can include one or more `//go:debug` directives at the top of the file
(preceding the `package` statement).
Continuing the `panicnil` example, if the module or workspace is updated
to say `go` `1.21`, the program can opt back into the old `panic(nil)`
behavior by including this directive:

	//go:debug panicnil=1

Starting in Go 1.21, the Go toolchain treats a `//go:debug` directive

invoking the C compiler to compile the C parts of the package.

The following options are available when running cgo directly:

	-V
		Print cgo version and exit.
	-debug-define
		Debugging option. Print #defines.
	-debug-gcc
		Debugging option. Trace C compiler execution and output.
	-dynimport file
		Write list of symbols imported by file. Write to
		-dynout argument or to standard output. Used by go

	if symName != funcName {
		t.Fatalf("expected function name %v; got %v", symName, funcName)
	}
	fi1, err := os.Stat("addr2line_test.go")
	if err != nil {
		t.Fatalf("Stat failed: %v", err)
	}

	// Debug paths are stored slash-separated, so convert to system-native.
	srcPath = filepath.FromSlash(srcPath)
	fi2, err := os.Stat(srcPath)

	if err != nil {
		t.Fatalf("Stat failed: %v", err)
	}

The `debug/elf` package now defines [PT_OPENBSD_NOBTCFI]. This [ProgType] is
used to disable Branch Tracking Control Flow Integrity (BTCFI) enforcement

pkg debug/elf, const R_LARCH_CFA R_LARCH #63725
pkg debug/elf, const R_LARCH_DELETE = 101 #63725
pkg debug/elf, const R_LARCH_DELETE R_LARCH #63725
pkg debug/elf, const R_LARCH_PCREL20_S2 = 103 #63725
pkg debug/elf, const R_LARCH_PCREL20_S2 R_LARCH #63725
pkg debug/elf, const R_LARCH_SUB6 = 106 #63725
pkg debug/elf, const R_LARCH_SUB6 R_LARCH #63725
pkg debug/elf, const R_LARCH_SUB_ULEB128 = 108 #63725

pkg debug/elf, const STT_GNU_IFUNC = 10 #66836
pkg debug/elf, const STT_GNU_IFUNC SymType #66836
pkg debug/elf, const STT_RELC = 8 #66836
pkg debug/elf, const STT_RELC SymType #66836
pkg debug/elf, const STT_SRELC = 9 #66836

pkg debug/elf, const PT_OPENBSD_NOBTCFI = 1705237480 #66054

pkg runtime/debug, func SetCrashOutput(*os.File) error #42888...

and less like C and C++, where the meaning of any program with a race
is entirely undefined, and the compiler may do anything at all.
Go's approach aims to make errant programs more reliable and easier to debug,
while still insisting that races are errors and that tools can diagnose and report them.
</p>

<h2 id="model">Memory Model</h2>

<p>
The following formal definition of Go's memory model closely follows

			if p.labels[label] != nil {
				p.errorf("label %q multiply defined", label)
				return
			}
			p.labels[label] = prog
		}
		p.pendingLabels = p.pendingLabels[0:0]
	}
	prog.Pc = p.pc
	if *flags.Debug {
		fmt.Println(p.lineNum, prog)
	}
	if testOut != nil {
		fmt.Fprintln(testOut, prog)
	}
}

// validSymbol checks that addr represents a valid name for a pseudo-op.