Commit 018daa028e

Andrew Kelley <andrew@ziglang.org>
2020-04-19 01:41:45
rework types and values data layout
1 parent f92ccf3
Changed files (3)
src-self-hosted
src-self-hosted/ir.zig
@@ -2,6 +2,7 @@ const std = @import("std");
 const mem = std.mem;
 const Allocator = std.mem.Allocator;
 const Value = @import("value.zig").Value;
+const Type = @import("type.zig").Type;
 const assert = std.debug.assert;
 
 pub const Inst = struct {
@@ -17,6 +18,7 @@ pub const Inst = struct {
         Fn,
     };
 
+    /// These names are used for the IR text format.
     pub const Tag = enum {
         constant,
         ptrtoint,
@@ -32,9 +34,11 @@ pub const Inst = struct {
     /// a memory location for the value to survive after a const instruction.
     pub const Constant = struct {
         base: Inst = Inst{ .tag = .constant },
-        value: *Value,
+        ty: Type,
 
-        positionals: struct {},
+        positionals: struct {
+            value: Value,
+        },
         kw_args: struct {},
     };
 
@@ -241,6 +245,31 @@ fn parseInstructionGeneric(ctx: *ParseContext, comptime fn_name: []const u8, com
 }
 
 fn parseParameterGeneric(ctx: *ParseContext, comptime T: type) !T {
+    if (@typeInfo(T) == .Enum) {
+        const start = ctx.i;
+        while (ctx.i < ctx.source.len) : (ctx.i += 1) switch (ctx.source[ctx.i]) {
+            ' ', '\n', ',', ')' => {
+                const enum_name = ctx.source[start..ctx.i];
+                ctx.i += 1;
+                return std.meta.stringToEnum(T, enum_name) orelse {
+                    return parseError(ctx, "tag '{}' not a member of enum '{}'", .{ enum_name, @typeName(T) });
+                };
+            },
+            else => continue,
+        };
+        return parseError(ctx, "unexpected EOF in enum parameter", .{});
+    }
+    switch (T) {
+        Inst.Fn.Body => {
+            var instructions = std.ArrayList(*Inst).init(ctx.allocator);
+            try requireEatBytes(ctx, "{");
+            return T{
+                .instructions = instructions.toOwnedSlice(),
+            };
+        },
+        Value => return parseError(ctx, "TODO implement parseParameterGeneric for type Value", .{}),
+        else => @compileError("Unimplemented: ir parseParameterGeneric for type " ++ @typeName(T)),
+    }
     return parseError(ctx, "TODO parse parameter {}", .{@typeName(T)});
 }
 
@@ -261,12 +290,18 @@ fn parseStringLiteralConst(ctx: *ParseContext) !*Inst {
                 },
                 else => |e| return e,
             };
-            const bytes_val = try ctx.allocator.create(Value.Bytes);
-            bytes_val.* = .{ .data = parsed };
+            const bytes_payload = try ctx.allocator.create(Value.Payload.Bytes);
+            errdefer ctx.allocator.destroy(bytes_payload);
+            bytes_payload.* = .{ .data = parsed };
+
+            const ty_payload = try ctx.allocator.create(Type.Payload.Array_u8_Sentinel0);
+            errdefer ctx.allocator.destroy(ty_payload);
+            ty_payload.* = .{ .len = parsed.len };
+
             const const_inst = try ctx.allocator.create(Inst.Constant);
             const_inst.* = .{
-                .value = &bytes_val.base,
-                .positionals = .{},
+                .ty = Type.initPayload(&ty_payload.base),
+                .positionals = .{ .value = Value.initPayload(&bytes_payload.base) },
                 .kw_args = .{},
             };
             return &const_inst.base;
src-self-hosted/type.zig
@@ -1,1075 +1,112 @@
 const std = @import("std");
-const builtin = std.builtin;
-const Scope = @import("scope.zig").Scope;
-const Compilation = @import("compilation.zig").Compilation;
 const Value = @import("value.zig").Value;
-const llvm = @import("llvm.zig");
-const event = std.event;
-const Allocator = std.mem.Allocator;
 const assert = std.debug.assert;
 
-pub const Type = struct {
-    base: Value,
-    id: Id,
-    name: []const u8,
-    abi_alignment: AbiAlignment,
-
-    pub const AbiAlignment = event.Future(error{OutOfMemory}!u32);
-
-    pub const Id = builtin.TypeId;
-
-    pub fn destroy(base: *Type, comp: *Compilation) void {
-        switch (base.id) {
-            .Struct => @fieldParentPtr(Struct, "base", base).destroy(comp),
-            .Fn => @fieldParentPtr(Fn, "base", base).destroy(comp),
-            .Type => @fieldParentPtr(MetaType, "base", base).destroy(comp),
-            .Void => @fieldParentPtr(Void, "base", base).destroy(comp),
-            .Bool => @fieldParentPtr(Bool, "base", base).destroy(comp),
-            .NoReturn => @fieldParentPtr(NoReturn, "base", base).destroy(comp),
-            .Int => @fieldParentPtr(Int, "base", base).destroy(comp),
-            .Float => @fieldParentPtr(Float, "base", base).destroy(comp),
-            .Pointer => @fieldParentPtr(Pointer, "base", base).destroy(comp),
-            .Array => @fieldParentPtr(Array, "base", base).destroy(comp),
-            .ComptimeFloat => @fieldParentPtr(ComptimeFloat, "base", base).destroy(comp),
-            .ComptimeInt => @fieldParentPtr(ComptimeInt, "base", base).destroy(comp),
-            .EnumLiteral => @fieldParentPtr(EnumLiteral, "base", base).destroy(comp),
-            .Undefined => @fieldParentPtr(Undefined, "base", base).destroy(comp),
-            .Null => @fieldParentPtr(Null, "base", base).destroy(comp),
-            .Optional => @fieldParentPtr(Optional, "base", base).destroy(comp),
-            .ErrorUnion => @fieldParentPtr(ErrorUnion, "base", base).destroy(comp),
-            .ErrorSet => @fieldParentPtr(ErrorSet, "base", base).destroy(comp),
-            .Enum => @fieldParentPtr(Enum, "base", base).destroy(comp),
-            .Union => @fieldParentPtr(Union, "base", base).destroy(comp),
-            .BoundFn => @fieldParentPtr(BoundFn, "base", base).destroy(comp),
-            .Opaque => @fieldParentPtr(Opaque, "base", base).destroy(comp),
-            .Frame => @fieldParentPtr(Frame, "base", base).destroy(comp),
-            .AnyFrame => @fieldParentPtr(AnyFrame, "base", base).destroy(comp),
-            .Vector => @fieldParentPtr(Vector, "base", base).destroy(comp),
+/// This is the raw data, with no bookkeeping, no memory awareness, no de-duplication.
+/// It's important for this struct to be small.
+/// It is not copyable since it may contain references to its inner data.
+/// Types are not de-duplicated, which helps with multi-threading since it obviates the requirement
+/// of obtaining a lock on a global type table, as well as making the
+/// garbage collection bookkeeping simpler.
+/// This union takes advantage of the fact that the first page of memory
+/// is unmapped, giving us 4096 possible enum tags that have no payload.
+pub const Type = extern union {
+    /// If the tag value is less than Tag.no_payload_count, then no pointer
+    /// dereference is needed.
+    tag_if_small_enough: usize,
+    ptr_otherwise: *Payload,
+
+    pub fn zigTypeTag(self: Type) std.builtin.TypeId {
+        switch (self.tag()) {
+            .int_u8, .int_usize => return .Int,
+            .array_u8, .array_u8_sentinel_0 => return .Array,
+            .single_const_pointer => return .Pointer,
         }
     }
 
