sema: Support reinterpreting extern/packed unions at comptime via field access

src/Module.zig

@@ -6607,6 +6607,7 @@ pub fn unionFieldNormalAlignment(mod: *Module, u: InternPool.UnionType, field_in
     return field_ty.abiAlignment(mod);
 }
 
+/// Returns the index of the active field, given the current tag value
 pub fn unionTagFieldIndex(mod: *Module, u: InternPool.UnionType, enum_tag: Value) ?u32 {
     const ip = &mod.intern_pool;
     if (enum_tag.toIntern() == .none) return null;

src/Sema.zig

@@ -27251,7 +27251,7 @@ fn unionFieldVal(
                     return sema.failWithOwnedErrorMsg(block, msg);
                 }
             },
-            .Packed, .Extern => {
+            .Packed, .Extern => |layout| {
                 if (tag_matches) {
                     return Air.internedToRef(un.val);
                 } else {
@@ -27260,7 +27260,7 @@ fn unionFieldVal(
                     else
                         union_ty.unionFieldType(un.tag.toValue(), mod).?;
 
-                    if (try sema.bitCastVal(block, src, un.val.toValue(), old_ty, field_ty, 0)) |new_val| {
+                    if (try sema.bitCastUnionFieldVal(block, src, un.val.toValue(), old_ty, field_ty, layout)) |new_val| {
                         return Air.internedToRef(new_val.toIntern());
                     }
                 }
@@ -29781,13 +29781,19 @@ fn storePtrVal(
                 error.IllDefinedMemoryLayout => unreachable, // Sema was supposed to emit a compile error already
                 error.Unimplemented => return sema.fail(block, src, "TODO: implement writeToMemory for type '{}'", .{mut_kit.ty.fmt(mod)}),
             };
-            operand_val.writeToMemory(operand_ty, mod, buffer[reinterpret.byte_offset..]) catch |err| switch (err) {
-                error.OutOfMemory => return error.OutOfMemory,
-                error.ReinterpretDeclRef => unreachable,
-                error.IllDefinedMemoryLayout => unreachable, // Sema was supposed to emit a compile error already
-                error.Unimplemented => return sema.fail(block, src, "TODO: implement writeToMemory for type '{}'", .{operand_ty.fmt(mod)}),
-            };
-
+            if (reinterpret.write_packed) {
+                operand_val.writeToPackedMemory(operand_ty, mod, buffer[reinterpret.byte_offset..], 0) catch |err| switch (err) {
+                    error.OutOfMemory => return error.OutOfMemory,
+                    error.ReinterpretDeclRef => unreachable,
+                };
+            } else {
+                operand_val.writeToMemory(operand_ty, mod, buffer[reinterpret.byte_offset..]) catch |err| switch (err) {
+                    error.OutOfMemory => return error.OutOfMemory,
+                    error.ReinterpretDeclRef => unreachable,
+                    error.IllDefinedMemoryLayout => unreachable, // Sema was supposed to emit a compile error already
+                    error.Unimplemented => return sema.fail(block, src, "TODO: implement writeToMemory for type '{}'", .{operand_ty.fmt(mod)}),
+                };
+            }
             const val = Value.readFromMemory(mut_kit.ty, mod, buffer, sema.arena) catch |err| switch (err) {
                 error.OutOfMemory => return error.OutOfMemory,
                 error.IllDefinedMemoryLayout => unreachable,
@@ -29819,6 +29825,8 @@ const ComptimePtrMutationKit = struct {
         reinterpret: struct {
             val_ptr: *Value,
             byte_offset: usize,
+            /// If set, write the operand to packed memory
+            write_packed: bool = false,
         },
         /// If the root decl could not be used as parent, this means `ty` is the type that
         /// caused that by not having a well-defined layout.
@@ -30182,21 +30190,43 @@ fn beginComptimePtrMutation(
                             );
                         },
                         .@"union" => {
-                            // We need to set the active field of the union.
-                            const union_tag_ty = base_child_ty.unionTagTypeHypothetical(mod);
-
                             const payload = &val_ptr.castTag(.@"union").?.data;
-                            payload.tag = try mod.enumValueFieldIndex(union_tag_ty, field_index);
+                            const layout = base_child_ty.containerLayout(mod);
 
