Commit 62d178e91a

Xavier Bouchoux <xavierb@gmail.com>
2023-07-29 20:08:08
codegen: fix field offsets in packed structs
* add nested packed struct/union behavior tests * use ptr_info.packed_offset rather than trying to duplicate the logic from Sema.structFieldPtrByIndex() * use the container_ptr_info.packed_offset to account for non-aligned nested structs. * dedup type.packedStructFieldBitOffset() and module.structPackedFieldBitOffset()
1 parent 412d863
Changed files (7)
src/arch/wasm/CodeGen.zig
@@ -3090,12 +3090,19 @@ fn lowerParentPtr(func: *CodeGen, ptr_val: Value, offset: u32) InnerError!WValue
             return func.lowerParentPtr(elem.base.toValue(), @as(u32, @intCast(elem_offset + offset)));
         },
         .field => |field| {
-            const parent_ty = mod.intern_pool.typeOf(field.base).toType().childType(mod);
+            const parent_ptr_ty = mod.intern_pool.typeOf(field.base).toType();
+            const parent_ty = parent_ptr_ty.childType(mod);
+            const field_index: u32 = @intCast(field.index);
 
             const field_offset = switch (parent_ty.zigTypeTag(mod)) {
-                .Struct => switch (parent_ty.containerLayout(mod)) {
-                    .Packed => parent_ty.packedStructFieldByteOffset(@as(usize, @intCast(field.index)), mod),
-                    else => parent_ty.structFieldOffset(@as(usize, @intCast(field.index)), mod),
+                .Struct => blk: {
+                    if (mod.typeToPackedStruct(parent_ty)) |struct_type| {
+                        if (ptr.ty.toType().ptrInfo(mod).packed_offset.host_size == 0)
+                            break :blk @divExact(mod.structPackedFieldBitOffset(struct_type, field_index) + parent_ptr_ty.ptrInfo(mod).packed_offset.bit_offset, 8)
+                        else
+                            break :blk 0;
+                    }
+                    break :blk parent_ty.structFieldOffset(field_index, mod);
                 },
                 .Union => switch (parent_ty.containerLayout(mod)) {
                     .Packed => 0,
src/arch/x86_64/CodeGen.zig
@@ -5774,14 +5774,15 @@ fn fieldPtr(self: *Self, inst: Air.Inst.Index, operand: Air.Inst.Ref, index: u32
     const ptr_container_ty_info = ptr_container_ty.ptrInfo(mod);
     const container_ty = ptr_container_ty.childType(mod);
 
-    const field_offset: i32 = @intCast(switch (container_ty.containerLayout(mod)) {
-        .Auto, .Extern => container_ty.structFieldOffset(index, mod),
-        .Packed => if (container_ty.zigTypeTag(mod) == .Struct and
-            ptr_field_ty.ptrInfo(mod).packed_offset.host_size == 0)
-            container_ty.packedStructFieldByteOffset(index, mod) + @divExact(ptr_container_ty_info.packed_offset.bit_offset, 8)
-        else
-            0,
-    });
+    const field_offset: i32 = blk: {
+        if (mod.typeToPackedStruct(container_ty)) |struct_type| {
+            break :blk if (ptr_field_ty.ptrInfo(mod).packed_offset.host_size == 0)
+                @divExact(mod.structPackedFieldBitOffset(struct_type, index) + ptr_container_ty_info.packed_offset.bit_offset, 8)
+            else
+                0;
+        }
+        break :blk @intCast(container_ty.structFieldOffset(index, mod));
+    };
 