-    pub fn getLlvmType(
-        base: *Type,
-        allocator: *Allocator,
-        llvm_context: *llvm.Context,
-    ) error{OutOfMemory}!*llvm.Type {
-        switch (base.id) {
-            .Struct => return @fieldParentPtr(Struct, "base", base).getLlvmType(allocator, llvm_context),
-            .Fn => return @fieldParentPtr(Fn, "base", base).getLlvmType(allocator, llvm_context),
-            .Type => unreachable,
-            .Void => unreachable,
-            .Bool => return @fieldParentPtr(Bool, "base", base).getLlvmType(allocator, llvm_context),
-            .NoReturn => unreachable,
-            .Int => return @fieldParentPtr(Int, "base", base).getLlvmType(allocator, llvm_context),
-            .Float => return @fieldParentPtr(Float, "base", base).getLlvmType(allocator, llvm_context),
-            .Pointer => return @fieldParentPtr(Pointer, "base", base).getLlvmType(allocator, llvm_context),
-            .Array => return @fieldParentPtr(Array, "base", base).getLlvmType(allocator, llvm_context),
-            .ComptimeFloat => unreachable,
-            .ComptimeInt => unreachable,
-            .EnumLiteral => unreachable,
-            .Undefined => unreachable,
-            .Null => unreachable,
-            .Optional => return @fieldParentPtr(Optional, "base", base).getLlvmType(allocator, llvm_context),
-            .ErrorUnion => return @fieldParentPtr(ErrorUnion, "base", base).getLlvmType(allocator, llvm_context),
-            .ErrorSet => return @fieldParentPtr(ErrorSet, "base", base).getLlvmType(allocator, llvm_context),
-            .Enum => return @fieldParentPtr(Enum, "base", base).getLlvmType(allocator, llvm_context),
-            .Union => return @fieldParentPtr(Union, "base", base).getLlvmType(allocator, llvm_context),
-            .BoundFn => return @fieldParentPtr(BoundFn, "base", base).getLlvmType(allocator, llvm_context),
-            .Opaque => return @fieldParentPtr(Opaque, "base", base).getLlvmType(allocator, llvm_context),
-            .Frame => return @fieldParentPtr(Frame, "base", base).getLlvmType(allocator, llvm_context),
-            .AnyFrame => return @fieldParentPtr(AnyFrame, "base", base).getLlvmType(allocator, llvm_context),
-            .Vector => return @fieldParentPtr(Vector, "base", base).getLlvmType(allocator, llvm_context),
-        }
+    pub fn initTag(comptime tag: Tag) Type {
+        comptime assert(@enumToInt(tag) < Tag.no_payload_count);
+        return .{ .tag_if_small_enough = @enumToInt(tag) };
     }
 
-    pub fn handleIsPtr(base: *Type) bool {
-        switch (base.id) {
-            .Type,
-            .ComptimeFloat,
-            .ComptimeInt,
-            .EnumLiteral,
-            .Undefined,
-            .Null,
-            .BoundFn,
-            .Opaque,
-            => unreachable,
-
-            .NoReturn,
-            .Void,
-            .Bool,
-            .Int,
-            .Float,
-            .Pointer,
-            .ErrorSet,
-            .Enum,
-            .Fn,
-            .Frame,
-            .AnyFrame,
-            .Vector,
-            => return false,
-
-            .Struct => @panic("TODO"),
-            .Array => @panic("TODO"),
-            .Optional => @panic("TODO"),
-            .ErrorUnion => @panic("TODO"),
-            .Union => @panic("TODO"),
-        }
+    pub fn initPayload(payload: *Payload) Type {
+        assert(@enumToInt(payload.tag) >= Tag.no_payload_count);
+        return .{ .ptr_otherwise = payload };
     }
 
