Merged with master

13 files changed, 341 insertions, 84 deletions

diff --git a/src/build_settings.cpp b/src/build_settings.cpp
index 5a1ec0f30..c264fbaaa 100644
--- a/src/build_settings.cpp
+++ b/src/build_settings.cpp
@@ -126,6 +126,7 @@ struct BuildContext {
 	bool   use_lld;
 	bool   vet;
 	bool   cross_compiling;
+	bool   use_subsystem_windows;
 
 	QueryDataSetSettings query_data_set_settings;
 
diff --git a/src/check_decl.cpp b/src/check_decl.cpp
index 0e669e473..ece38e84f 100644
--- a/src/check_decl.cpp
+++ b/src/check_decl.cpp
@@ -1207,7 +1207,7 @@ void check_proc_body(CheckerContext *ctx_, Token token, DeclInfo *decl, Type *ty
 	}
 
 
-	bool where_clause_ok = evaluate_where_clauses(ctx, decl->scope, &decl->proc_lit->ProcLit.where_clauses, true);
+	bool where_clause_ok = evaluate_where_clauses(ctx, nullptr, decl->scope, &decl->proc_lit->ProcLit.where_clauses, true);
 	if (!where_clause_ok) {
 		// NOTE(bill, 2019-08-31): Don't check the body as the where clauses failed
 		return;
diff --git a/src/check_expr.cpp b/src/check_expr.cpp
index b854a693b..1a01eef31 100644
--- a/src/check_expr.cpp
+++ b/src/check_expr.cpp
@@ -2260,6 +2260,8 @@ bool check_cast_internal(CheckerContext *c, Operand *x, Type *type) {
 			x->mode = Addressing_Value;
 		} else if (is_type_slice(type) && is_type_string(x->type)) {
 			x->mode = Addressing_Value;
+		} else if (is_type_union(type)) {
+			x->mode = Addressing_Value;
 		}
 		return true;
 	}
@@ -5278,6 +5280,10 @@ bool check_builtin_procedure(CheckerContext *c, Operand *operand, Ast *call, i32
 		break;
 	}
 
+	case BuiltinProc_cpu_relax:
+		operand->mode = Addressing_NoValue;
+		break;
+
 	case BuiltinProc_atomic_fence:
 	case BuiltinProc_atomic_fence_acq:
 	case BuiltinProc_atomic_fence_rel:
@@ -5942,7 +5948,9 @@ CALL_ARGUMENT_CHECKER(check_call_arguments_internal) {
 				Entity *e = sig_params[operand_index];
 				Type *t = e->type;
 				Operand o = operands[operand_index];
-				call->viral_state_flags |= o.expr->viral_state_flags;
+				if (o.expr != nullptr) {
+					call->viral_state_flags |= o.expr->viral_state_flags;
+				}
 
 				if (e->kind == Entity_TypeName) {
 					// GB_ASSERT(!variadic);
@@ -5983,6 +5991,15 @@ CALL_ARGUMENT_CHECKER(check_call_arguments_internal) {
 				}
 				score += s;
 
+				if (e->flags & EntityFlag_ConstInput) {
+					if (o.mode != Addressing_Constant) {
+						if (show_error) {
+							error(o.expr, "Expected a constant value for the argument '%.*s'", LIT(e->token.string));
+						}
+						err = CallArgumentError_NoneConstantParameter;
+					}
+				}
+
 				if (o.mode == Addressing_Type && is_type_typeid(e->type)) {
 					add_type_info_type(c, o.type);
 					add_type_and_value(c->info, o.expr, Addressing_Value, e->type, exact_value_typeid(o.type));
@@ -6238,6 +6255,15 @@ CALL_ARGUMENT_CHECKER(check_named_call_arguments) {
 					}
 					err = CallArgumentError_WrongTypes;
 				}
+
+				if (e->flags & EntityFlag_ConstInput) {
+					if (o->mode != Addressing_Constant) {
+						if (show_error) {
+							error(o->expr, "Expected a constant value for the argument '%.*s'", LIT(e->token.string));
+						}
+						err = CallArgumentError_NoneConstantParameter;
+					}
+				}
 			}
 			score += s;
 		}
@@ -6296,7 +6322,7 @@ Entity **populate_proc_parameter_list(CheckerContext *c, Type *proc_type, isize
 }
 
 
-bool evaluate_where_clauses(CheckerContext *ctx, Scope *scope, Array<Ast *> *clauses, bool print_err) {
+bool evaluate_where_clauses(CheckerContext *ctx, Ast *call_expr, Scope *scope, Array<Ast *> *clauses, bool print_err) {
 	if (clauses != nullptr) {
 		for_array(i, *clauses) {
 			Ast *clause = (*clauses)[i];
@@ -6304,9 +6330,11 @@ bool evaluate_where_clauses(CheckerContext *ctx, Scope *scope, Array<Ast *> *cla
 			check_expr(ctx, &o, clause);
 			if (o.mode != Addressing_Constant) {
 				if (print_err) error(clause, "'where' clauses expect a constant boolean evaluation");
+				if (print_err && call_expr) error(call_expr, "at caller location");
 				return false;
 			} else if (o.value.kind != ExactValue_Bool) {
 				if (print_err) error(clause, "'where' clauses expect a constant boolean evaluation");
+				if (print_err && call_expr) error(call_expr, "at caller location");
 				return false;
 			} else if (!o.value.value_bool) {
 				if (print_err) {
@@ -6348,6 +6376,7 @@ bool evaluate_where_clauses(CheckerContext *ctx, Scope *scope, Array<Ast *> *cla
 						}
 					}
 
+					if (call_expr) error(call_expr, "at caller location");
 				}
 				return false;
 			}
@@ -6613,7 +6642,7 @@ CallArgumentData check_call_arguments(CheckerContext *c, Operand *operand, Type
 					ctx.curr_proc_sig  = e->type;
 
