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AstGen: properly generate errdefer expressions when returning
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`return` statements use a new function `nodeMayEvalToError` which does
some basic checks on the AST node to return never, always, or maybe.

Depending on this result, AstGen skips the errdefers, always includes
the errdefers, or emits a conditional branch to check whether the return
value is an error that Sema will have to evaluate.

Closes #8821
Unblocks #9047
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@andrewrk
andrewrk committed Jun 20, 2021
1 parent 76102ea commit 1fe0142
Showing 1 changed file with 259 additions and 13 deletions.
272 changes: 259 additions & 13 deletions src/AstGen.zig
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Original file line number Diff line number Diff line change
Expand Up @@ -5998,22 +5998,55 @@ fn ret(gz: *GenZir, scope: *Scope, node: ast.Node.Index) InnerError!Zir.Inst.Ref
if (gz.in_defer) return astgen.failNode(node, "cannot return from defer expression", .{});

const operand_node = node_datas[node].lhs;
if (operand_node != 0) {
const rl: ResultLoc = if (nodeMayNeedMemoryLocation(tree, operand_node)) .{
.ptr = try gz.addNodeExtended(.ret_ptr, node),
} else .{
.ty = try gz.addNodeExtended(.ret_type, node),
};
const operand = try expr(gz, scope, rl, operand_node);
// TODO check operand to see if we need to generate errdefers
if (operand_node == 0) {
// Returning a void value; skip error defers.
try genDefers(gz, &astgen.fn_block.?.base, scope, .none);
_ = try gz.addUnNode(.ret_node, operand, node);
_ = try gz.addUnNode(.ret_node, .void_value, node);
return Zir.Inst.Ref.unreachable_value;
}
// Returning a void value; skip error defers.
try genDefers(gz, &astgen.fn_block.?.base, scope, .none);
_ = try gz.addUnNode(.ret_node, .void_value, node);
return Zir.Inst.Ref.unreachable_value;

const rl: ResultLoc = if (nodeMayNeedMemoryLocation(tree, operand_node)) .{
.ptr = try gz.addNodeExtended(.ret_ptr, node),
} else .{
.ty = try gz.addNodeExtended(.ret_type, node),
};
const operand = try expr(gz, scope, rl, operand_node);

switch (nodeMayEvalToError(tree, operand_node)) {
.never => {
// Returning a value that cannot be an error; skip error defers.
try genDefers(gz, &astgen.fn_block.?.base, scope, .none);
_ = try gz.addUnNode(.ret_node, operand, node);
return Zir.Inst.Ref.unreachable_value;
},
.always => {
// Value is always an error. Emit both error defers and regular defers.
const err_code = try gz.addUnNode(.err_union_code, operand, node);
try genDefers(gz, &astgen.fn_block.?.base, scope, err_code);
_ = try gz.addUnNode(.ret_node, operand, node);
return Zir.Inst.Ref.unreachable_value;
},
.maybe => {
// Emit conditional branch for generating errdefers.
const is_err = try gz.addUnNode(.is_err, operand, node);
const condbr = try gz.addCondBr(.condbr, node);

var then_scope = gz.makeSubBlock(scope);
defer then_scope.instructions.deinit(astgen.gpa);
const err_code = try then_scope.addUnNode(.err_union_code, operand, node);
try genDefers(&then_scope, &astgen.fn_block.?.base, scope, err_code);
_ = try then_scope.addUnNode(.ret_node, operand, node);

var else_scope = gz.makeSubBlock(scope);
defer else_scope.instructions.deinit(astgen.gpa);
try genDefers(&else_scope, &astgen.fn_block.?.base, scope, .none);
_ = try else_scope.addUnNode(.ret_node, operand, node);

try setCondBrPayload(condbr, is_err, &then_scope, &else_scope);

return Zir.Inst.Ref.unreachable_value;
},
}
}

fn identifier(
Expand Down Expand Up @@ -7555,6 +7588,219 @@ fn nodeMayNeedMemoryLocation(tree: *const ast.Tree, start_node: ast.Node.Index)
}
}

fn nodeMayEvalToError(tree: *const ast.Tree, start_node: ast.Node.Index) enum { never, always, maybe } {
const node_tags = tree.nodes.items(.tag);
const node_datas = tree.nodes.items(.data);
const main_tokens = tree.nodes.items(.main_token);
const token_tags = tree.tokens.items(.tag);

var node = start_node;
while (true) {
switch (node_tags[node]) {
.root,
.@"usingnamespace",
.test_decl,
.switch_case,
.switch_case_one,
.container_field_init,
.container_field_align,
.container_field,
.asm_output,
.asm_input,
=> unreachable,

.error_value => return .always,

.@"asm",
.asm_simple,
.identifier,
.field_access,
.deref,
.array_access,
.while_simple,
.while_cont,
.for_simple,
.if_simple,
.@"while",
.@"if",
.@"for",
.@"switch",
.switch_comma,
.call_one,
.call_one_comma,
.async_call_one,
.async_call_one_comma,
.call,
.call_comma,
.async_call,
.async_call_comma,
=> return .maybe,

.@"return",
.@"break",
.@"continue",
.bit_not,
.bool_not,
.global_var_decl,
.local_var_decl,
.simple_var_decl,
.aligned_var_decl,
.@"defer",
.@"errdefer",
.address_of,
.optional_type,
.negation,
.negation_wrap,
.@"resume",
.array_type,
.array_type_sentinel,
.ptr_type_aligned,
.ptr_type_sentinel,
.ptr_type,
.ptr_type_bit_range,
.@"suspend",
.@"anytype",
.fn_proto_simple,
.fn_proto_multi,
.fn_proto_one,
.fn_proto,
.fn_decl,
.anyframe_type,
.anyframe_literal,
.integer_literal,
.float_literal,
.enum_literal,
.string_literal,
.multiline_string_literal,
.char_literal,
.true_literal,
.false_literal,
.null_literal,
.undefined_literal,
.unreachable_literal,
.error_set_decl,
.container_decl,
.container_decl_trailing,
.container_decl_two,
.container_decl_two_trailing,
.container_decl_arg,
.container_decl_arg_trailing,
.tagged_union,
.tagged_union_trailing,
.tagged_union_two,
.tagged_union_two_trailing,
.tagged_union_enum_tag,
.tagged_union_enum_tag_trailing,
.add,
.add_wrap,
.array_cat,
.array_mult,
.assign,
.assign_bit_and,
.assign_bit_or,
.assign_bit_shift_left,
.assign_bit_shift_right,
.assign_bit_xor,
.assign_div,
.assign_sub,
.assign_sub_wrap,
.assign_mod,
.assign_add,
.assign_add_wrap,
.assign_mul,
.assign_mul_wrap,
.bang_equal,
.bit_and,
.bit_or,
.bit_shift_left,
.bit_shift_right,
.bit_xor,
.bool_and,
.bool_or,
.div,
.equal_equal,
.error_union,
.greater_or_equal,
.greater_than,
.less_or_equal,
.less_than,
.merge_error_sets,
.mod,
.mul,
.mul_wrap,
.switch_range,
.sub,
.sub_wrap,
.slice,
.slice_open,
.slice_sentinel,
.array_init_one,
.array_init_one_comma,
.array_init_dot_two,
.array_init_dot_two_comma,
.array_init_dot,
.array_init_dot_comma,
.array_init,
.array_init_comma,
.struct_init_one,
.struct_init_one_comma,
.struct_init_dot_two,
.struct_init_dot_two_comma,
.struct_init_dot,
.struct_init_dot_comma,
.struct_init,
.struct_init_comma,
=> return .never,

// Forward the question to the LHS sub-expression.
.grouped_expression,
.@"try",
.@"await",
.@"comptime",
.@"nosuspend",
.unwrap_optional,
=> node = node_datas[node].lhs,

// Forward the question to the RHS sub-expression.
.@"catch",
.@"orelse",
=> node = node_datas[node].rhs,

.block_two,
.block_two_semicolon,
.block,
.block_semicolon,
=> {
const lbrace = main_tokens[node];
if (token_tags[lbrace - 1] == .colon) {
// Labeled blocks may need a memory location to forward
// to their break statements.
return .maybe;
} else {
return .never;
}
},

.builtin_call,
.builtin_call_comma,
.builtin_call_two,
.builtin_call_two_comma,
=> {
const builtin_token = main_tokens[node];
const builtin_name = tree.tokenSlice(builtin_token);
// If the builtin is an invalid name, we don't cause an error here; instead
// let it pass, and the error will be "invalid builtin function" later.
const builtin_info = BuiltinFn.list.get(builtin_name) orelse return .maybe;
if (builtin_info.tag == .err_set_cast) {
return .always;
} else {
return .never;
}
},
}
}
}

/// Applies `rl` semantics to `inst`. Expressions which do not do their own handling of
/// result locations must call this function on their result.
/// As an example, if the `ResultLoc` is `ptr`, it will write the result to the pointer.
Expand Down

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