incremental: debug line number updates

[mlugg]

No description provided.

[jacobly0]

Using the change_line_number incremental test as a demonstration, the source code of the initial incremental update is:

const std = @import("std");
pub fn main() !void {
    try std.io.getStdOut().writeAll("foo\n");
}

After which we have a binary containing this debug info for main:

0x0000002d:     DW_TAG_subprogram
                  DW_AT_ZIG_parent      (0x0000000000000072)
                  DW_AT_decl_line       (2)
                  DW_AT_decl_column     (1)
                  DW_AT_accessibility   (DW_ACCESS_public)
                  DW_AT_name    ("main")
                  DW_AT_linkage_name    ("main.main")
                  DW_AT_type    (0x0000000000094be8 "std::@typeInfo(@typeInfo(@TypeOf(main.main)).@"fn".return_type.?).error_union.error_set!void")
                  DW_AT_low_pc  (0x0000000001014021)
                  DW_AT_high_pc (0x0000000001014090)
                  DW_AT_alignment       (16)
                  DW_AT_external        (0x00)
                  DW_AT_noreturn        (0x00)

0x00000053:       DW_TAG_lexical_block
                    DW_AT_low_pc        (0x0000000001014075)
                    DW_AT_high_pc       (0x000000000101407a)

0x00000060:       NULL

Which when debugged with a breakpoint on main.main displays:

* thread #1, name = 'main', stop reason = breakpoint 1.1
    frame #0: 0x0000000001014032 main`main.main at main.zig:3:25
   1   	const std = @import("std");
   2   	pub fn main() !void {
-> 3   	   try std.io.getStdOut().writeAll("foo\n");
   4   	}

---

The source code for the second incremental update is:

const std = @import("std");

pub fn main() !void {
    try std.io.getStdOut().writeAll("foo\n");
}

Which calls the following exhaustive list of dwarf update functions:

info: emit_digest included stderr:
debug(dwarf): updateLineNumber(main.zig:3:1 %4 = main)

Which directly modifies the debug info in the binary for main to:

0x0000002d:     DW_TAG_subprogram
                  DW_AT_ZIG_parent      (0x0000000000000072)
                  DW_AT_decl_line       (3)
                  DW_AT_decl_column     (1)
                  DW_AT_accessibility   (DW_ACCESS_public)
                  DW_AT_name    ("main")
                  DW_AT_linkage_name    ("main.main")
                  DW_AT_type    (0x0000000000094be8 "std::@typeInfo(@typeInfo(@TypeOf(main.main)).@"fn".return_type.?).error_union.error_set!void")
                  DW_AT_low_pc  (0x0000000001014021)
                  DW_AT_high_pc (0x0000000001014090)
                  DW_AT_alignment       (16)
                  DW_AT_external        (0x00)
                  DW_AT_noreturn        (0x00)

0x00000053:       DW_TAG_lexical_block
                    DW_AT_low_pc        (0x0000000001014075)
                    DW_AT_high_pc       (0x000000000101407a)

0x00000060:       NULL

Which when debugged again displays:

* thread #1, name = 'main', stop reason = breakpoint 1.1
    frame #0: 0x0000000001014032 main`main.main at main.zig:4:25
   1   	const std = @import("std");
   2   	
   3   	pub fn main() !void {
-> 4   	   try std.io.getStdOut().writeAll("foo\n");
   5   	}

[jacobly0]

Using the change_generic_line_number incremental test as a demonstration, the source code of the initial incremental update is:

const std = @import("std");
fn Printer(message: []const u8) type {
    return struct {
        fn print() !void {
            try std.io.getStdOut().writeAll(message);
        }
    };
}
pub fn main() !void {
    try Printer("foo\n").print();
    try Printer("bar\n").print();
}

After which we have a binary containing this debug info for print:

0x0000002d:     DW_TAG_subprogram
                  DW_AT_ZIG_parent      (0x0000000000000111)
                  DW_AT_decl_line       (4)
                  DW_AT_decl_column     (9)
                  DW_AT_accessibility   (DW_ACCESS_private)
                  DW_AT_name    ("print")
                  DW_AT_declaration     (true)