     const src_mcv = try self.resolveInst(operand);
     const dst_mcv = if (switch (src_mcv) {
src/codegen/c.zig
@@ -5269,22 +5269,26 @@ fn fieldLocation(
     const ip = &mod.intern_pool;
     const container_ty = container_ptr_ty.childType(mod);
     return switch (container_ty.zigTypeTag(mod)) {
-        .Struct => switch (container_ty.containerLayout(mod)) {
-            .Auto, .Extern => for (field_index..container_ty.structFieldCount(mod)) |next_field_index_usize| {
+        .Struct => blk: {
+            if (mod.typeToPackedStruct(container_ty)) |struct_type| {
+                if (field_ptr_ty.ptrInfo(mod).packed_offset.host_size == 0)
+                    break :blk .{ .byte_offset = @divExact(mod.structPackedFieldBitOffset(struct_type, field_index) + container_ptr_ty.ptrInfo(mod).packed_offset.bit_offset, 8) }
+                else
+                    break :blk .begin;
+            }
+
+            for (field_index..container_ty.structFieldCount(mod)) |next_field_index_usize| {
                 const next_field_index: u32 = @intCast(next_field_index_usize);
                 if (container_ty.structFieldIsComptime(next_field_index, mod)) continue;
                 const field_ty = container_ty.structFieldType(next_field_index, mod);
                 if (!field_ty.hasRuntimeBitsIgnoreComptime(mod)) continue;
 
-                break .{ .field = if (container_ty.isSimpleTuple(mod))
+                break :blk .{ .field = if (container_ty.isSimpleTuple(mod))
                     .{ .field = next_field_index }
                 else
                     .{ .identifier = ip.stringToSlice(container_ty.legacyStructFieldName(next_field_index, mod)) } };
-            } else if (container_ty.hasRuntimeBitsIgnoreComptime(mod)) .end else .begin,
-            .Packed => if (field_ptr_ty.ptrInfo(mod).packed_offset.host_size == 0)
-                .{ .byte_offset = container_ty.packedStructFieldByteOffset(field_index, mod) + @divExact(container_ptr_ty.ptrInfo(mod).packed_offset.bit_offset, 8) }
-            else
-                .begin,
+            }
+            break :blk if (container_ty.hasRuntimeBitsIgnoreComptime(mod)) .end else .begin;
         },
         .Union => {
             const union_obj = mod.typeToUnion(container_ty).?;
src/codegen/llvm.zig
@@ -4,7 +4,6 @@ const assert = std.debug.assert;
 const Allocator = std.mem.Allocator;
 const log = std.log.scoped(.codegen);
 const math = std.math;
-const native_endian = builtin.cpu.arch.endian();
 const DW = std.dwarf;
 
 const Builder = @import("llvm/Builder.zig");
@@ -3770,7 +3769,7 @@ pub const Object = struct {
                     .opt_payload,
                     .elem,
                     .field,
-                    => try o.lowerParentPtr(val, ty.ptrInfo(mod).packed_offset.bit_offset % 8 == 0),
+                    => try o.lowerParentPtr(val),
                     .comptime_field => unreachable,
                 };
                 switch (ptr.len) {
@@ -4230,15 +4229,16 @@ pub const Object = struct {
         return o.lowerDeclRefValue(ptr_ty, decl_index);
     }
 
-    fn lowerParentPtr(o: *Object, ptr_val: Value, byte_aligned: bool) Allocator.Error!Builder.Constant {
+    fn lowerParentPtr(o: *Object, ptr_val: Value) Allocator.Error!Builder.Constant {
         const mod = o.module;
         const ip = &mod.intern_pool;
-        return switch (ip.indexToKey(ptr_val.toIntern()).ptr.addr) {
+        const ptr = ip.indexToKey(ptr_val.toIntern()).ptr;
+        return switch (ptr.addr) {
             .decl => |decl| o.lowerParentPtrDecl(decl),
             .mut_decl => |mut_decl| o.lowerParentPtrDecl(mut_decl.decl),
             .int => |int| try o.lowerIntAsPtr(int),
             .eu_payload => |eu_ptr| {
-                const parent_ptr = try o.lowerParentPtr(eu_ptr.toValue(), true);
+                const parent_ptr = try o.lowerParentPtr(eu_ptr.toValue());
 
                 const eu_ty = ip.typeOf(eu_ptr).toType().childType(mod);
                 const payload_ty = eu_ty.errorUnionPayload(mod);
@@ -4256,7 +4256,7 @@ pub const Object = struct {
                 });
             },
             .opt_payload => |opt_ptr| {
-                const parent_ptr = try o.lowerParentPtr(opt_ptr.toValue(), true);
+                const parent_ptr = try o.lowerParentPtr(opt_ptr.toValue());
 