 					GB_ASSERT(decl->proc_lit->kind == Ast_ProcLit);
-					if (!evaluate_where_clauses(&ctx, decl->scope, &decl->proc_lit->ProcLit.where_clauses, false)) {
+					if (!evaluate_where_clauses(&ctx, operand->expr, decl->scope, &decl->proc_lit->ProcLit.where_clauses, false)) {
 						continue;
 					}
 				}
@@ -8572,8 +8601,6 @@ ExprKind check_expr_base_internal(CheckerContext *c, Operand *o, Ast *node, Type
 			o->expr = node;
 			return kind;
 		}
-		Type *t = check_type(c, ta->type);
-
 		if (o->mode == Addressing_Constant) {
 			gbString expr_str = expr_to_string(o->expr);
 			error(o->expr, "A type assertion cannot be applied to a constant expression: '%s'", expr_str);
@@ -8594,54 +8621,80 @@ ExprKind check_expr_base_internal(CheckerContext *c, Operand *o, Ast *node, Type
 
 		bool src_is_ptr = is_type_pointer(o->type);
 		Type *src = type_deref(o->type);
-		Type *dst = t;
 		Type *bsrc = base_type(src);
-		Type *bdst = base_type(dst);
 
 
-		if (is_type_union(src)) {
-			bool ok = false;
-			for_array(i, bsrc->Union.variants) {
-				Type *vt = bsrc->Union.variants[i];
-				if (are_types_identical(vt, dst)) {
-					ok = true;
-					break;
-				}
+		if (ta->type != nullptr && ta->type->kind == Ast_UnaryExpr && ta->type->UnaryExpr.op.kind == Token_Question) {
+			if (!is_type_union(src)) {
+				gbString str = type_to_string(o->type);
+				error(o->expr, "Type assertions with .? can only operate on unions with 1 variant, got %s", str);
+				gb_string_free(str);
+				o->mode = Addressing_Invalid;
+				o->expr = node;
+				return kind;
 			}
-
-			if (!ok) {
-				gbString expr_str = expr_to_string(o->expr);
-				gbString dst_type_str = type_to_string(t);
-				defer (gb_string_free(expr_str));
-				defer (gb_string_free(dst_type_str));
-				if (bsrc->Union.variants.count == 0) {
-					error(o->expr, "Cannot type assert '%s' to '%s' as this is an empty union", expr_str, dst_type_str);
-				} else {
-					error(o->expr, "Cannot type assert '%s' to '%s' as it is not a variant of that union", expr_str, dst_type_str);
-				}
+			if (bsrc->Union.variants.count != 1) {
+				error(o->expr, "Type assertions with .? can only operate on unions with 1 variant, got %lld", cast(long long)bsrc->Union.variants.count);
 				o->mode = Addressing_Invalid;
 				o->expr = node;
 				return kind;
 			}
 
 			add_type_info_type(c, o->type);
-			add_type_info_type(c, t);
+			add_type_info_type(c, bsrc->Union.variants[0]);
 
-			o->type = t;
+			o->type = bsrc->Union.variants[0];
 			o->mode = Addressing_OptionalOk;
-		} else if (is_type_any(src)) {
-			o->type = t;
-			o->mode = Addressing_OptionalOk;
-
-			add_type_info_type(c, o->type);
-			add_type_info_type(c, t);
 		} else {
-			gbString str = type_to_string(o->type);
-			error(o->expr, "Type assertions can only operate on unions and 'any', got %s", str);
-			gb_string_free(str);
-			o->mode = Addressing_Invalid;
-			o->expr = node;
-			return kind;
+			Type *t = check_type(c, ta->type);
+			Type *dst = t;
+			Type *bdst = base_type(dst);
+
+
+			if (is_type_union(src)) {
+				bool ok = false;
+				for_array(i, bsrc->Union.variants) {
+					Type *vt = bsrc->Union.variants[i];
+					if (are_types_identical(vt, dst)) {
+						ok = true;
+						break;
+					}
+				}
+
+				if (!ok) {
+					gbString expr_str = expr_to_string(o->expr);
+					gbString dst_type_str = type_to_string(t);
+					defer (gb_string_free(expr_str));
+					defer (gb_string_free(dst_type_str));
+					if (bsrc->Union.variants.count == 0) {
+						error(o->expr, "Cannot type assert '%s' to '%s' as this is an empty union", expr_str, dst_type_str);
+					} else {
+						error(o->expr, "Cannot type assert '%s' to '%s' as it is not a variant of that union", expr_str, dst_type_str);
+					}
+					o->mode = Addressing_Invalid;
+					o->expr = node;
+					return kind;
+				}
+
+				add_type_info_type(c, o->type);
+				add_type_info_type(c, t);
+
+				o->type = t;
+				o->mode = Addressing_OptionalOk;
+			} else if (is_type_any(src)) {
+				o->type = t;
+				o->mode = Addressing_OptionalOk;
+
+				add_type_info_type(c, o->type);
+				add_type_info_type(c, t);
+			} else {
+				gbString str = type_to_string(o->type);
+				error(o->expr, "Type assertions can only operate on unions and 'any', got %s", str);
+				gb_string_free(str);
+				o->mode = Addressing_Invalid;
+				o->expr = node;
+				return kind;
+			}
 		}
 
 		add_package_dependency(c, "runtime", "type_assertion_check");
diff --git a/src/check_stmt.cpp b/src/check_stmt.cpp
index c365dca84..4b250c6a6 100644
--- a/src/check_stmt.cpp
+++ b/src/check_stmt.cpp
@@ -314,7 +314,11 @@ Type *check_assignment_variable(CheckerContext *ctx, Operand *lhs, Operand *rhs)
 