0x00000041:     DW_TAG_subprogram
                  DW_AT_ZIG_parent      (0x000000000000013a)
                  DW_AT_abstract_origin (0x000000000000002d "print")
                  DW_AT_linkage_name    ("main.Printer(\"foo\\n\"[0..4]).print")
                  DW_AT_type    (0x0000000000094d89 "std::@typeInfo(@typeInfo(@TypeOf(main.Printer("foo\n"[0..4]).print)).@"fn".return_type.?).error_union.error_set!void")
                  DW_AT_low_pc  (0x0000000001014021)
                  DW_AT_high_pc (0x0000000001014090)
                  DW_AT_alignment       (16)
                  DW_AT_external        (0x00)
                  DW_AT_noreturn        (0x00)

0x00000061:       DW_TAG_lexical_block
                    DW_AT_low_pc        (0x0000000001014075)
                    DW_AT_high_pc       (0x000000000101407a)

0x0000006e:       NULL

0x0000007e:     DW_TAG_subprogram
                  DW_AT_ZIG_parent      (0x000000000000015c)
                  DW_AT_abstract_origin (0x000000000000002d "print")
                  DW_AT_linkage_name    ("main.Printer(\"bar\\n\"[0..4]).print")
                  DW_AT_type    (0x0000000000094e7d "std::@typeInfo(@typeInfo(@TypeOf(main.Printer("bar\n"[0..4]).print)).@"fn".return_type.?).error_union.error_set!void")
                  DW_AT_low_pc  (0x00000000010140b5)
                  DW_AT_high_pc (0x0000000001014124)
                  DW_AT_alignment       (16)
                  DW_AT_external        (0x00)
                  DW_AT_noreturn        (0x00)

0x0000009e:       DW_TAG_lexical_block
                    DW_AT_low_pc        (0x0000000001014109)
                    DW_AT_high_pc       (0x000000000101410e)

0x000000ab:       NULL

Which when debugged displays:

* thread #1, name = 'main', stop reason = breakpoint 1.1
    frame #0: 0x000000000101415a main`main.main at main.zig:10:31
   7   	   };
   8   	}
   9   	pub fn main() !void {
-> 10  	   try Printer("foo\n").print();
   11  	   try Printer("bar\n").print();
   12  	}
(lldb) s
Process 366773 stopped
* thread #1, name = 'main', stop reason = step in
    frame #0: 0x0000000001014032 main`main.Printer("foo\n"[0..4]).print at main.zig:5:33
   2   	fn Printer(message: []const u8) type {
   3   	   return struct {
   4   	       fn print() !void {
-> 5   	           try std.io.getStdOut().writeAll(message);
   6   	       }
   7   	   };
   8   	}
(lldb) n
foo
Process 366773 stopped
* thread #1, name = 'main', stop reason = step over
    frame #0: 0x000000000101415f main`main.main at main.zig:10:31
   7   	   };
   8   	}
   9   	pub fn main() !void {
-> 10  	   try Printer("foo\n").print();
   11  	   try Printer("bar\n").print();
   12  	}
(lldb) s
Process 366773 stopped
* thread #1, name = 'main', stop reason = step in
    frame #0: 0x0000000001014177 main`main.main at main.zig:11:31
   8   	}
   9   	pub fn main() !void {
   10  	   try Printer("foo\n").print();
-> 11  	   try Printer("bar\n").print();
   12  	}
(lldb) s
Process 366773 stopped
* thread #1, name = 'main', stop reason = step in
    frame #0: 0x00000000010140c6 main`main.Printer("bar\n"[0..4]).print at main.zig:5:33
   2   	fn Printer(message: []const u8) type {
   3   	   return struct {
   4   	       fn print() !void {
-> 5   	           try std.io.getStdOut().writeAll(message);
   6   	       }
   7   	   };
   8   	}
(lldb) n
bar
Process 366773 stopped
* thread #1, name = 'main', stop reason = step over
    frame #0: 0x000000000101417c main`main.main at main.zig:11:31
   8   	}
   9   	pub fn main() !void {
   10  	   try Printer("foo\n").print();
-> 11  	   try Printer("bar\n").print();
   12  	}

---

The source code for the second incremental update is:

const std = @import("std");

fn Printer(message: []const u8) type {
    return struct {
        fn print() !void {
            try std.io.getStdOut().writeAll(message);
        }
    };
}
pub fn main() !void {
    try Printer("foo\n").print();
    try Printer("bar\n").print();
}

Which calls the following exhaustive list of dwarf update functions:

debug(dwarf): updateLineNumber(main.zig:3:1 %4 = Printer)
debug(dwarf): updateLineNumber(main.zig:10:1 %37 = main)
debug(dwarf): updateLineNumber(main.zig:5:9 %10 = print)

Which directly modifies the debug info in the binary for the generic origin of print to:

0x0000002d:     DW_TAG_subprogram
                  DW_AT_ZIG_parent      (0x0000000000000111)
                  DW_AT_decl_line       (5)
                  DW_AT_decl_column     (9)
                  DW_AT_accessibility   (DW_ACCESS_private)
                  DW_AT_name    ("print")
                  DW_AT_declaration     (true)

0x00000041:     DW_TAG_subprogram
                  DW_AT_ZIG_parent      (0x000000000000013a)
                  DW_AT_abstract_origin (0x000000000000002d "print")
                  DW_AT_linkage_name    ("main.Printer(\"foo\\n\"[0..4]).print")
                  DW_AT_type    (0x0000000000094d89 "std::@typeInfo(@typeInfo(@TypeOf(main.Printer("foo\n"[0..4]).print)).@"fn".return_type.?).error_union.error_set!void")
                  DW_AT_low_pc  (0x0000000001014021)
                  DW_AT_high_pc (0x0000000001014090)
                  DW_AT_alignment       (16)
                  DW_AT_external        (0x00)
                  DW_AT_noreturn        (0x00)

0x00000061:       DW_TAG_lexical_block
                    DW_AT_low_pc        (0x0000000001014075)
                    DW_AT_high_pc       (0x000000000101407a)

0x0000006e:       NULL

0x0000007e:     DW_TAG_subprogram
                  DW_AT_ZIG_parent      (0x000000000000015c)
                  DW_AT_abstract_origin (0x000000000000002d "print")
                  DW_AT_linkage_name    ("main.Printer(\"bar\\n\"[0..4]).print")
                  DW_AT_type    (0x0000000000094e7d "std::@typeInfo(@typeInfo(@TypeOf(main.Printer("bar\n"[0..4]).print)).@"fn".return_type.?).error_union.error_set!void")
                  DW_AT_low_pc  (0x00000000010140b5)
                  DW_AT_high_pc (0x0000000001014124)
                  DW_AT_alignment       (16)
                  DW_AT_external        (0x00)
                  DW_AT_noreturn        (0x00)

0x0000009e:       DW_TAG_lexical_block
                    DW_AT_low_pc        (0x0000000001014109)
                    DW_AT_high_pc       (0x000000000101410e)

0x000000ab:       NULL

Which when debugged again displays:

* thread #1, name = 'main', stop reason = breakpoint 1.1
    frame #0: 0x000000000101415a main`main.main at main.zig:11:31
   8   	   };
   9   	}
   10  	pub fn main() !void {
-> 11  	   try Printer("foo\n").print();
   12  	   try Printer("bar\n").print();
   13  	}
(lldb) s
Process 366903 stopped
* thread #1, name = 'main', stop reason = step in
    frame #0: 0x0000000001014032 main`main.Printer("foo\n"[0..4]).print at main.zig:6:33
   3   	fn Printer(message: []const u8) type {
   4   	   return struct {
   5   	       fn print() !void {
-> 6   	           try std.io.getStdOut().writeAll(message);
   7   	       }
   8   	   };
   9   	}
(lldb) n
foo
Process 366903 stopped
* thread #1, name = 'main', stop reason = step over
    frame #0: 0x000000000101415f main`main.main at main.zig:11:31
   8   	   };
   9   	}
   10  	pub fn main() !void {
-> 11  	   try Printer("foo\n").print();
   12  	   try Printer("bar\n").print();
   13  	}
(lldb) s
Process 366903 stopped
* thread #1, name = 'main', stop reason = step in
    frame #0: 0x0000000001014177 main`main.main at main.zig:12:31
   9   	}
   10  	pub fn main() !void {
   11  	   try Printer("foo\n").print();
-> 12  	   try Printer("bar\n").print();
   13  	}
(lldb) s
Process 366903 stopped
* thread #1, name = 'main', stop reason = step in
    frame #0: 0x00000000010140c6 main`main.Printer("bar\n"[0..4]).print at main.zig:6:33
   3   	fn Printer(message: []const u8) type {
   4   	   return struct {
   5   	       fn print() !void {
-> 6   	           try std.io.getStdOut().writeAll(message);
   7   	       }
   8   	   };
   9   	}
(lldb) n
bar
Process 366903 stopped
* thread #1, name = 'main', stop reason = step over
    frame #0: 0x000000000101417c main`main.main at main.zig:12:31
   9   	}
   10  	pub fn main() !void {
   11  	   try Printer("foo\n").print();
-> 12  	   try Printer("bar\n").print();
   13  	}