-                            return beginComptimePtrMutationInner(
-                                sema,
-                                block,
-                                src,
-                                parent.ty.structFieldType(field_index, mod),
-                                &payload.val,
-                                ptr_elem_ty,
-                                parent.mut_decl,
-                            );
+                            const tag_type = base_child_ty.unionTagTypeHypothetical(mod);
+                            const hypothetical_tag = try mod.enumValueFieldIndex(tag_type, field_index);
+                            if (layout == .Auto or (payload.tag != null and hypothetical_tag.eql(payload.tag.?, tag_type, mod))) {
+                                // We need to set the active field of the union.
+                                payload.tag = hypothetical_tag;
+
+                                const field_ty = parent.ty.structFieldType(field_index, mod);
+                                return beginComptimePtrMutationInner(
+                                    sema,
+                                    block,
+                                    src,
+                                    field_ty,
+                                    &payload.val,
+                                    ptr_elem_ty,
+                                    parent.mut_decl,
+                                );
+                            } else {
+                                // Writing to a different field (a different or unknown tag is active) requires reinterpreting
+                                // memory of the entire union, which requires knowing its abiSize.
+                                try sema.resolveTypeLayout(parent.ty);
+
+                                // This union value no longer has a well-defined tag type.
+                                // The reinterpretation will read it back out as .none.
+                                payload.val = try payload.val.unintern(sema.arena, mod);
+                                return ComptimePtrMutationKit{
+                                    .mut_decl = parent.mut_decl,
+                                    .pointee = .{ .reinterpret = .{
+                                        .val_ptr = val_ptr,
+                                        .byte_offset = 0,
+                                        .write_packed = layout == .Packed,
+                                    } },
+                                    .ty = parent.ty,
+                                };
+                            }
                         },
                         .slice => switch (field_index) {
                             Value.slice_ptr_index => return beginComptimePtrMutationInner(
@@ -30697,6 +30727,7 @@ fn bitCastVal(
     // For types with well-defined memory layouts, we serialize them a byte buffer,
     // then deserialize to the new type.
     const abi_size = try sema.usizeCast(block, src, old_ty.abiSize(mod));
+
     const buffer = try sema.gpa.alloc(u8, abi_size);
     defer sema.gpa.free(buffer);
     val.writeToMemory(old_ty, mod, buffer) catch |err| switch (err) {
@@ -30713,6 +30744,63 @@ fn bitCastVal(
     };
 }
 
+fn bitCastUnionFieldVal(
+    sema: *Sema,
+    block: *Block,
+    src: LazySrcLoc,
+    val: Value,
+    old_ty: Type,
+    field_ty: Type,
+    layout: std.builtin.Type.ContainerLayout,
+) !?Value {
+    const mod = sema.mod;
+    if (old_ty.eql(field_ty, mod)) return val;
+
+    const old_size = try sema.usizeCast(block, src, old_ty.abiSize(mod));
+    const field_size = try sema.usizeCast(block, src, field_ty.abiSize(mod));
+    const endian = mod.getTarget().cpu.arch.endian();
+
+    const buffer = try sema.gpa.alloc(u8, @max(old_size, field_size));
+    defer sema.gpa.free(buffer);
+
+    // Reading a larger value means we need to reinterpret from undefined bytes.
+    const offset = switch (layout) {
+        .Extern => offset: {
+            if (field_size > old_size) @memset(buffer[old_size..], 0xaa);
+            val.writeToMemory(old_ty, mod, buffer) catch |err| switch (err) {
+                error.OutOfMemory => return error.OutOfMemory,
+                error.ReinterpretDeclRef => return null,
+                error.IllDefinedMemoryLayout => unreachable, // Sema was supposed to emit a compile error already
+                error.Unimplemented => return sema.fail(block, src, "TODO: implement writeToMemory for type '{}'", .{old_ty.fmt(mod)}),
+            };
+            break :offset 0;
+        },
+        .Packed => offset: {
+            if (field_size > old_size) {
+                const min_size = @max(old_size, 1);
+                switch (endian) {
+                    .Little => @memset(buffer[min_size - 1 ..], 0xaa),
+                    .Big => @memset(buffer[0 .. buffer.len - min_size + 1], 0xaa),
+                }
+            }
+
+            val.writeToPackedMemory(old_ty, mod, buffer, 0) catch |err| switch (err) {
+                error.OutOfMemory => return error.OutOfMemory,
+                error.ReinterpretDeclRef => return null,
+            };
+
+            break :offset if (endian == .Big) buffer.len - field_size else 0;
+        },
+        .Auto => unreachable,
+    };
+
+    return Value.readFromMemory(field_ty, mod, buffer[offset..], sema.arena) catch |err| switch (err) {
+        error.OutOfMemory => return error.OutOfMemory,
+        error.IllDefinedMemoryLayout => unreachable,
+        error.Unimplemented => return sema.fail(block, src, "TODO: implement readFromMemory for type '{}'", .{field_ty.fmt(mod)}),
+    };
+}
+
 fn coerceArrayPtrToSlice(
     sema: *Sema,
     block: *Block,