-    pub fn hasBits(base: *Type) bool {
-        switch (base.id) {
-            .Type,
-            .ComptimeFloat,
-            .ComptimeInt,
-            .EnumLiteral,
-            .Undefined,
-            .Null,
-            .BoundFn,
-            .Opaque,
-            => unreachable,
-
-            .Void,
-            .NoReturn,
-            => return false,
-
-            .Bool,
-            .Int,
-            .Float,
-            .Fn,
-            .Frame,
-            .AnyFrame,
-            .Vector,
-            => return true,
-
-            .Pointer => {
-                const ptr_type = @fieldParentPtr(Pointer, "base", base);
-                return ptr_type.key.child_type.hasBits();
-            },
-
-            .ErrorSet => @panic("TODO"),
-            .Enum => @panic("TODO"),
-            .Struct => @panic("TODO"),
-            .Array => @panic("TODO"),
-            .Optional => @panic("TODO"),
-            .ErrorUnion => @panic("TODO"),
-            .Union => @panic("TODO"),
+    pub fn tag(self: Type) Tag {
+        if (self.tag_if_small_enough < Tag.no_payload_count) {
+            return @intToEnum(self.tag_if_small_enough);
+        } else {
+            return self.ptr_otherwise.tag;
         }
     }
 
-    pub fn cast(base: *Type, comptime T: type) ?*T {
-        if (base.id != @field(Id, @typeName(T))) return null;
-        return @fieldParentPtr(T, "base", base);
-    }
-
-    pub fn dump(base: *const Type) void {
-        std.debug.warn("{}", .{@tagName(base.id)});
-    }
-
-    fn init(base: *Type, comp: *Compilation, id: Id, name: []const u8) void {
-        base.* = Type{
-            .base = Value{
-                .id = .Type,
-                .typ = &MetaType.get(comp).base,
-                .ref_count = std.atomic.Int(usize).init(1),
+    pub fn format(
+        self: Type,
+        comptime fmt: []const u8,
+        options: std.fmt.FormatOptions,
+        out_stream: var,
+    ) !void {
+        comptime assert(fmt.len == 0);
+        switch (self.tag()) {
+            .int_u8 => return out_stream.writeAll("u8"),
+            .int_usize => return out_stream.writeAll("usize"),
+            .array_u8_sentinel_0 => {
+                const payload = @fieldParentPtr(Payload.Array_u8_Sentinel0, "base", self.ptr_otherwise);
+                return out_stream.print("[{}:0]u8", .{payload.len});
+            },
+            .array => {
+                const payload = @fieldParentPtr(Payload.Array, "base", self.ptr_otherwise);
+                return out_stream.print("[{}]{}", .{ payload.len, payload.elem_type });
+            },
+            .single_const_pointer => {
+                const payload = @fieldParentPtr(Payload.SingleConstPointer, "base", self.ptr_otherwise);
+                return out_stream.print("*const {}", .{payload.pointee_type});
             },
-            .id = id,
-            .name = name,
-            .abi_alignment = AbiAlignment.init(),
-        };
-    }
-
-    /// If you happen to have an llvm context handy, use getAbiAlignmentInContext instead.
-    /// Otherwise, this one will grab one from the pool and then release it.
-    pub fn getAbiAlignment(base: *Type, comp: *Compilation) !u32 {
-        if (base.abi_alignment.start()) |ptr| return ptr.*;
-
-        {
-            const held = try comp.zig_compiler.getAnyLlvmContext();
-            defer held.release(comp.zig_compiler);
-
-            const llvm_context = held.node.data;
-
-            base.abi_alignment.data = base.resolveAbiAlignment(comp, llvm_context);
         }
-        base.abi_alignment.resolve();
-        return base.abi_alignment.data;
     }
 
-    /// If you have an llvm conext handy, you can use it here.
-    pub fn getAbiAlignmentInContext(base: *Type, comp: *Compilation, llvm_context: *llvm.Context) !u32 {
-        if (base.abi_alignment.start()) |ptr| return ptr.*;
-
-        base.abi_alignment.data = base.resolveAbiAlignment(comp, llvm_context);
-        base.abi_alignment.resolve();
-        return base.abi_alignment.data;
-    }
-
-    /// Lower level function that does the work. See getAbiAlignment.
-    fn resolveAbiAlignment(base: *Type, comp: *Compilation, llvm_context: *llvm.Context) !u32 {
-        const llvm_type = try base.getLlvmType(comp.gpa(), llvm_context);
-        return @intCast(u32, llvm.ABIAlignmentOfType(comp.target_data_ref, llvm_type));
-    }
-
-    pub const Struct = struct {
-        base: Type,
-        decls: *Scope.Decls,
-
-        pub fn destroy(self: *Struct, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-
-        pub fn getLlvmType(self: *Struct, allocator: *Allocator, llvm_context: *llvm.Context) *llvm.Type {
-            @panic("TODO");
-        }
-    };
-
-    pub const Fn = struct {
-        base: Type,
-        key: Key,
-        non_key: NonKey,
-        garbage_node: std.atomic.Stack(*Fn).Node,
-
-        pub const Kind = enum {
-            Normal,
-            Generic,
-        };
-
-        pub const NonKey = union {
-            Normal: Normal,
-            Generic: void,
-
-            pub const Normal = struct {
-                variable_list: std.ArrayList(*Scope.Var),
-            };
-        };
-
-        pub const Key = struct {
-            data: Data,
-            alignment: ?u32,
-
-            pub const Data = union(Kind) {
-                Generic: Generic,
-                Normal: Normal,
-            };
-
-            pub const Normal = struct {
-                params: []Param,
-                return_type: *Type,
-                is_var_args: bool,
-                cc: CallingConvention,
-            };
-
-            pub const Generic = struct {
-                param_count: usize,
-                cc: CallingConvention,
-            };
-
-            pub fn hash(self: *const Key) u32 {
-                var result: u32 = 0;
-                result +%= hashAny(self.alignment, 0);
-                switch (self.data) {
-                    .Generic => |generic| {
-                        result +%= hashAny(generic.param_count, 1);
-                        result +%= hashAny(generic.cc, 3);
-                    },
-                    .Normal => |normal| {
-                        result +%= hashAny(normal.return_type, 4);
-                        result +%= hashAny(normal.is_var_args, 5);
-                        result +%= hashAny(normal.cc, 6);
-                        for (normal.params) |param| {
-                            result +%= hashAny(param.is_noalias, 7);
-                            result +%= hashAny(param.typ, 8);
-                        }
-                    },
-                }
-                return result;
-            }
-
-            pub fn eql(self: *const Key, other: *const Key) bool {
-                if ((self.alignment == null) != (other.alignment == null)) return false;
-                if (self.alignment) |self_align| {
-                    if (self_align != other.alignment.?) return false;
-                }
-                if (@as(@TagType(Data), self.data) != @as(@TagType(Data), other.data)) return false;
-                switch (self.data) {
-                    .Generic => |*self_generic| {
-                        const other_generic = &other.data.Generic;
-                        if (self_generic.param_count != other_generic.param_count) return false;
-                        if (self_generic.cc != other_generic.cc) return false;
-                    },
-                    .Normal => |*self_normal| {
-                        const other_normal = &other.data.Normal;
-                        if (self_normal.cc != other_normal.cc) return false;
-                        if (self_normal.is_var_args != other_normal.is_var_args) return false;
-                        if (self_normal.return_type != other_normal.return_type) return false;
-                        for (self_normal.params) |*self_param, i| {
-                            const other_param = &other_normal.params[i];
-                            if (self_param.is_noalias != other_param.is_noalias) return false;
-                            if (self_param.typ != other_param.typ) return false;
-                        }
-                    },
-                }
-                return true;
-            }
-
-            pub fn deref(key: Key, comp: *Compilation) void {
-                switch (key.data) {
-                    .Generic => {},
-                    .Normal => |normal| {
-                        normal.return_type.base.deref(comp);
-                        for (normal.params) |param| {
-                            param.typ.base.deref(comp);
-                        }
-                    },
-                }
-            }
-
-            pub fn ref(key: Key) void {
-                switch (key.data) {
-                    .Generic => {},
-                    .Normal => |normal| {
-                        normal.return_type.base.ref();
-                        for (normal.params) |param| {
-                            param.typ.base.ref();
-                        }
-                    },
-                }
-            }
-        };
-
-        const CallingConvention = builtin.CallingConvention;
-
-        pub const Param = struct {
-            is_noalias: bool,
-            typ: *Type,
-        };
-
-        fn ccFnTypeStr(cc: CallingConvention) []const u8 {
-            return switch (cc) {
-                .Unspecified => "",
-                .C => "extern ",
-                .Cold => "coldcc ",
-                .Naked => "nakedcc ",
-                .Stdcall => "stdcallcc ",
-                .Async => "async ",
-                else => unreachable,
-            };
-        }
-
-        pub fn paramCount(self: *Fn) usize {
-            return switch (self.key.data) {
-                .Generic => |generic| generic.param_count,
-                .Normal => |normal| normal.params.len,
-            };
-        }
-
-        /// takes ownership of key.Normal.params on success
-        pub fn get(comp: *Compilation, key: Key) !*Fn {
-            {
-                const held = comp.fn_type_table.acquire();
-                defer held.release();
-
-                if (held.value.get(&key)) |entry| {
-                    entry.value.base.base.ref();
-                    return entry.value;
-                }
-            }
-
-            key.ref();
-            errdefer key.deref(comp);
-
-            const self = try comp.gpa().create(Fn);
-            self.* = Fn{
-                .base = undefined,
-                .key = key,
-                .non_key = undefined,
-                .garbage_node = undefined,
-            };
-            errdefer comp.gpa().destroy(self);
-
-            var name_buf = std.ArrayList(u8).init(comp.gpa());
-            defer name_buf.deinit();
-
-            const name_stream = name_buf.outStream();
-
-            switch (key.data) {
-                .Generic => |generic| {
-                    self.non_key = NonKey{ .Generic = {} };
-                    const cc_str = ccFnTypeStr(generic.cc);
-                    try name_stream.print("{}fn(", .{cc_str});
-                    var param_i: usize = 0;
-                    while (param_i < generic.param_count) : (param_i += 1) {
-                        const arg = if (param_i == 0) "var" else ", var";
-                        try name_stream.write(arg);
-                    }
-                    try name_stream.write(")");
-                    if (key.alignment) |alignment| {
-                        try name_stream.print(" align({})", .{alignment});
-                    }
-                    try name_stream.write(" var");
-                },
-                .Normal => |normal| {
-                    self.non_key = NonKey{
-                        .Normal = NonKey.Normal{ .variable_list = std.ArrayList(*Scope.Var).init(comp.gpa()) },
-                    };
-                    const cc_str = ccFnTypeStr(normal.cc);
-                    try name_stream.print("{}fn(", .{cc_str});
-                    for (normal.params) |param, i| {
-                        if (i != 0) try name_stream.write(", ");
-                        if (param.is_noalias) try name_stream.write("noalias ");
-                        try name_stream.write(param.typ.name);
-                    }
-                    if (normal.is_var_args) {
-                        if (normal.params.len != 0) try name_stream.write(", ");
-                        try name_stream.write("...");
-                    }
-                    try name_stream.write(")");
-                    if (key.alignment) |alignment| {
-                        try name_stream.print(" align({})", .{alignment});
-                    }