                 const opt_ty = ip.typeOf(opt_ptr).toType().childType(mod);
                 const payload_ty = opt_ty.optionalChild(mod);
@@ -4274,7 +4274,7 @@ pub const Object = struct {
             },
             .comptime_field => unreachable,
             .elem => |elem_ptr| {
-                const parent_ptr = try o.lowerParentPtr(elem_ptr.base.toValue(), true);
+                const parent_ptr = try o.lowerParentPtr(elem_ptr.base.toValue());
                 const elem_ty = ip.typeOf(elem_ptr.base).toType().elemType2(mod);
 
                 return o.builder.gepConst(.inbounds, try o.lowerType(elem_ty), parent_ptr, null, &.{
@@ -4282,9 +4282,9 @@ pub const Object = struct {
                 });
             },
             .field => |field_ptr| {
-                const parent_ptr = try o.lowerParentPtr(field_ptr.base.toValue(), byte_aligned);
-                const parent_ty = ip.typeOf(field_ptr.base).toType().childType(mod);
-
+                const parent_ptr = try o.lowerParentPtr(field_ptr.base.toValue());
+                const parent_ptr_ty = ip.typeOf(field_ptr.base).toType();
+                const parent_ty = parent_ptr_ty.childType(mod);
                 const field_index: u32 = @intCast(field_ptr.index);
                 switch (parent_ty.zigTypeTag(mod)) {
                     .Union => {
@@ -4309,22 +4309,14 @@ pub const Object = struct {
                     },
                     .Struct => {
                         if (mod.typeToPackedStruct(parent_ty)) |struct_type| {
-                            if (!byte_aligned) return parent_ptr;
+                            const ptr_info = ptr.ty.toType().ptrInfo(mod);
+                            if (ptr_info.packed_offset.host_size != 0) return parent_ptr;
+
+                            const parent_ptr_info = parent_ptr_ty.ptrInfo(mod);
+                            const bit_offset = mod.structPackedFieldBitOffset(struct_type, field_index) + parent_ptr_info.packed_offset.bit_offset;
                             const llvm_usize = try o.lowerType(Type.usize);
-                            const base_addr =
-                                try o.builder.castConst(.ptrtoint, parent_ptr, llvm_usize);
-                            // count bits of fields before this one
-                            // TODO https://github.com/ziglang/zig/issues/17178
-                            const prev_bits = b: {
-                                var b: usize = 0;
-                                for (0..field_index) |i| {
-                                    const field_ty = struct_type.field_types.get(ip)[i].toType();
-                                    if (!field_ty.hasRuntimeBitsIgnoreComptime(mod)) continue;
-                                    b += @intCast(field_ty.bitSize(mod));
-                                }
-                                break :b b;
-                            };
-                            const byte_offset = try o.builder.intConst(llvm_usize, prev_bits / 8);
+                            const base_addr = try o.builder.castConst(.ptrtoint, parent_ptr, llvm_usize);
+                            const byte_offset = try o.builder.intConst(llvm_usize, @divExact(bit_offset, 8));
                             const field_addr = try o.builder.binConst(.add, base_addr, byte_offset);
                             return o.builder.castConst(.inttoptr, field_addr, .ptr);
                         }
src/type.zig
@@ -3028,24 +3028,10 @@ pub const Type = struct {
         };
     }
 
-    pub fn packedStructFieldBitOffset(ty: Type, field_index: usize, mod: *Module) u32 {
+    pub fn packedStructFieldByteOffset(ty: Type, field_index: u32, mod: *Module) u32 {
         const ip = &mod.intern_pool;
         const struct_type = ip.indexToKey(ty.toIntern()).struct_type;
-        assert(struct_type.layout == .Packed);
-        comptime assert(Type.packed_struct_layout_version == 2);
-
-        var running_bits: u32 = 0;
-        for (struct_type.field_types.get(ip), 0..) |field_ty, i| {
-            if (i == field_index) break;
-            if (!field_ty.toType().hasRuntimeBits(mod)) continue;
-            const field_bits: u32 = @intCast(field_ty.toType().bitSize(mod));
-            running_bits += field_bits;
-        }
-        return running_bits;
-    }
-
-    pub fn packedStructFieldByteOffset(ty: Type, field_index: usize, mod: *Module) u32 {
-        return packedStructFieldBitOffset(ty, field_index, mod) / 8;
+        return @divExact(mod.structPackedFieldBitOffset(struct_type, field_index), 8);
     }
 
     pub const FieldOffset = struct {
test/behavior/packed-struct.zig
@@ -253,6 +253,79 @@ test "regular in irregular packed struct" {
     try expectEqual(@as(u8, 42), foo.bar.b);
 }
 