 		gbString str = expr_to_string(lhs->expr);
 		if (e != nullptr && e->flags & EntityFlag_Param) {
-			error(lhs->expr, "Cannot assign to '%s' which is a procedure parameter", str);
+			if (e->flags & EntityFlag_Using) {
+				error(lhs->expr, "Cannot assign to '%s' which is from a 'using' procedure parameter", str);
+			} else {
+				error(lhs->expr, "Cannot assign to '%s' which is a procedure parameter", str);
+			}
 		} else {
 			error(lhs->expr, "Cannot assign to '%s'", str);
 		}
@@ -497,6 +501,8 @@ bool check_using_stmt_entity(CheckerContext *ctx, AstUsingStmt *us, Ast *expr, b
 				Entity *f = found->elements.entries[i].value;
 				if (f->kind == Entity_Variable) {
 					Entity *uvar = alloc_entity_using_variable(e, f->token, f->type, expr);
+					if (e->flags & EntityFlag_Value) uvar->flags |= EntityFlag_Value;
+					if (e->flags & EntityFlag_Param) uvar->flags |= EntityFlag_Param;
 					Entity *prev = scope_insert(ctx->scope, uvar);
 					if (prev != nullptr) {
 						gbString expr_str = expr_to_string(expr);
@@ -1102,6 +1108,12 @@ void check_type_switch_stmt(CheckerContext *ctx, Ast *node, u32 mod_flags) {
 			if (type_expr != nullptr) { // Otherwise it's a default expression
 				Operand y = {};
 				check_expr_or_type(ctx, &y, type_expr);
+				if (y.mode != Addressing_Type) {
+					gbString str = expr_to_string(type_expr);
+					error(type_expr, "Expected a type as a case, got %s", str);
+					gb_string_free(str);
+					continue;
+				}
 
 				if (switch_kind == TypeSwitch_Union) {
 					GB_ASSERT(is_type_union(bt));
diff --git a/src/check_type.cpp b/src/check_type.cpp
index f21c1563b..6194951c9 100644
--- a/src/check_type.cpp
+++ b/src/check_type.cpp
@@ -527,7 +527,7 @@ void check_struct_type(CheckerContext *ctx, Type *struct_type, Ast *node, Array<
 		if (st->where_clauses.count > 0 && st->polymorphic_params == nullptr) {
 			error(st->where_clauses[0], "'where' clauses can only be used on structures with polymorphic parameters");
 		} else {
-			bool where_clause_ok = evaluate_where_clauses(ctx, ctx->scope, &st->where_clauses, true);
+			bool where_clause_ok = evaluate_where_clauses(ctx, node, ctx->scope, &st->where_clauses, true);
 		}
 		check_struct_fields(ctx, node, &struct_type->Struct.fields, &struct_type->Struct.tags, st->fields, min_field_count, struct_type, context);
 	}
@@ -714,7 +714,7 @@ void check_union_type(CheckerContext *ctx, Type *union_type, Ast *node, Array<Op
 	if (ut->where_clauses.count > 0 && ut->polymorphic_params == nullptr) {
 		error(ut->where_clauses[0], "'where' clauses can only be used on unions with polymorphic parameters");
 	} else {
-		bool where_clause_ok = evaluate_where_clauses(ctx, ctx->scope, &ut->where_clauses, true);
+		bool where_clause_ok = evaluate_where_clauses(ctx, node, ctx->scope, &ut->where_clauses, true);
 	}
 
 
@@ -748,11 +748,17 @@ void check_union_type(CheckerContext *ctx, Type *union_type, Ast *node, Array<Op
 
 	union_type->Union.variants = variants;
 	union_type->Union.no_nil = ut->no_nil;
+	union_type->Union.maybe = ut->maybe;
 	if (union_type->Union.no_nil) {
 		if (variants.count < 2) {
 			error(ut->align, "A union with #no_nil must have at least 2 variants");
 		}
 	}
+	if (union_type->Union.maybe) {
+		if (variants.count != 1) {
+			error(ut->align, "A union with #maybe must have at 1 variant, got %lld", cast(long long)variants.count);
+		}
+	}
 
 	if (ut->align != nullptr) {
 		i64 custom_align = 1;
@@ -1716,8 +1722,11 @@ Type *check_get_params(CheckerContext *ctx, Scope *scope, Ast *_params, bool *is
 			if (p->flags&FieldFlag_auto_cast) {
 				param->flags |= EntityFlag_AutoCast;
 			}
-			param->state = EntityState_Resolved; // NOTE(bill): This should have be resolved whilst determining it
+			if (p->flags&FieldFlag_const) {
+				param->flags |= EntityFlag_ConstInput;
+			}
 
+			param->state = EntityState_Resolved; // NOTE(bill): This should have be resolved whilst determining it
 			add_entity(ctx->checker, scope, name, param);
 			if (is_using) {
 				add_entity_use(ctx, name, param);
diff --git a/src/checker_builtin_procs.hpp b/src/checker_builtin_procs.hpp
index 4981fdedb..7ef1be8b8 100644
--- a/src/checker_builtin_procs.hpp
+++ b/src/checker_builtin_procs.hpp
@@ -36,6 +36,8 @@ enum BuiltinProcId {
 	BuiltinProc_simd_vector,
 	BuiltinProc_soa_struct,
 
+	BuiltinProc_cpu_relax,
+
 	BuiltinProc_atomic_fence,
 	BuiltinProc_atomic_fence_acq,
 	BuiltinProc_atomic_fence_rel,
@@ -214,6 +216,8 @@ gb_global BuiltinProc builtin_procs[BuiltinProc_COUNT] = {
 	{STR_LIT("simd_vector"), 2, false, Expr_Expr, BuiltinProcPkg_intrinsics}, // Type
 	{STR_LIT("soa_struct"),  2, false, Expr_Expr, BuiltinProcPkg_intrinsics}, // Type
 
+	{STR_LIT("cpu_relax"),  0, false, Expr_Stmt, BuiltinProcPkg_intrinsics},
+
 	{STR_LIT("atomic_fence"),        0, false, Expr_Stmt, BuiltinProcPkg_intrinsics},
 	{STR_LIT("atomic_fence_acq"),    0, false, Expr_Stmt, BuiltinProcPkg_intrinsics},
 	{STR_LIT("atomic_fence_rel"),    0, false, Expr_Stmt, BuiltinProcPkg_intrinsics},
diff --git a/src/entity.cpp b/src/entity.cpp
index 8273af3f1..b89522b07 100644
--- a/src/entity.cpp
+++ b/src/entity.cpp
@@ -46,6 +46,7 @@ enum EntityFlag {
 	EntityFlag_BitFieldValue = 1<<12,
 	EntityFlag_PolyConst     = 1<<13,
 	EntityFlag_NotExported   = 1<<14,
+	EntityFlag_ConstInput    = 1<<15,
 