-                    try name_stream.print(" {}", .{normal.return_type.name});
-                },
-            }
-
-            self.base.init(comp, .Fn, name_buf.toOwnedSlice());
-
-            {
-                const held = comp.fn_type_table.acquire();
-                defer held.release();
-
-                _ = try held.value.put(&self.key, self);
-            }
-            return self;
-        }
-
-        pub fn destroy(self: *Fn, comp: *Compilation) void {
-            self.key.deref(comp);
-            switch (self.key.data) {
-                .Generic => {},
-                .Normal => {
-                    self.non_key.Normal.variable_list.deinit();
-                },
-            }
-            comp.gpa().destroy(self);
-        }
-
-        pub fn getLlvmType(self: *Fn, allocator: *Allocator, llvm_context: *llvm.Context) !*llvm.Type {
-            const normal = &self.key.data.Normal;
-            const llvm_return_type = switch (normal.return_type.id) {
-                .Void => llvm.VoidTypeInContext(llvm_context) orelse return error.OutOfMemory,
-                else => try normal.return_type.getLlvmType(allocator, llvm_context),
-            };
-            const llvm_param_types = try allocator.alloc(*llvm.Type, normal.params.len);
-            defer allocator.free(llvm_param_types);
-            for (llvm_param_types) |*llvm_param_type, i| {
-                llvm_param_type.* = try normal.params[i].typ.getLlvmType(allocator, llvm_context);
-            }
-
-            return llvm.FunctionType(
-                llvm_return_type,
-                llvm_param_types.ptr,
-                @intCast(c_uint, llvm_param_types.len),
-                @boolToInt(normal.is_var_args),
-            ) orelse error.OutOfMemory;
-        }
-    };
-
-    pub const MetaType = struct {
-        base: Type,
-        value: *Type,
-
-        /// Adds 1 reference to the resulting type
-        pub fn get(comp: *Compilation) *MetaType {
-            comp.meta_type.base.base.ref();
-            return comp.meta_type;
-        }
-
-        pub fn destroy(self: *MetaType, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-    };
-
-    pub const Void = struct {
-        base: Type,
-
-        /// Adds 1 reference to the resulting type
-        pub fn get(comp: *Compilation) *Void {
-            comp.void_type.base.base.ref();
-            return comp.void_type;
-        }
-
-        pub fn destroy(self: *Void, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-    };
-
-    pub const Bool = struct {
-        base: Type,
-
-        /// Adds 1 reference to the resulting type
-        pub fn get(comp: *Compilation) *Bool {
-            comp.bool_type.base.base.ref();
-            return comp.bool_type;
-        }
-
-        pub fn destroy(self: *Bool, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-
-        pub fn getLlvmType(self: *Bool, allocator: *Allocator, llvm_context: *llvm.Context) *llvm.Type {
-            @panic("TODO");
-        }
-    };
-
-    pub const NoReturn = struct {
-        base: Type,
-
-        /// Adds 1 reference to the resulting type
-        pub fn get(comp: *Compilation) *NoReturn {
-            comp.noreturn_type.base.base.ref();
-            return comp.noreturn_type;
-        }
-
-        pub fn destroy(self: *NoReturn, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-    };
-
-    pub const Int = struct {
-        base: Type,
-        key: Key,
-        garbage_node: std.atomic.Stack(*Int).Node,
-
-        pub const Key = struct {
-            bit_count: u32,
-            is_signed: bool,
-
-            pub fn hash(self: *const Key) u32 {
-                var result: u32 = 0;
-                result +%= hashAny(self.is_signed, 0);
-                result +%= hashAny(self.bit_count, 1);
-                return result;
-            }
-
-            pub fn eql(self: *const Key, other: *const Key) bool {
-                return self.bit_count == other.bit_count and self.is_signed == other.is_signed;
-            }
-        };
-
-        pub fn get_u8(comp: *Compilation) *Int {
-            comp.u8_type.base.base.ref();
-            return comp.u8_type;
-        }
-
-        pub fn get(comp: *Compilation, key: Key) !*Int {
-            {
-                const held = comp.int_type_table.acquire();
-                defer held.release();
-
-                if (held.value.get(&key)) |entry| {
-                    entry.value.base.base.ref();
-                    return entry.value;
-                }
-            }
-
-            const self = try comp.gpa().create(Int);
-            self.* = Int{
-                .base = undefined,
-                .key = key,
-                .garbage_node = undefined,
-            };
-            errdefer comp.gpa().destroy(self);
-
-            const u_or_i = "ui"[@boolToInt(key.is_signed)];
-            const name = try std.fmt.allocPrint(comp.gpa(), "{c}{}", .{ u_or_i, key.bit_count });
-            errdefer comp.gpa().free(name);
-
-            self.base.init(comp, .Int, name);
-
-            {
-                const held = comp.int_type_table.acquire();
-                defer held.release();
-
-                _ = try held.value.put(&self.key, self);
-            }
-            return self;
-        }
-
-        pub fn destroy(self: *Int, comp: *Compilation) void {
-            self.garbage_node = std.atomic.Stack(*Int).Node{
-                .data = self,
-                .next = undefined,
-            };
-            comp.registerGarbage(Int, &self.garbage_node);
-        }
-
-        pub fn gcDestroy(self: *Int, comp: *Compilation) void {
-            {
-                const held = comp.int_type_table.acquire();
-                defer held.release();
-
-                _ = held.value.remove(&self.key).?;
-            }
-            // we allocated the name
-            comp.gpa().free(self.base.name);
-            comp.gpa().destroy(self);
-        }
-
-        pub fn getLlvmType(self: *Int, allocator: *Allocator, llvm_context: *llvm.Context) !*llvm.Type {
-            return llvm.IntTypeInContext(llvm_context, self.key.bit_count) orelse return error.OutOfMemory;
-        }
-    };
-
-    pub const Float = struct {
-        base: Type,
-
-        pub fn destroy(self: *Float, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-
-        pub fn getLlvmType(self: *Float, allocator: *Allocator, llvm_context: *llvm.Context) *llvm.Type {
-            @panic("TODO");
-        }
-    };
-    pub const Pointer = struct {
-        base: Type,
-        key: Key,
-        garbage_node: std.atomic.Stack(*Pointer).Node,
-
-        pub const Key = struct {
-            child_type: *Type,
-            mut: Mut,
-            vol: Vol,
-            size: Size,
-            alignment: Align,
-
-            pub fn hash(self: *const Key) u32 {
-                var result: u32 = 0;
-                result +%= switch (self.alignment) {
-                    .Abi => 0xf201c090,
-                    .Override => |x| hashAny(x, 0),
-                };
-                result +%= hashAny(self.child_type, 1);
-                result +%= hashAny(self.mut, 2);
-                result +%= hashAny(self.vol, 3);
-                result +%= hashAny(self.size, 4);
-                return result;
-            }
-
-            pub fn eql(self: *const Key, other: *const Key) bool {
-                if (self.child_type != other.child_type or
-                    self.mut != other.mut or
-                    self.vol != other.vol or
-                    self.size != other.size or
-                    @as(@TagType(Align), self.alignment) != @as(@TagType(Align), other.alignment))
-                {
-                    return false;
-                }
-                switch (self.alignment) {
-                    .Abi => return true,
-                    .Override => |x| return x == other.alignment.Override,
-                }
-            }
+    /// This enum does not directly correspond to `std.builtin.TypeId` because
+    /// it has extra enum tags in it, as a way of using less memory. For example,
+    /// even though Zig recognizes `*align(10) i32` and `*i32` both as Pointer types
+    /// but with different alignment values, in this data structure they are represented
+    /// with different enum tags, because the the former requires more payload data than the latter.
+    /// See `zigTypeTag` for the function that corresponds to `std.builtin.TypeId`.
+    pub const Tag = enum {
+        // The first section of this enum are tags that require no payload.
+        int_u8,
+        int_usize,
+        // Bump this when adding items above.
+        pub const last_no_payload_tag = Tag.int_usize;
+        pub const no_payload_count = @enumToInt(last_no_payload_tag) + 1;
+        // After this, the tag requires a payload.
+
+        array_u8_sentinel_0,
+        array,
+        single_const_pointer,
+    };
+
+    pub const Payload = struct {
+        tag: Tag,
+
+        pub const Array_u8_Sentinel0 = struct {
+            base: Payload = Payload{ .tag = .array_u8_sentinel_0 },
+
+            len: u64,
         };
 
-        pub const Mut = enum {
-            Mut,
-            Const,
-        };
-
-        pub const Vol = enum {
-            Non,
-            Volatile,
-        };
+        pub const Array = struct {
+            base: Payload = Payload{ .tag = .array },
 
-        pub const Align = union(enum) {
-            Abi,
-            Override: u32,
+            elem_type: Type,
+            len: u64,
         };
 
-        pub const Size = builtin.TypeInfo.Pointer.Size;
-
-        pub fn destroy(self: *Pointer, comp: *Compilation) void {
-            self.garbage_node = std.atomic.Stack(*Pointer).Node{
-                .data = self,
-                .next = undefined,
-            };
-            comp.registerGarbage(Pointer, &self.garbage_node);
-        }
-
-        pub fn gcDestroy(self: *Pointer, comp: *Compilation) void {
-            {
-                const held = comp.ptr_type_table.acquire();
-                defer held.release();
-
-                _ = held.value.remove(&self.key).?;
-            }
-            self.key.child_type.base.deref(comp);
-            comp.gpa().destroy(self);
-        }
-
-        pub fn getAlignAsInt(self: *Pointer, comp: *Compilation) u32 {
-            switch (self.key.alignment) {
-                .Abi => return self.key.child_type.getAbiAlignment(comp),
-                .Override => |alignment| return alignment,
-            }
-        }
-
-        pub fn get(
-            comp: *Compilation,
-            key: Key,
-        ) !*Pointer {
-            var normal_key = key;
-            switch (key.alignment) {
-                .Abi => {},
-                .Override => |alignment| {
-                    // TODO https://github.com/ziglang/zig/issues/3190
-                    var align_spill = alignment;
-                    const abi_align = try key.child_type.getAbiAlignment(comp);
-                    if (abi_align == align_spill) {
-                        normal_key.alignment = .Abi;
-                    }
-                },
-            }
-            {
-                const held = comp.ptr_type_table.acquire();
-                defer held.release();
-
-                if (held.value.get(&normal_key)) |entry| {
-                    entry.value.base.base.ref();
-                    return entry.value;
-                }
-            }
+        pub const SingleConstPointer = struct {
+            base: Payload = Payload{ .tag = .single_const_pointer },
 
