamed = true;
-                    const field_size = field_ty.abiSize(zcu);
+                    if (padding_len == 0) break :p payload;
-                    if (field_size == layout.payload_size) break :p payload;
-                    const padding_len = layout.payload_size - field_size;
                     const padding_ty = try o.builder.arrayType(padding_len, .i8);
                     break :p try o.builder.structConst(
                         try o.builder.structType(.@"packed", &.{ payload_ty, padding_ty }),

test/behavior/union.zig+16

...	@@ -1162,6 +1162,22 @@ test "extern union most-aligned field is smaller" {	...	@@ -1162,6 +1162,22 @@ test "extern union most-aligned field is smaller" {
1162	try expect(a != null);	1162	try expect(a != null);
1163	}	1163	}
1164		1164
		1165	test "union constant trailing padding" {
		1166	const S = struct {
		1167	fn doTheTest() !void {
		1168	const U = extern union { x: u32, y: [5]u8 };
		1169	var u = [2]U{ .{ .x = 0 }, .{ .y = @splat(0xff) } };
		1170	_ = &u;
		1171	try expect(u[0].x == 0);
		1172	try expect(u[1].y[0] == 0xff);
		1173	try expect(u[1].y[4] == 0xff);
		1174	}
		1175	};
		1176
		1177	try S.doTheTest();
		1178	try comptime S.doTheTest();
		1179	}
		1180
1165	test "return an extern union from C calling convention" {	1181	test "return an extern union from C calling convention" {
1166	if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO	1182	if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
1167	if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO	1183	if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO