description: "Please also describe the change informally, as in a class teaching Go."
    validations:
      required: false

  - type: textarea
    id: go-backwards-compatiblity
    attributes:
      label: Is this change backward compatible?
      description: Breaking the Go 1 compatibility guarantee is a large cost and requires a large benefit.
      placeholder: |
       Yes or No

       If yes, 

imaginary_lit = (decimal_digits | int_lit | float_lit) "i" .
</pre>

<p>
For backward compatibility, an imaginary literal's integer part consisting
entirely of decimal digits (and possibly underscores) is considered a decimal
integer, even if it starts with a leading <code>0</code>.
</p>

<pre>
0i
0123i         // == 123i for backward-compatibility
0o123i        // == 0o123 * 1i == 83i
0xabci        // == 0xabc * 1i == 2748i

// they follow the rules for passing pointers between Go and C.
// This reports whether the package needs to import unsafe as _cgo_unsafe.
func (p *Package) rewriteCalls(f *File) bool {
	needsUnsafe := false
	// Walk backward so that in C.f1(C.f2()) we rewrite C.f2 first.
	for _, call := range f.Calls {
		if call.Done {
			continue
		}
		start := f.offset(call.Call.Pos())
		end := f.offset(call.Call.End())

---
title: "Go, Backwards Compatibility, and GODEBUG"
layout: article
---

<!--
This document is kept in the Go repo, not x/website,
because it documents the full list of known GODEBUG settings,
which are tied to a specific release.
-->

## Introduction {#intro}

Go's emphasis on backwards compatibility is one of its key strengths.
There are, however, times when we cannot maintain complete compatibility.

/*
Package zip provides support for reading and writing ZIP archives.

See the [ZIP specification] for details.

This package does not support disk spanning.

A note about ZIP64:

To be backwards compatible the FileHeader has both 32 and 64 bit Size
fields. The 64 bit fields will always contain the correct value and
for normal archives both fields will be the same. For files requiring

imaginary_lit = (decimal_digits | int_lit | float_lit) "i" .
</pre>

<p>
For backward compatibility, an imaginary literal's integer part consisting
entirely of decimal digits (and possibly underscores) is considered a decimal
integer, even if it starts with a leading <code>0</code>.
</p>

<pre>
0i
0123i         // == 123i for backward-compatibility
0o123i        // == 0o123 * 1i == 83i
0xabci        // == 0xabc * 1i == 2748i

	// Hex constant 0xFFFFFFFE00000002 (load of constant on < power10, pli on >= power10
	MOVD $-8589934590, R5           // 3ca00000e8a50000 or 0602000038a00002

	// For backwards compatibility, MOVW $const,Rx and MOVWZ $const,Rx assemble identically
	// and accept the same constants.
	MOVW $2147483648, R5            // 64058000
	MOVWZ $-2147483648, R5          // 3ca08000

		[]byte("ab\xc0a\xc0")},
	{not(isValidRune), "\xc0a\xc0",
		[]byte("a"),
		[]byte("a\xc0"),
		[]byte("\xc0a")},
	// The nils returned by TrimLeftFunc are odd behavior, but we need
	// to preserve backwards compatibility.
	{isSpace, "",
		nil,
		nil,
		[]byte("")},
	{isSpace, " ",
		nil,
		nil,
		[]byte("")},
}

func TestTrimFunc(t *testing.T) {

	MOVQ M0, 8(SP) // 0f7f442408
	MOVQ 8(SP), M0 // 0f6f442408
	MOVQ M0, (AX)  // 0f7f00
	MOVQ M0, (BX)  // 0f7f03
	// On non-64bit arch, Go asm allowed uint32 offsets instead of int32.
	// These tests check that property for backwards-compatibility.
	MOVL 2147483648(AX), AX  // 8b8000000080
	MOVL -2147483648(AX), AX // 8b8000000080
	ADDL 2147483648(AX), AX  // 038000000080
	ADDL -2147483648(AX), AX // 038000000080

)

// IsRegisterShift reports whether the token is one of the ARM register shift operators.
func IsRegisterShift(r ScanToken) bool {
	return ROT <= r && r <= LSH // Order looks backwards because these are negative.
}

func (t ScanToken) String() string {
	switch t {
	case scanner.EOF:
		return "EOF"
	case scanner.Ident:
		return "identifier"
	case scanner.Int:
		return "integer constant"