+test "nested packed struct unaligned" {
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
+    if (native_endian != .Little) return error.SkipZigTest; // Byte aligned packed struct field pointers have not been implemented yet
+
+    const S1 = packed struct {
+        a: u4,
+        b: u4,
+        c: u8,
+    };
+    const S2 = packed struct {
+        base: u8,
+        p0: S1,
+        bit0: u1,
+        p1: packed struct {
+            a: u8,
+        },
+        p2: packed struct {
+            a: u7,
+            b: u8,
+        },
+        p3: S1,
+
+        var s: @This() = .{
+            .base = 1,
+            .p0 = .{ .a = 2, .b = 3, .c = 4 },
+            .bit0 = 0,
+            .p1 = .{ .a = 5 },
+            .p2 = .{ .a = 6, .b = 7 },
+            .p3 = .{ .a = 8, .b = 9, .c = 10 },
+        };
+    };
+
+    try expect(S2.s.base == 1);
+    try expect(S2.s.p0.a == 2);
+    try expect(S2.s.p0.b == 3);
+    try expect(S2.s.p0.c == 4);
+    try expect(S2.s.bit0 == 0);
+    try expect(S2.s.p1.a == 5);
+    try expect(S2.s.p2.a == 6);
+    try expect(S2.s.p2.b == 7);
+    try expect(S2.s.p3.a == 8);
+    try expect(S2.s.p3.b == 9);
+    try expect(S2.s.p3.c == 10);
+
+    const S3 = packed struct {
+        pad: u8,
+        v: u2,
+        s: packed struct {
+            v: u3,
+            s: packed struct {
+                v: u2,
+                s: packed struct {
+                    bit0: u1,
+                    byte: u8,
+                    bit1: u1,
+                },
+            },
+        },
+        var v0: @This() = .{ .pad = 0, .v = 1, .s = .{ .v = 2, .s = .{ .v = 3, .s = .{ .bit0 = 0, .byte = 4, .bit1 = 1 } } } };
+    };
+
+    try expect(S3.v0.v == 1);
+    try expect(S3.v0.s.v == 2);
+    try expect(S3.v0.s.s.v == 3);
+    try expect(S3.v0.s.s.s.bit0 == 0);
+    try expect(S3.v0.s.s.s.byte == 4);
+    try expect(S3.v0.s.s.s.bit1 == 1);
+}
+
 test "byte-aligned field pointer offsets" {
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
@@ -354,6 +427,45 @@ test "byte-aligned field pointer offsets" {
     try comptime S.doTheTest();
 }
 
+test "nested packed struct field pointers" {
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_c) return error.SkipZigTest; // ubsan unaligned pointer access
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
+    if (native_endian != .Little) return error.SkipZigTest; // Byte aligned packed struct field pointers have not been implemented yet
+
+    const S2 = packed struct {
+        base: u8,
+        p0: packed struct {
+            a: u4,
+            b: u4,
+            c: u8,
+        },
+        bit: u1,
+        p1: packed struct {
+            a: u7,
+            b: u8,
+        },
+
+        var s: @This() = .{ .base = 1, .p0 = .{ .a = 2, .b = 3, .c = 4 }, .bit = 0, .p1 = .{ .a = 5, .b = 6 } };
+    };
+
+    const ptr_base = &S2.s.base;
+    const ptr_p0_a = &S2.s.p0.a;
+    const ptr_p0_b = &S2.s.p0.b;
+    const ptr_p0_c = &S2.s.p0.c;
+    const ptr_p1_a = &S2.s.p1.a;
+    const ptr_p1_b = &S2.s.p1.b;
+    try expectEqual(@as(u8, 1), ptr_base.*);
+    try expectEqual(@as(u4, 2), ptr_p0_a.*);
+    try expectEqual(@as(u4, 3), ptr_p0_b.*);
+    try expectEqual(@as(u8, 4), ptr_p0_c.*);
+    try expectEqual(@as(u7, 5), ptr_p1_a.*);
+    try expectEqual(@as(u8, 6), ptr_p1_b.*);
+}
+
 test "load pointer from packed struct" {
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
@@ -380,6 +492,7 @@ test "@intFromPtr on a packed struct field" {
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
+    if (native_endian != .Little) return error.SkipZigTest;
 