-            const self = try comp.gpa().create(Pointer);
-            self.* = Pointer{
-                .base = undefined,
-                .key = normal_key,
-                .garbage_node = undefined,
-            };
-            errdefer comp.gpa().destroy(self);
-
-            const size_str = switch (self.key.size) {
-                .One => "*",
-                .Many => "[*]",
-                .Slice => "[]",
-                .C => "[*c]",
-            };
-            const mut_str = switch (self.key.mut) {
-                .Const => "const ",
-                .Mut => "",
-            };
-            const vol_str = switch (self.key.vol) {
-                .Volatile => "volatile ",
-                .Non => "",
-            };
-            const name = switch (self.key.alignment) {
-                .Abi => try std.fmt.allocPrint(comp.gpa(), "{}{}{}{}", .{
-                    size_str,
-                    mut_str,
-                    vol_str,
-                    self.key.child_type.name,
-                }),
-                .Override => |alignment| try std.fmt.allocPrint(comp.gpa(), "{}align<{}> {}{}{}", .{
-                    size_str,
-                    alignment,
-                    mut_str,
-                    vol_str,
-                    self.key.child_type.name,
-                }),
-            };
-            errdefer comp.gpa().free(name);
-
-            self.base.init(comp, .Pointer, name);
-
-            {
-                const held = comp.ptr_type_table.acquire();
-                defer held.release();
-
-                _ = try held.value.put(&self.key, self);
-            }
-            return self;
-        }
-
-        pub fn getLlvmType(self: *Pointer, allocator: *Allocator, llvm_context: *llvm.Context) !*llvm.Type {
-            const elem_llvm_type = try self.key.child_type.getLlvmType(allocator, llvm_context);
-            return llvm.PointerType(elem_llvm_type, 0) orelse return error.OutOfMemory;
-        }
-    };
-
-    pub const Array = struct {
-        base: Type,
-        key: Key,
-        garbage_node: std.atomic.Stack(*Array).Node,
-
-        pub const Key = struct {
-            elem_type: *Type,
-            len: usize,
-
-            pub fn hash(self: *const Key) u32 {
-                var result: u32 = 0;
-                result +%= hashAny(self.elem_type, 0);
-                result +%= hashAny(self.len, 1);
-                return result;
-            }
-
-            pub fn eql(self: *const Key, other: *const Key) bool {
-                return self.elem_type == other.elem_type and self.len == other.len;
-            }
+            pointee_type: Type,
         };
-
-        pub fn destroy(self: *Array, comp: *Compilation) void {
-            self.key.elem_type.base.deref(comp);
-            comp.gpa().destroy(self);
-        }
-
-        pub fn get(comp: *Compilation, key: Key) !*Array {
-            key.elem_type.base.ref();
-            errdefer key.elem_type.base.deref(comp);
-
-            {
-                const held = comp.array_type_table.acquire();
-                defer held.release();
-
-                if (held.value.get(&key)) |entry| {
-                    entry.value.base.base.ref();
-                    return entry.value;
-                }
-            }
-
-            const self = try comp.gpa().create(Array);
-            self.* = Array{
-                .base = undefined,
-                .key = key,
-                .garbage_node = undefined,
-            };
-            errdefer comp.gpa().destroy(self);
-
-            const name = try std.fmt.allocPrint(comp.gpa(), "[{}]{}", .{ key.len, key.elem_type.name });
-            errdefer comp.gpa().free(name);
-
-            self.base.init(comp, .Array, name);
-
-            {
-                const held = comp.array_type_table.acquire();
-                defer held.release();
-
-                _ = try held.value.put(&self.key, self);
-            }
-            return self;
-        }
-
-        pub fn getLlvmType(self: *Array, allocator: *Allocator, llvm_context: *llvm.Context) !*llvm.Type {
-            const elem_llvm_type = try self.key.elem_type.getLlvmType(allocator, llvm_context);
-            return llvm.ArrayType(elem_llvm_type, @intCast(c_uint, self.key.len)) orelse return error.OutOfMemory;
-        }
-    };
-
-    pub const Vector = struct {
-        base: Type,
-
-        pub fn destroy(self: *Vector, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-
-        pub fn getLlvmType(self: *Vector, allocator: *Allocator, llvm_context: *llvm.Context) *llvm.Type {
-            @panic("TODO");
-        }
-    };
-
-    pub const ComptimeFloat = struct {
-        base: Type,
-
-        pub fn destroy(self: *ComptimeFloat, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-    };
-
-    pub const ComptimeInt = struct {
-        base: Type,
-
-        /// Adds 1 reference to the resulting type
-        pub fn get(comp: *Compilation) *ComptimeInt {
-            comp.comptime_int_type.base.base.ref();
-            return comp.comptime_int_type;
-        }
-
-        pub fn destroy(self: *ComptimeInt, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-    };
-
-    pub const EnumLiteral = struct {
-        base: Type,
-
-        /// Adds 1 reference to the resulting type
-        pub fn get(comp: *Compilation) *EnumLiteral {
-            comp.comptime_int_type.base.base.ref();
-            return comp.comptime_int_type;
-        }
-
-        pub fn destroy(self: *EnumLiteral, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-    };
-
-    pub const Undefined = struct {
-        base: Type,
-
-        pub fn destroy(self: *Undefined, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-    };
-
-    pub const Null = struct {
-        base: Type,
-
-        pub fn destroy(self: *Null, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-    };
-
-    pub const Optional = struct {
-        base: Type,
-
-        pub fn destroy(self: *Optional, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-
-        pub fn getLlvmType(self: *Optional, allocator: *Allocator, llvm_context: *llvm.Context) *llvm.Type {
-            @panic("TODO");
-        }
-    };
-
-    pub const ErrorUnion = struct {
-        base: Type,
-
-        pub fn destroy(self: *ErrorUnion, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-
-        pub fn getLlvmType(self: *ErrorUnion, allocator: *Allocator, llvm_context: *llvm.Context) *llvm.Type {
-            @panic("TODO");
-        }
-    };
-
-    pub const ErrorSet = struct {
-        base: Type,
-
-        pub fn destroy(self: *ErrorSet, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-
-        pub fn getLlvmType(self: *ErrorSet, allocator: *Allocator, llvm_context: *llvm.Context) *llvm.Type {
-            @panic("TODO");
-        }
-    };
-
-    pub const Enum = struct {
-        base: Type,
-
-        pub fn destroy(self: *Enum, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-
-        pub fn getLlvmType(self: *Enum, allocator: *Allocator, llvm_context: *llvm.Context) *llvm.Type {
-            @panic("TODO");
-        }
-    };
-
-    pub const Union = struct {
-        base: Type,
-
-        pub fn destroy(self: *Union, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-
-        pub fn getLlvmType(self: *Union, allocator: *Allocator, llvm_context: *llvm.Context) *llvm.Type {
-            @panic("TODO");
-        }
-    };
-
-    pub const BoundFn = struct {
-        base: Type,
-
-        pub fn destroy(self: *BoundFn, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-
-        pub fn getLlvmType(self: *BoundFn, allocator: *Allocator, llvm_context: *llvm.Context) *llvm.Type {
-            @panic("TODO");
-        }
-    };
-
-    pub const Opaque = struct {
-        base: Type,
-
-        pub fn destroy(self: *Opaque, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-
-        pub fn getLlvmType(self: *Opaque, allocator: *Allocator, llvm_context: *llvm.Context) *llvm.Type {
-            @panic("TODO");
-        }
-    };
-
-    pub const Frame = struct {
-        base: Type,
-
-        pub fn destroy(self: *Frame, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-
-        pub fn getLlvmType(self: *Frame, allocator: *Allocator, llvm_context: *llvm.Context) *llvm.Type {
-            @panic("TODO");
-        }
-    };
-
-    pub const AnyFrame = struct {
-        base: Type,
-
-        pub fn destroy(self: *AnyFrame, comp: *Compilation) void {
-            comp.gpa().destroy(self);
-        }
-
-        pub fn getLlvmType(self: *AnyFrame, allocator: *Allocator, llvm_context: *llvm.Context) *llvm.Type {
-            @panic("TODO");
-        }
     };
 };
-
-fn hashAny(x: var, comptime seed: u64) u32 {
-    switch (@typeInfo(@TypeOf(x))) {
-        .Int => |info| {
-            comptime var rng = comptime std.rand.DefaultPrng.init(seed);
-            const unsigned_x = @bitCast(std.meta.IntType(false, info.bits), x);
-            if (info.bits <= 32) {
-                return @as(u32, unsigned_x) *% comptime rng.random.scalar(u32);
-            } else {
-                return @truncate(u32, unsigned_x *% comptime rng.random.scalar(@TypeOf(unsigned_x)));
-            }
-        },
-        .Pointer => |info| {
-            switch (info.size) {
-                .One => return hashAny(@ptrToInt(x), seed),
-                .Many => @compileError("implement hash function"),
-                .Slice => @compileError("implement hash function"),
-                .C => unreachable,
-            }
-        },
-        .Enum => return hashAny(@enumToInt(x), seed),
-        .Bool => {
-            comptime var rng = comptime std.rand.DefaultPrng.init(seed);
-            const vals = comptime [2]u32{ rng.random.scalar(u32), rng.random.scalar(u32) };
-            return vals[@boolToInt(x)];
-        },
-        .Optional => {
-            if (x) |non_opt| {
-                return hashAny(non_opt, seed);
-            } else {
-                return hashAny(@as(u32, 1), seed);
-            }
-        },
-        else => @compileError("implement hash function for " ++ @typeName(@TypeOf(x))),
-    }
-}
src-self-hosted/value.zig
@@ -1,26 +1,35 @@
 const std = @import("std");
+const Type = @import("type.zig").Type;
+const log2 = std.math.log2;
+const assert = std.debug.assert;
 