 	EntityFlag_Static        = 1<<16,
 
diff --git a/src/ir.cpp b/src/ir.cpp
index 5d075d7dd..6a56eb387 100644
--- a/src/ir.cpp
+++ b/src/ir.cpp
@@ -197,6 +197,7 @@ gbAllocator ir_allocator(void) {
 	IR_INSTR_KIND(ZeroInit, struct { irValue *address; })             \
 	IR_INSTR_KIND(Store,    struct { irValue *address, *value; bool is_volatile; }) \
 	IR_INSTR_KIND(Load,     struct { Type *type; irValue *address; i64 custom_align; }) \
+	IR_INSTR_KIND(InlineCode, struct { BuiltinProcId id; Array<irValue *> operands; }) \
 	IR_INSTR_KIND(AtomicFence, struct { BuiltinProcId id; })          \
 	IR_INSTR_KIND(AtomicStore, struct {                               \
 		irValue *address, *value;                                     \
@@ -1063,6 +1064,14 @@ irValue *ir_instr_load(irProcedure *p, irValue *address) {
 	return v;
 }
 
+irValue *ir_instr_inline_code(irProcedure *p, BuiltinProcId id, Array<irValue *> operands) {
+	irValue *v = ir_alloc_instr(p, irInstr_InlineCode);
+	irInstr *i = &v->Instr;
+	i->InlineCode.id = id;
+	i->InlineCode.operands = operands;
+	return v;
+}
+
 irValue *ir_instr_atomic_fence(irProcedure *p, BuiltinProcId id) {
 	irValue *v = ir_alloc_instr(p, irInstr_AtomicFence);
 	irInstr *i = &v->Instr;
@@ -1222,7 +1231,11 @@ irValue *ir_instr_union_tag_ptr(irProcedure *p, irValue *address) {
 
 	// i->UnionTagPtr.type = alloc_type_pointer(t_type_info_ptr);
 	Type *u = type_deref(ir_type(address));
+	if (is_type_union_maybe_pointer(u)) {
+		GB_PANIC("union #maybe UnionTagPtr");
+	}
 	i->UnionTagPtr.type = alloc_type_pointer(union_tag_type(u));
+
 	return v;
 }
 
@@ -1234,6 +1247,9 @@ irValue *ir_instr_union_tag_value(irProcedure *p, irValue *address) {
 	if (address) address->uses += 1;
 
 	Type *u = type_deref(ir_type(address));
+	if (is_type_union_maybe_pointer(u)) {
+		GB_PANIC("union #maybe UnionTagValue");
+	}
 	i->UnionTagPtr.type = union_tag_type(u);
 	return v;
 }
@@ -4351,7 +4367,9 @@ irValue *ir_emit_union_tag_value(irProcedure *proc, irValue *u) {
 
 irValue *ir_emit_comp_against_nil(irProcedure *proc, TokenKind op_kind, irValue *x) {
 	Type *t = ir_type(x);
-	if (is_type_cstring(t)) {
+	if (is_type_pointer(t)) {
+		return ir_emit_comp(proc, op_kind, x, v_raw_nil);
+	} else if (is_type_cstring(t)) {
 		irValue *ptr = ir_emit_conv(proc, x, t_u8_ptr);
 		return ir_emit_comp(proc, op_kind, ptr, v_raw_nil);
 	} else if (is_type_any(t)) {
@@ -4396,6 +4414,12 @@ irValue *ir_emit_comp_against_nil(irProcedure *proc, TokenKind op_kind, irValue
 	} else if (is_type_union(t)) {
 		if (type_size_of(t) == 0) {
 			return ir_emit_comp(proc, op_kind, v_zero, v_zero);
+		} else if (is_type_union_maybe_pointer(t)) {
+			Type *bt = base_type(t);
+			irValue *ptr = ir_address_from_load_or_generate_local(proc, x);
+			ptr = ir_emit_bitcast(proc, ptr, alloc_type_pointer(bt->Union.variants[0]));
+			irValue *data = ir_emit_load(proc, ptr);
+			return ir_emit_comp_against_nil(proc, op_kind, data);
 		} else {
 			irValue *tag = ir_emit_union_tag_value(proc, x);
 			return ir_emit_comp(proc, op_kind, tag, v_zero);
@@ -5232,8 +5256,12 @@ void ir_emit_store_union_variant(irProcedure *proc, irValue *parent, irValue *va
 
 	Type *t = type_deref(ir_type(parent));
 
-	irValue *tag_ptr = ir_emit_union_tag_ptr(proc, parent);
-	ir_emit_store(proc, tag_ptr, ir_const_union_tag(t, variant_type));
+	if (is_type_union_maybe_pointer(t)) {
+		// No tag needed!
+	} else {
+		irValue *tag_ptr = ir_emit_union_tag_ptr(proc, parent);
+		ir_emit_store(proc, tag_ptr, ir_const_union_tag(t, variant_type));
+	}
 }
 
 irValue *ir_emit_conv(irProcedure *proc, irValue *value, Type *t) {
@@ -5701,7 +5729,7 @@ irValue *ir_emit_transmute(irProcedure *proc, irValue *value, Type *t) {
 	}
 
 	// TODO(bill): Actually figure out what the conversion needs to be correctly 'cause LLVM
-	return ir_emit_bitcast(proc, value, dst);
+	return ir_emit_bitcast(proc, value, t);
 }
 
 
@@ -5728,22 +5756,41 @@ irValue *ir_emit_union_cast(irProcedure *proc, irValue *value, Type *type, Token
 	GB_ASSERT_MSG(is_type_union(src), "%s", type_to_string(src_type));
 	Type *dst = tuple->Tuple.variables[0]->type;
 
-
 	irValue *value_  = ir_address_from_load_or_generate_local(proc, value);
-	irValue *tag     = ir_emit_load(proc, ir_emit_union_tag_ptr(proc, value_));
-	irValue *dst_tag = ir_const_union_tag(src, dst);
 
+	irValue *tag = nullptr;
+	irValue *dst_tag = nullptr;
+	irValue *cond = nullptr;
+	irValue *data = nullptr;
+
+	irValue *gep0 = ir_emit_struct_ep(proc, v, 0);
+	irValue *gep1 = ir_emit_struct_ep(proc, v, 1);
+
+	if (is_type_union_maybe_pointer(src)) {
+		data = ir_emit_load(proc, ir_emit_conv(proc, value_, ir_type(gep0)));
+	} else {
+		tag     = ir_emit_load(proc, ir_emit_union_tag_ptr(proc, value_));
+		dst_tag = ir_const_union_tag(src, dst);
+	}
 
 	irBlock *ok_block = ir_new_block(proc, nullptr, "union_cast.ok");
 	irBlock *end_block = ir_new_block(proc, nullptr, "union_cast.end");
-	irValue *cond = ir_emit_comp(proc, Token_CmpEq, tag, dst_tag);
+
+	if (data != nullptr) {
+		GB_ASSERT(is_type_union_maybe_pointer(src));
+		cond = ir_emit_comp_against_nil(proc, Token_NotEq, data);
+	} else {
+		cond = ir_emit_comp(proc, Token_CmpEq, tag, dst_tag);
+	}
+
 	ir_emit_if(proc, cond, ok_block, end_block);
 	ir_start_block(proc, ok_block);
 
-	irValue *gep0 = ir_emit_struct_ep(proc, v, 0);
-	irValue *gep1 = ir_emit_struct_ep(proc, v, 1);
 
-	irValue *data = ir_emit_load(proc, ir_emit_conv(proc, value_, ir_type(gep0)));
+
+	if (data == nullptr) {
+		data = ir_emit_load(proc, ir_emit_conv(proc, value_, ir_type(gep0)));
+	}
 	ir_emit_store(proc, gep0, data);
 	ir_emit_store(proc, gep1, v_true);
 
@@ -6876,6 +6923,9 @@ irValue *ir_build_builtin_proc(irProcedure *proc, Ast *expr, TypeAndValue tv, Bu
 
 
 	// "Intrinsics"
+	case BuiltinProc_cpu_relax:
+		return ir_emit(proc, ir_instr_inline_code(proc, id, {}));
+
 	case BuiltinProc_atomic_fence:
 	case BuiltinProc_atomic_fence_acq:
 	case BuiltinProc_atomic_fence_rel:
@@ -7344,10 +7394,11 @@ irValue *ir_build_expr_internal(irProcedure *proc, Ast *expr) {
 					GB_ASSERT(are_types_identical(ir_type(left), key_type));
 
 					Type *it = bit_set_to_int(rt);
+					left = ir_emit_conv(proc, left, it);
 
 					irValue *lower = ir_value_constant(it, exact_value_i64(rt->BitSet.lower));
-					irValue *key = ir_emit_arith(proc, Token_Sub, left, lower, ir_type(left));
-					irValue *bit = ir_emit_arith(proc, Token_Shl, v_one, key, ir_type(left));
+					irValue *key = ir_emit_arith(proc, Token_Sub, left, lower, it);
+					irValue *bit = ir_emit_arith(proc, Token_Shl, v_one, key, it);
 					bit = ir_emit_conv(proc, bit, it);
 
 					irValue *old_value = ir_emit_bitcast(proc, right, it);
@@ -11465,6 +11516,7 @@ void ir_setup_type_info_data(irProcedure *proc) { // NOTE(bill): Setup type_info
 				irValue *tag_type_ptr     = ir_emit_struct_ep(proc, tag, 2);
 				irValue *custom_align_ptr = ir_emit_struct_ep(proc, tag, 3);
 				irValue *no_nil_ptr       = ir_emit_struct_ep(proc, tag, 4);
+				irValue *maybe_ptr        = ir_emit_struct_ep(proc, tag, 5);
 
 				isize variant_count = gb_max(0, t->Union.variants.count);
 				irValue *memory_types = ir_type_info_member_types_offset(proc, variant_count);
@@ -11494,6 +11546,7 @@ void ir_setup_type_info_data(irProcedure *proc) { // NOTE(bill): Setup type_info
 				ir_emit_store(proc, custom_align_ptr, is_custom_align);
 
 				ir_emit_store(proc, no_nil_ptr, ir_const_bool(t->Union.no_nil));
+				ir_emit_store(proc, maybe_ptr, ir_const_bool(t->Union.maybe));
 			}
 
 			break;
diff --git a/src/ir_print.cpp b/src/ir_print.cpp
index 2baf1c2a0..bcbdca615 100644
--- a/src/ir_print.cpp
+++ b/src/ir_print.cpp
@@ -508,13 +508,25 @@ void ir_print_type(irFileBuffer *f, irModule *m, Type *t, bool in_struct) {
 			// NOTE(bill): The zero size array is used to fix the alignment used in a structure as
 			// LLVM takes the first element's alignment as the entire alignment (like C)
 			i64 align = type_align_of(t);
+
+			if (is_type_union_maybe_pointer_original_alignment(t)) {
+				ir_write_byte(f, '{');
+				ir_print_type(f, m, t->Union.variants[0]);
+				ir_write_byte(f, '}');
+				return;
+			}
+
 			i64 block_size =  t->Union.variant_block_size;
 
 			ir_write_byte(f, '{');
 			ir_print_alignment_prefix_hack(f, align);
-			ir_fprintf(f, ", [%lld x i8], ", block_size);
-			// ir_print_type(f, m, t_type_info_ptr);
-			ir_print_type(f, m, union_tag_type(t));
+			if (is_type_union_maybe_pointer(t)) {
+				ir_fprintf(f, ", ");
+				ir_print_type(f, m, t->Union.variants[0]);
+			} else {
+				ir_fprintf(f, ", [%lld x i8], ", block_size);
+				ir_print_type(f, m, union_tag_type(t));
+			}
 			ir_write_byte(f, '}');
 		}
 		return;
@@ -1471,6 +1483,18 @@ void ir_print_instr(irFileBuffer *f, irModule *m, irValue *value) {
 		break;
 	}
 