     const S = struct {
         const P = packed struct {
@@ -387,6 +500,7 @@ test "@intFromPtr on a packed struct field" {
             y: u8,
             z: u32,
         };
+
         var p0: P = P{
             .x = 1,
             .y = 2,
@@ -396,6 +510,138 @@ test "@intFromPtr on a packed struct field" {
     try expect(@intFromPtr(&S.p0.z) - @intFromPtr(&S.p0.x) == 2);
 }
 
+test "@intFromPtr on a packed struct field unaligned and nested" {
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest;
+    if (native_endian != .Little) return error.SkipZigTest; // Byte aligned packed struct field pointers have not been implemented yet
+
+    const S1 = packed struct {
+        a: u4,
+        b: u4,
+        c: u8,
+    };
+    const S2 = packed struct {
+        base: u8,
+        p0: S1,
+        bit0: u1,
+        p1: packed struct {
+            a: u8,
+        },
+        p2: packed struct {
+            a: u7,
+            b: u8,
+        },
+        p3: S1,
+
+        var s: @This() = .{
+            .base = 1,
+            .p0 = .{ .a = 2, .b = 3, .c = 4 },
+            .bit0 = 0,
+            .p1 = .{ .a = 5 },
+            .p2 = .{ .a = 6, .b = 7 },
+            .p3 = .{ .a = 8, .b = 9, .c = 10 },
+        };
+    };
+
+    switch (comptime @alignOf(S2)) {
+        4 => {
+            comptime assert(@TypeOf(&S2.s.base) == *align(4) u8);
+            comptime assert(@TypeOf(&S2.s.p0.a) == *align(1:0:2) u4);
+            comptime assert(@TypeOf(&S2.s.p0.b) == *align(1:4:2) u4);
+            comptime assert(@TypeOf(&S2.s.p0.c) == *u8);
+            comptime assert(@TypeOf(&S2.s.bit0) == *align(4:24:8) u1);
+            comptime assert(@TypeOf(&S2.s.p1.a) == *align(4:25:8) u8);
+            comptime assert(@TypeOf(&S2.s.p2.a) == *align(4:33:8) u7);
+            comptime assert(@TypeOf(&S2.s.p2.b) == *u8);
+            comptime assert(@TypeOf(&S2.s.p3.a) == *align(2:0:2) u4);
+            comptime assert(@TypeOf(&S2.s.p3.b) == *align(2:4:2) u4);
+            comptime assert(@TypeOf(&S2.s.p3.c) == *u8);
+        },
+        8 => {
+            comptime assert(@TypeOf(&S2.s.base) == *align(8) u8);
+            comptime assert(@TypeOf(&S2.s.p0.a) == *align(1:0:2) u4);
+            comptime assert(@TypeOf(&S2.s.p0.b) == *align(1:4:2) u4);
+            comptime assert(@TypeOf(&S2.s.p0.c) == *u8);
+            comptime assert(@TypeOf(&S2.s.bit0) == *align(8:24:8) u1);
+            comptime assert(@TypeOf(&S2.s.p1.a) == *align(8:25:8) u8);
+            comptime assert(@TypeOf(&S2.s.p2.a) == *align(8:33:8) u7);
+            comptime assert(@TypeOf(&S2.s.p2.b) == *u8);
+            comptime assert(@TypeOf(&S2.s.p3.a) == *align(2:0:2) u4);
+            comptime assert(@TypeOf(&S2.s.p3.b) == *align(2:4:2) u4);
+            comptime assert(@TypeOf(&S2.s.p3.c) == *u8);
+        },
+        else => {},
+    }
+    try expect(@intFromPtr(&S2.s.base) - @intFromPtr(&S2.s) == 0);