src/TypedValue.zig

@@ -84,22 +84,27 @@ pub fn print(
                 if (level == 0) {
                     return writer.writeAll(".{ ... }");
                 }
-                const union_val = val.castTag(.@"union").?.data;
+                const payload = val.castTag(.@"union").?.data;
                 try writer.writeAll(".{ ");
 
-                if (union_val.tag.toIntern() != .none) {
+                if (payload.tag) |tag| {
                     try print(.{
                         .ty = ip.indexToKey(ty.toIntern()).union_type.enum_tag_ty.toType(),
-                        .val = union_val.tag,
+                        .val = tag,
                     }, writer, level - 1, mod);
                     try writer.writeAll(" = ");
-                    const field_ty = ty.unionFieldType(union_val.tag, mod).?;
+                    const field_ty = ty.unionFieldType(tag, mod).?;
                     try print(.{
                         .ty = field_ty,
-                        .val = union_val.val,
+                        .val = payload.val,
                     }, writer, level - 1, mod);
                 } else {
-                    return writer.writeAll("(unknown tag)");
+                    try writer.writeAll("(unknown tag) = ");
+                    const backing_ty = try ty.unionBackingType(mod);
+                    try print(.{
+                        .ty = backing_ty,
+                        .val = payload.val,
+                    }, writer, level - 1, mod);
                 }
 
                 return writer.writeAll(" }");
@@ -421,7 +426,12 @@ pub fn print(
                             .val = un.val.toValue(),
                         }, writer, level - 1, mod);
                     } else {
-                        try writer.writeAll("(unknown tag)");
+                        try writer.writeAll("(unknown tag) = ");
+                        const backing_ty = try ty.unionBackingType(mod);
+                        try print(.{
+                            .ty = backing_ty,
+                            .val = un.val.toValue(),
+                        }, writer, level - 1, mod);
                     }
                 } else try writer.writeAll("...");
                 return writer.writeAll(" }");

src/type.zig

@@ -1954,6 +1954,16 @@ pub const Type = struct {
         return true;
     }
 
+    /// Returns the type used for backing storage of this union during comptime operations.
+    /// Asserts the type is either an extern or packed union.
+    pub fn unionBackingType(ty: Type, mod: *Module) !Type {
+        return switch (ty.containerLayout(mod)) {
+            .Extern => try mod.arrayType(.{ .len = ty.abiSize(mod), .child = .u8_type }),
+            .Packed => try mod.intType(.unsigned, @intCast(ty.bitSize(mod))),
+            .Auto => unreachable,
+        };
+    }
+
     pub fn unionGetLayout(ty: Type, mod: *Module) Module.UnionLayout {
         const ip = &mod.intern_pool;
         const union_type = ip.indexToKey(ty.toIntern()).union_type;

src/value.zig

@@ -327,11 +327,19 @@ pub const Value = struct {
             },
             .@"union" => {
                 const pl = val.castTag(.@"union").?.data;
-                return mod.intern(.{ .un = .{
-                    .ty = ty.toIntern(),
-                    .tag = try pl.tag.intern(ty.unionTagTypeHypothetical(mod), mod),
-                    .val = try pl.val.intern(ty.unionFieldType(pl.tag, mod).?, mod),
-                } });
+                if (pl.tag) |pl_tag| {
+                    return mod.intern(.{ .un = .{
+                        .ty = ty.toIntern(),
+                        .tag = try pl_tag.intern(ty.unionTagTypeHypothetical(mod), mod),
+                        .val = try pl.val.intern(ty.unionFieldType(pl_tag, mod).?, mod),
+                    } });
+                } else {
+                    return mod.intern(.{ .un = .{
+                        .ty = ty.toIntern(),
+                        .tag = .none,
+                        .val = try pl.val.intern(try ty.unionBackingType(mod), mod),
+                    } });
+                }
             },
         }
     }
@@ -399,10 +407,7 @@ pub const Value = struct {
 