 /// This is the raw data, with no bookkeeping, no memory awareness,
 /// no de-duplication, and no type system awareness.
 /// It's important for this struct to be small.
-/// It is not copyable since it may contain references to its inner data.
-pub const Value = struct {
-    tag: Tag,
+/// This union takes advantage of the fact that the first page of memory
+/// is unmapped, giving us 4096 possible enum tags that have no payload.
+pub const Value = extern union {
+    /// If the tag value is less than Tag.no_payload_count, then no pointer
+    /// dereference is needed.
+    tag_if_small_enough: usize,
+    ptr_otherwise: *Payload,
 
     pub const Tag = enum {
+        // The first section of this enum are tags that require no payload.
         void_type,
         noreturn_type,
         bool_type,
         usize_type,
-
         void_value,
         noreturn_value,
         bool_true,
         bool_false,
+        // Bump this when adding items above.
+        pub const last_no_payload_tag = Tag.bool_false;
+        pub const no_payload_count = @enumToInt(last_no_payload_tag) + 1;
+        // After this, the tag requires a payload.
 
-        array_sentinel_0_u8_type,
-        single_const_ptr_type,
-
+        ty,
         int_u64,
         int_i64,
         function,
@@ -28,44 +37,76 @@ pub const Value = struct {
         bytes,
     };
 
-    pub const Int_u64 = struct {
-        base: Value = Value{ .tag = .int_u64 },
-        int: u64,
-    };
+    pub fn initTag(comptime tag: Tag) Value {
+        comptime assert(@enumToInt(tag) < Tag.no_payload_count);
+        return .{ .tag_if_small_enough = @enumToInt(tag) };
+    }
 
-    pub const Int_i64 = struct {
-        base: Value = Value{ .tag = .int_i64 },
-        int: i64,
-    };
+    pub fn initPayload(payload: *Payload) Value {
+        assert(@enumToInt(payload.tag) >= Tag.no_payload_count);
+        return .{ .ptr_otherwise = payload };
+    }
 
-    pub const Function = struct {
-        base: Value = Value{ .tag = .function },
-    };
+    pub fn tag(self: Value) Tag {
+        if (self.tag_if_small_enough < Tag.no_payload_count) {
+            return @intToEnum(self.tag_if_small_enough);
+        } else {
+            return self.ptr_otherwise.tag;
+        }
+    }
 
-    pub const ArraySentinel0_u8_Type = struct {
-        base: Value = Value{ .tag = .array_sentinel_0_u8_type },
-        len: u64,
-    };
+    /// This type is not copyable since it may contain pointers to its inner data.
+    pub const Payload = struct {
+        tag: Tag,
 
-    pub const SingleConstPtrType = struct {
-        base: Value = Value{ .tag = .single_const_ptr_type },
-        elem_type: *Value,
-    };
+        pub const Int_u64 = struct {
+            base: Payload = Payload{ .tag = .int_u64 },
+            int: u64,
+        };
 
-    pub const Ref = struct {
-        base: Value = Value{ .tag = .ref },
-        pointee: *MemoryCell,
-    };
+        pub const Int_i64 = struct {
+            base: Payload = Payload{ .tag = .int_i64 },
+            int: i64,
+        };
+
+        pub const Function = struct {
+            base: Payload = Payload{ .tag = .function },
+        };
+
+        pub const ArraySentinel0_u8_Type = struct {
+            base: Payload = Payload{ .tag = .array_sentinel_0_u8_type },
+            len: u64,
+        };
+
+        pub const SingleConstPtrType = struct {
+            base: Payload = Payload{ .tag = .single_const_ptr_type },
+            elem_type: *Type,
+        };
+
+        pub const Ref = struct {
+            base: Payload = Payload{ .tag = .ref },
+            pointee: *MemoryCell,
+        };
+
+        pub const Bytes = struct {
+            base: Payload = Payload{ .tag = .bytes },
+            data: []u8,
+        };
 
-    pub const Bytes = struct {
-        base: Value = Value{ .tag = .bytes },
-        data: []u8,
+        pub const Ty = struct {
+            base: Payload = Payload{ .tag = .fully_qualified_type },
+            ptr: *Type,
+        };
     };
 };
 
+/// This is the heart of resource management of the Zig compiler. The Zig compiler uses
+/// stop-the-world mark-and-sweep garbage collection during compilation to manage the resources
+/// associated with evaluating compile-time code and semantic analysis. Each `MemoryCell` represents
+/// a root.
 pub const MemoryCell = struct {
     parent: Parent,
-    contents: *Value,
+    contents: Value,
 
     pub const Parent = union(enum) {
         none,