+    try expect(@intFromPtr(&S2.s.p0.a) - @intFromPtr(&S2.s) == 1);
+    try expect(@intFromPtr(&S2.s.p0.b) - @intFromPtr(&S2.s) == 1);
+    try expect(@intFromPtr(&S2.s.p0.c) - @intFromPtr(&S2.s) == 2);
+    try expect(@intFromPtr(&S2.s.bit0) - @intFromPtr(&S2.s) == 0);
+    try expect(@intFromPtr(&S2.s.p1.a) - @intFromPtr(&S2.s) == 0);
+    try expect(@intFromPtr(&S2.s.p2.a) - @intFromPtr(&S2.s) == 0);
+    try expect(@intFromPtr(&S2.s.p2.b) - @intFromPtr(&S2.s) == 5);
+    try expect(@intFromPtr(&S2.s.p3.a) - @intFromPtr(&S2.s) == 6);
+    try expect(@intFromPtr(&S2.s.p3.b) - @intFromPtr(&S2.s) == 6);
+    try expect(@intFromPtr(&S2.s.p3.c) - @intFromPtr(&S2.s) == 7);
+
+    const S3 = packed struct {
+        pad: u8,
+        v: u2,
+        s: packed struct {
+            v: u3,
+            s: packed struct {
+                v: u2,
+                s: packed struct {
+                    bit0: u1,
+                    byte: u8,
+                    bit1: u1,
+                },
+            },
+        },
+        var v0: @This() = .{ .pad = 0, .v = 1, .s = .{ .v = 2, .s = .{ .v = 3, .s = .{ .bit0 = 0, .byte = 4, .bit1 = 1 } } } };
+    };
+
+    comptime assert(@TypeOf(&S3.v0.v) == *align(4:8:4) u2);
+    comptime assert(@TypeOf(&S3.v0.s.v) == *align(4:10:4) u3);
+    comptime assert(@TypeOf(&S3.v0.s.s.v) == *align(4:13:4) u2);
+    comptime assert(@TypeOf(&S3.v0.s.s.s.bit0) == *align(4:15:4) u1);
+    comptime assert(@TypeOf(&S3.v0.s.s.s.byte) == *align(2) u8);
+    comptime assert(@TypeOf(&S3.v0.s.s.s.bit1) == *align(4:24:4) u1);
+    try expect(@intFromPtr(&S3.v0.v) - @intFromPtr(&S3.v0) == 0);
+    try expect(@intFromPtr(&S3.v0.s) - @intFromPtr(&S3.v0) == 0);
+    try expect(@intFromPtr(&S3.v0.s.v) - @intFromPtr(&S3.v0) == 0);
+    try expect(@intFromPtr(&S3.v0.s.s) - @intFromPtr(&S3.v0) == 0);
+    try expect(@intFromPtr(&S3.v0.s.s.v) - @intFromPtr(&S3.v0) == 0);
+    try expect(@intFromPtr(&S3.v0.s.s.s) - @intFromPtr(&S3.v0) == 0);
+    try expect(@intFromPtr(&S3.v0.s.s.s.bit0) - @intFromPtr(&S3.v0) == 0);
+    try expect(@intFromPtr(&S3.v0.s.s.s.byte) - @intFromPtr(&S3.v0) == 2);
+    try expect(@intFromPtr(&S3.v0.s.s.s.bit1) - @intFromPtr(&S3.v0) == 0);
+}
+
+test "packed struct fields modification" {
+    const Small = packed struct {
+        val: u8 = 0,
+        lo: u4 = 0,
+        hi: u4 = 0,
+
+        var p: @This() = undefined;
+    };
+    Small.p = .{
+        .val = 0x12,
+        .lo = 3,
+        .hi = 4,
+    };
+    try expect(@as(u16, @bitCast(Small.p)) == 0x4312);
+
+    Small.p.val -= Small.p.lo;
+    Small.p.val += Small.p.hi;
+    Small.p.hi -= Small.p.lo;
+    try expect(@as(u16, @bitCast(Small.p)) == 0x1313);
+}
+
 test "optional pointer in packed struct" {
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
@@ -410,7 +656,7 @@ test "optional pointer in packed struct" {
 