+	case irInstr_InlineCode:
+		{
+			switch (instr->InlineCode.id) {
+			case BuiltinProc_cpu_relax:
+				ir_write_str_lit(f, "call void asm sideeffect \"pause\", \"\"()");
+				break;
+			default: GB_PANIC("Unknown inline code %d", instr->InlineCode.id); break;
+			}
+		}
+		break;
+
+
 	case irInstr_AtomicFence:
 		ir_write_str_lit(f, "fence ");
 		switch (instr->AtomicFence.id) {
@@ -1850,6 +1874,12 @@ void ir_print_instr(irFileBuffer *f, irModule *m, irValue *value) {
 
 	case irInstr_UnionTagPtr: {
 		Type *et = ir_type(instr->UnionTagPtr.address);
+
+		Type *ut = type_deref(et);
+		if (is_type_union_maybe_pointer(ut)) {
+			GB_PANIC("union #maybe UnionTagPtr");
+		}
+
 		ir_fprintf(f, "%%%d = getelementptr inbounds ", value->index);
 		Type *t = base_type(type_deref(et));
 		GB_ASSERT(is_type_union(t));
@@ -1869,10 +1899,16 @@ void ir_print_instr(irFileBuffer *f, irModule *m, irValue *value) {
 
 	case irInstr_UnionTagValue: {
 		Type *et = ir_type(instr->UnionTagValue.address);
-		ir_fprintf(f, "%%%d = extractvalue ", value->index);
 		Type *t = base_type(et);
+
+		if (is_type_union_maybe_pointer(t)) {
+			GB_PANIC("union #maybe UnionTagValue");
+		}
+
+		ir_fprintf(f, "%%%d = extractvalue ", value->index);
 		GB_ASSERT(is_type_union(t));
 
+
 		ir_print_type(f, m, et);
 		ir_write_byte(f, ' ');
 		ir_print_value(f, m, instr->UnionTagValue.address, et);
diff --git a/src/main.cpp b/src/main.cpp
index 87669f8e3..077603b30 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -243,6 +243,7 @@ enum BuildFlagKind {
 #if defined(GB_SYSTEM_WINDOWS)
 	BuildFlag_ResourceFile,
 	BuildFlag_WindowsPdbName,
+	BuildFlag_Subsystem,
 #endif
 
 	BuildFlag_COUNT,
@@ -331,8 +332,9 @@ bool parse_build_flags(Array<String> args) {
 	add_flag(&build_flags, BuildFlag_GoToDefinitions, str_lit("go-to-definitions"), BuildFlagParam_None);
 
 #if defined(GB_SYSTEM_WINDOWS)
-	add_flag(&build_flags, BuildFlag_ResourceFile,   str_lit("resource"), BuildFlagParam_String);
-	add_flag(&build_flags, BuildFlag_WindowsPdbName, str_lit("pdb-name"), BuildFlagParam_String);
+	add_flag(&build_flags, BuildFlag_ResourceFile,   str_lit("resource"),  BuildFlagParam_String);
+	add_flag(&build_flags, BuildFlag_WindowsPdbName, str_lit("pdb-name"),  BuildFlagParam_String);
+	add_flag(&build_flags, BuildFlag_Subsystem,      str_lit("subsystem"), BuildFlagParam_String);
 #endif
 
 	GB_ASSERT(args.count >= 3);
@@ -764,7 +766,19 @@ bool parse_build_flags(Array<String> args) {
 								bad_flags = true;
 							}
 							break;
+						}
 
+						case BuildFlag_Subsystem: {
+							GB_ASSERT(value.kind == ExactValue_String);
+							String subsystem = value.value_string;
+							if (str_eq_ignore_case(subsystem, str_lit("console"))) {
+								build_context.use_subsystem_windows = false;
+							} else  if (str_eq_ignore_case(subsystem, str_lit("windows"))) {
+								build_context.use_subsystem_windows = true;
+							} else {
+								gb_printf_err("Invalid -subsystem string, got %.*s, expected either 'console' or 'windows'\n", LIT(subsystem));
+								bad_flags = true;
+							}
 							break;
 						}
 					#endif
@@ -1180,6 +1194,10 @@ int main(int arg_count, char const **arg_ptr) {
 		return 1;
 	}
 
+	if (init_filename == "-help") {
+		build_context.show_help = true;
+	}
+
 	build_context.command = command;
 
 	if (!parse_build_flags(args)) {
@@ -1372,8 +1390,8 @@ int main(int arg_count, char const **arg_ptr) {
 		}
 
 
+		char const *subsystem_str = build_context.use_subsystem_windows ? "WINDOWS" : "CONSOLE";
 		if (!build_context.use_lld) { // msvc
-
 			if (build_context.has_resource) {
 				exit_code = system_exec_command_line_app("msvc-link",
 					"\"%.*src.exe\" /nologo /fo \"%.*s.res\" \"%.*s.rc\"",
@@ -1388,36 +1406,42 @@ int main(int arg_count, char const **arg_ptr) {
 