             .un => |un| Tag.@"union".create(arena, .{
                 // toValue asserts that the value cannot be .none which is valid on unions.
-                .tag = .{
-                    .ip_index = un.tag,
-                    .legacy = undefined,
-                },
+                .tag = if (un.tag == .none) null else un.tag.toValue(),
                 .val = un.val.toValue(),
             }),
 
@@ -709,21 +714,22 @@ pub const Value = struct {
             .Union => switch (ty.containerLayout(mod)) {
                 .Auto => return error.IllDefinedMemoryLayout, // Sema is supposed to have emitted a compile error already
                 .Extern => {
-                    const union_obj = mod.typeToUnion(ty).?;
                     if (val.unionTag(mod)) |union_tag| {
+                        const union_obj = mod.typeToUnion(ty).?;
                         const field_index = mod.unionTagFieldIndex(union_obj, union_tag).?;
                         const field_type = union_obj.field_types.get(&mod.intern_pool)[field_index].toType();
                         const field_val = try val.fieldValue(mod, field_index);
                         const byte_count = @as(usize, @intCast(field_type.abiSize(mod)));
                         return writeToMemory(field_val, field_type, mod, buffer[0..byte_count]);
                     } else {
-                        const union_size = ty.abiSize(mod);
-                        const array_type = try mod.arrayType(.{ .len = union_size, .child = .u8_type });
-                        return writeToMemory(val.unionValue(mod), array_type, mod, buffer[0..@as(usize, @intCast(union_size))]);
+                        const backing_ty = try ty.unionBackingType(mod);
+                        const byte_count: usize = @intCast(backing_ty.abiSize(mod));
+                        return writeToMemory(val.unionValue(mod), backing_ty, mod, buffer[0..byte_count]);
                     }
                 },
                 .Packed => {
-                    const byte_count = (@as(usize, @intCast(ty.bitSize(mod))) + 7) / 8;
+                    const backing_ty = try ty.unionBackingType(mod);
+                    const byte_count: usize = @intCast(backing_ty.abiSize(mod));
                     return writeToPackedMemory(val, ty, mod, buffer[0..byte_count], 0);
                 },
             },
@@ -842,9 +848,8 @@ pub const Value = struct {
                             const field_val = try val.fieldValue(mod, field_index);
                             return field_val.writeToPackedMemory(field_type, mod, buffer, bit_offset);
                         } else {
-                            const union_bits: u16 = @intCast(ty.bitSize(mod));
-                            const int_ty = try mod.intType(.unsigned, union_bits);
-                            return val.unionValue(mod).writeToPackedMemory(int_ty, mod, buffer, bit_offset);
+                            const backing_ty = try ty.unionBackingType(mod);
+                            return val.unionValue(mod).writeToPackedMemory(backing_ty, mod, buffer, bit_offset);
                         }
                     },
                 }
@@ -1146,10 +1151,8 @@ pub const Value = struct {
             .Union => switch (ty.containerLayout(mod)) {
                 .Auto, .Extern => unreachable, // Handled by non-packed readFromMemory
                 .Packed => {
-                    const union_bits: u16 = @intCast(ty.bitSize(mod));
-                    assert(union_bits != 0);
-                    const int_ty = try mod.intType(.unsigned, union_bits);
-                    const val = (try readFromPackedMemory(int_ty, mod, buffer, bit_offset, arena)).toIntern();
+                    const backing_ty = try ty.unionBackingType(mod);
+                    const val = (try readFromPackedMemory(backing_ty, mod, buffer, bit_offset, arena)).toIntern();
                     return (try mod.intern(.{ .un = .{
                         .ty = ty.toIntern(),
                         .tag = .none,
@@ -4017,7 +4020,7 @@ pub const Value = struct {
             data: Data,
 
             pub const Data = struct {
-                tag: Value,
+                tag: ?Value,
                 val: Value,
             };
         };