 test "nested packed struct field access test" {
     if (builtin.zig_backend == .stage2_c) return error.SkipZigTest; // TODO
-    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO packed structs larger than 64 bits
     if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
@@ -562,7 +808,7 @@ test "nested packed struct at non-zero offset 2" {
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
-    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO packed structs larger than 64 bits
     if (builtin.zig_backend == .stage2_c) return error.SkipZigTest;
 
     const S = struct {
@@ -700,7 +946,6 @@ test "packed struct initialized in bitcast" {
 test "pointer to container level packed struct field" {
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
-    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest; // TODO
     if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
 
@@ -727,7 +972,6 @@ test "store undefined to packed result location" {
     if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
     if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
-    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest;
 
     var x: u4 = 0;
     var s = packed struct { x: u4, y: u4 }{ .x = x, .y = if (x > 0) x else undefined };
@@ -743,3 +987,19 @@ test "bitcast back and forth" {
     try expect(s.one == s2.one);
     try expect(s.two == s2.two);
 }
+
+test "field access of packed struct smaller than its abi size inside struct initialized with rls" {
+    if (builtin.zig_backend == .stage2_llvm and builtin.cpu.arch == .arm) return error.SkipZigTest;
+    const S = struct {
+        ps: packed struct { x: i2, y: i2 },
+
+        fn init(cond: bool) @This() {
+            return .{ .ps = .{ .x = 0, .y = if (cond) 1 else 0 } };
+        }
+    };
+
+    var s = S.init(true);
+    // note: this bug is triggered by the == operator, expectEqual will hide it
+    try expect(@as(i2, 0) == s.ps.x);
+    try expect(@as(i2, 1) == s.ps.y);
+}
test/behavior/packed-union.zig
@@ -38,3 +38,50 @@ test "flags in packed union" {
     try expectEqual(true, test_bits.enable_1);
     try expectEqual(false, test_bits.other_flags.flags.enable_1);
 }
+
+test "flags in packed union at offset" {
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_sparc64) return error.SkipZigTest;
+    if (builtin.zig_backend == .stage2_spirv64) return error.SkipZigTest;
+
+    const FlagBits = packed union {
+        base_flags: packed union {
+            flags: packed struct(u4) {
+                enable_1: bool = true,
+                enable_2: bool = false,
+                enable_3: bool = false,
+                enable_4: bool = false,
+            },
+            bits: u4,
+        },
+        adv_flags: packed struct(u12) {
+            pad: u8 = 0,
+            adv: packed union {
+                flags: packed struct(u4) {
+                    enable_1: bool = true,
+                    enable_2: bool = false,
+                    enable_3: bool = false,
+                    enable_4: bool = false,
+                },
+                bits: u4,
+            },
+        },
+    };
+    var test_bits: FlagBits = .{ .adv_flags = .{ .adv = .{ .flags = .{} } } };
+
+    try expectEqual(@as(u8, 0), test_bits.adv_flags.pad);
+    try expectEqual(true, test_bits.adv_flags.adv.flags.enable_1);
+    try expectEqual(false, test_bits.adv_flags.adv.flags.enable_2);
+
+    test_bits.adv_flags.adv.flags.enable_1 = false;
+    test_bits.adv_flags.adv.flags.enable_2 = true;
+    try expectEqual(@as(u8, 0), test_bits.adv_flags.pad);
+    try expectEqual(false, test_bits.adv_flags.adv.flags.enable_1);
+    try expectEqual(true, test_bits.adv_flags.adv.flags.enable_2);
+
+    test_bits.adv_flags.adv.bits = 12;
+    try expectEqual(@as(u8, 0), test_bits.adv_flags.pad);
+    try expectEqual(false, test_bits.adv_flags.adv.flags.enable_1);
+    try expectEqual(false, test_bits.adv_flags.adv.flags.enable_2);
+}