 				exit_code = system_exec_command_line_app("msvc-link",
 					"\"%.*slink.exe\" \"%.*s.obj\" \"%.*s.res\" -OUT:\"%.*s.%s\" %s "
-					"/nologo /incremental:no /opt:ref /subsystem:CONSOLE "
+					"/nologo /incremental:no /opt:ref /subsystem:%s "
 					" %.*s "
 					" %s "
 					"",
 					LIT(find_result.vs_exe_path), LIT(output_base), LIT(output_base), LIT(output_base), output_ext,
-					link_settings, LIT(build_context.link_flags),
+					link_settings,
+					subsystem_str,
+					LIT(build_context.link_flags),
 					lib_str
 				);
 			} else {
 				exit_code = system_exec_command_line_app("msvc-link",
 					"\"%.*slink.exe\" \"%.*s.obj\" -OUT:\"%.*s.%s\" %s "
-					"/nologo /incremental:no /opt:ref /subsystem:CONSOLE "
+					"/nologo /incremental:no /opt:ref /subsystem:%s "
 					" %.*s "
 					" %s "
 					"",
 					LIT(find_result.vs_exe_path), LIT(output_base), LIT(output_base), output_ext,
-					link_settings, LIT(build_context.link_flags),
+					link_settings,
+					subsystem_str,
+					LIT(build_context.link_flags),
 					lib_str
 				);
 			}
 		} else { // lld
 			exit_code = system_exec_command_line_app("msvc-link",
 				"\"%.*s\\bin\\lld-link\" \"%.*s.obj\" -OUT:\"%.*s.%s\" %s "
-				"/nologo /incremental:no /opt:ref /subsystem:CONSOLE "
+				"/nologo /incremental:no /opt:ref /subsystem:%s "
 				" %.*s "
 				" %s "
 				"",
 				LIT(build_context.ODIN_ROOT),
 				LIT(output_base), LIT(output_base), output_ext,
-				link_settings, LIT(build_context.link_flags),
+				link_settings,
+				subsystem_str,
+				LIT(build_context.link_flags),
 				lib_str
 			);
 		}
@@ -1500,11 +1524,11 @@ int main(int arg_count, char const **arg_ptr) {
 			// Shared libraries are .dylib on MacOS and .so on Linux.
 			#if defined(GB_SYSTEM_OSX)
 				output_ext = STR_LIT(".dylib");
+				link_settings = "-dylib -dynamic";
 			#else
 				output_ext = STR_LIT(".so");
+				link_settings = "-shared";
 			#endif
-
-			link_settings = "-shared";
 		} else {
 			// TODO: Do I need anything here?
 			link_settings = "";
diff --git a/src/parser.cpp b/src/parser.cpp
index ecb02c803..f89b5676b 100644
--- a/src/parser.cpp
+++ b/src/parser.cpp
@@ -929,7 +929,7 @@ Ast *ast_struct_type(AstFile *f, Token token, Array<Ast *> fields, isize field_c
 }
 
 
-Ast *ast_union_type(AstFile *f, Token token, Array<Ast *> variants, Ast *polymorphic_params, Ast *align, bool no_nil,
+Ast *ast_union_type(AstFile *f, Token token, Array<Ast *> variants, Ast *polymorphic_params, Ast *align, bool no_nil, bool maybe,
                     Token where_token, Array<Ast *> const &where_clauses) {
 	Ast *result = alloc_ast_node(f, Ast_UnionType);
 	result->UnionType.token              = token;
@@ -937,6 +937,7 @@ Ast *ast_union_type(AstFile *f, Token token, Array<Ast *> variants, Ast *polymor
 	result->UnionType.polymorphic_params = polymorphic_params;
 	result->UnionType.align              = align;
 	result->UnionType.no_nil             = no_nil;
+	result->UnionType.maybe             = maybe;
 	result->UnionType.where_token        = where_token;
 	result->UnionType.where_clauses      = where_clauses;
 	return result;
@@ -2091,6 +2092,7 @@ Ast *parse_operand(AstFile *f, bool lhs) {
 		Ast *polymorphic_params = nullptr;
 		Ast *align = nullptr;
 		bool no_nil = false;
+		bool maybe = false;
 
 		CommentGroup *docs = f->lead_comment;
 		Token start_token = f->curr_token;
@@ -2118,10 +2120,19 @@ Ast *parse_operand(AstFile *f, bool lhs) {
 					syntax_error(tag, "Duplicate union tag '#%.*s'", LIT(tag.string));
 				}
 				no_nil = true;
-			} else {
+			} else if (tag.string == "maybe") {
+				if (maybe) {
+					syntax_error(tag, "Duplicate union tag '#%.*s'", LIT(tag.string));
+				}
+				maybe = true;
+			}else {
 				syntax_error(tag, "Invalid union tag '#%.*s'", LIT(tag.string));
 			}
 		}
+		if (no_nil && maybe) {
+			syntax_error(f->curr_token, "#maybe and #no_nil cannot be applied together");
+		}
+
 
 		Token where_token = {};
 		Array<Ast *> where_clauses = {};
@@ -2150,7 +2161,7 @@ Ast *parse_operand(AstFile *f, bool lhs) {
 
 		Token close = expect_token(f, Token_CloseBrace);
 
-		return ast_union_type(f, token, variants, polymorphic_params, align, no_nil, where_token, where_clauses);
+		return ast_union_type(f, token, variants, polymorphic_params, align, no_nil, maybe, where_token, where_clauses);
 	} break;
 
 	case Token_enum: {
@@ -2350,6 +2361,12 @@ Ast *parse_atom_expr(AstFile *f, Ast *operand, bool lhs) {
 				operand = ast_type_assertion(f, operand, token, type);
 			} break;
 
+			case Token_Question: {
+				Token question = expect_token(f, Token_Question);
+				Ast *type = ast_unary_expr(f, question, nullptr);
+				operand = ast_type_assertion(f, operand, token, type);
+			} break;
+
 			default:
 				syntax_error(f->curr_token, "Expected a selector");
 				advance_token(f);
@@ -2981,6 +2998,7 @@ enum FieldPrefixKind {
 	FieldPrefix_Invalid = 0,
 
 	FieldPrefix_using,
+	FieldPrefix_const,
 	FieldPrefix_no_alias,
 	FieldPrefix_c_var_arg,
 	FieldPrefix_auto_cast,
@@ -3007,6 +3025,9 @@ FieldPrefixKind is_token_field_prefix(AstFile *f) {
 				return FieldPrefix_c_var_arg;
 			}
 			break;
+
+		case Token_const:
+			return FieldPrefix_const;
 		}
 		return FieldPrefix_Unknown;
 	}
@@ -3019,6 +3040,7 @@ u32 parse_field_prefixes(AstFile *f) {
 	i32 no_alias_count  = 0;
 	i32 c_vararg_count  = 0;
 	i32 auto_cast_count = 0;
+	i32 const_count     = 0;
 
 	for (;;) {
 		FieldPrefixKind kind = is_token_field_prefix(f);
@@ -3036,12 +3058,14 @@ u32 parse_field_prefixes(AstFile *f) {
 		case FieldPrefix_no_alias:  no_alias_count  += 1; advance_token(f); break;
 		case FieldPrefix_c_var_arg: c_vararg_count  += 1; advance_token(f); break;
 		case FieldPrefix_auto_cast: auto_cast_count += 1; advance_token(f); break;
+		case FieldPrefix_const:     const_count     += 1; advance_token(f); break;
 		}
 	}
 	if (using_count     > 1) syntax_error(f->curr_token, "Multiple 'using' in this field list");
 	if (no_alias_count  > 1) syntax_error(f->curr_token, "Multiple '#no_alias' in this field list");
 	if (c_vararg_count  > 1) syntax_error(f->curr_token, "Multiple '#c_vararg' in this field list");
 	if (auto_cast_count > 1) syntax_error(f->curr_token, "Multiple 'auto_cast' in this field list");
+	if (const_count     > 1) syntax_error(f->curr_token, "Multiple '#const' in this field list");
 
 
 	u32 field_flags = 0;
@@ -3049,6 +3073,7 @@ u32 parse_field_prefixes(AstFile *f) {
 	if (no_alias_count  > 0) field_flags |= FieldFlag_no_alias;
 	if (c_vararg_count  > 0) field_flags |= FieldFlag_c_vararg;
 	if (auto_cast_count > 0) field_flags |= FieldFlag_auto_cast;
+	if (const_count     > 0) field_flags |= FieldFlag_const;
 	return field_flags;
 }
 
diff --git a/src/parser.hpp b/src/parser.hpp
index 983db1042..6426cc96b 100644
--- a/src/parser.hpp
+++ b/src/parser.hpp
@@ -203,12 +203,13 @@ enum FieldFlag {
 	FieldFlag_no_alias  = 1<<2,
 	FieldFlag_c_vararg  = 1<<3,
 	FieldFlag_auto_cast = 1<<4,
+	FieldFlag_const     = 1<<5,
 
 	FieldFlag_Tags = 1<<10,
 
 	FieldFlag_Results   = 1<<16,
 
-	FieldFlag_Signature = FieldFlag_ellipsis|FieldFlag_using|FieldFlag_no_alias|FieldFlag_c_vararg|FieldFlag_auto_cast,
+	FieldFlag_Signature = FieldFlag_ellipsis|FieldFlag_using|FieldFlag_no_alias|FieldFlag_c_vararg|FieldFlag_auto_cast|FieldFlag_const,
 	FieldFlag_Struct    = FieldFlag_using|FieldFlag_Tags,
 };
 
@@ -515,6 +516,7 @@ AST_KIND(_TypeBegin, "", bool) \
 		Array<Ast *> variants;      \
 		Ast *polymorphic_params;    \
 		Ast *        align;         \
+		bool         maybe;         \
 		bool         no_nil;        \
 		Token where_token;          \
 		Array<Ast *> where_clauses; \
diff --git a/src/types.cpp b/src/types.cpp
index e120c77c0..d2a040b0b 100644
--- a/src/types.cpp
+++ b/src/types.cpp
@@ -152,6 +152,7 @@ struct TypeUnion {
 	Type *        polymorphic_params; // Type_Tuple
 	Type *        polymorphic_parent;
 	bool          no_nil;
+	bool          maybe;
 	bool          is_polymorphic;
 	bool          is_poly_specialized;
 };
@@ -1219,6 +1220,32 @@ bool is_type_map(Type *t) {
 	return t->kind == Type_Map;
 }
 
+bool is_type_union_maybe_pointer(Type *t) {
+	t = base_type(t);
+	if (t->kind == Type_Union && t->Union.maybe) {
+		if (t->Union.variants.count == 1) {
+			return is_type_pointer(t->Union.variants[0]);
+		}
+	}
+	return false;
+}
+
+
+bool is_type_union_maybe_pointer_original_alignment(Type *t) {
+	t = base_type(t);
+	if (t->kind == Type_Union && t->Union.maybe) {
+		if (t->Union.variants.count == 1) {
+			Type *v = t->Union.variants[0];
+			if (is_type_pointer(v)) {
+				return type_align_of(v) == type_align_of(t);
+			}
+		}
+	}
+	return false;
+}
+
+
+
 
 bool is_type_integer_endian_big(Type *t) {
 	t = core_type(t);
@@ -2024,6 +2051,7 @@ i64 union_tag_size(Type *u) {
 Type *union_tag_type(Type *u) {
 	i64 s = union_tag_size(u);
 	switch (s) {
+	case  0: return  t_u8;
 	case  1: return  t_u8;
 	case  2: return  t_u16;
 	case  4: return  t_u32;
@@ -2934,14 +2962,23 @@ i64 type_size_of_internal(Type *t, TypePath *path) {
 			}
 		}
 
-		// NOTE(bill): Align to tag
-		i64 tag_size = union_tag_size(t);
-		i64 size = align_formula(max, tag_size);
-		// NOTE(bill): Calculate the padding between the common fields and the tag
-		t->Union.tag_size = tag_size;
-		t->Union.variant_block_size = size - field_size;
+		i64 size = 0;
+
+		if (is_type_union_maybe_pointer(t)) {
+			size = max;
+			t->Union.tag_size = 0;
+			t->Union.variant_block_size = size;
+		} else {
+			// NOTE(bill): Align to tag
+			i64 tag_size = union_tag_size(t);
+			size = align_formula(max, tag_size);
+			// NOTE(bill): Calculate the padding between the common fields and the tag
+			t->Union.tag_size = tag_size;
+			t->Union.variant_block_size = size - field_size;
 
-		return align_formula(size + tag_size, align);
+			size += tag_size;
+		}
+		return align_formula(size, align);
 	} break;