---
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.

    // * The element with (index == from) should be kept.
    // * Everything with (index > from) has not been checked yet.

    // Check from the end of the list backwards (minimize expected cost of
    // moving elements when remove() is called). Stop before 'from' because
    // we already know that should be kept.
    for (int n = list.size() - 1; n > from; n--) {

                    .get());
            options.addOption(Option.builder(BATCH_MODE)
                    .longOpt("batch-mode")
                    .desc("Run in non-interactive mode. Alias for --non-interactive (kept for backwards compatability)")
                    .get());
            options.addOption(Option.builder()
                    .longOpt(NON_INTERACTIVE)
                    .desc("Run in non-interactive mode. Alias for --batch-mode")

              <include>org.apache.maven.toolchain</include>
              <include>org.apache.maven.usability</include>
            </includes>
            <!-- allowed non-binary backwards compatible changes -->
            <excludes>
              <!-- START default constructor on Plexus/JSR 330 components -->

                .build());
        options.addOption(Option.builder(Character.toString(BATCH_MODE))
                .longOpt("batch-mode")
                .desc("Run in non-interactive mode. Alias for --non-interactive (kept for backwards compatability)")
                .build());
        options.addOption(Option.builder()
                .longOpt(NON_INTERACTIVE)
                .desc("Run in non-interactive mode. Alias for --batch-mode")

        // First byte should be length of "test" (4) after processing
        assertEquals(4, dst[0]);

        // After processing, dots should be replaced with length markers
        // The scope processing happens backwards
        assertEquals(name.scope.length() + 2, length);
    }

    @Test
    void readScopeWireFormat_withMultipleLabels_shouldParseCorrectly() {
        byte[] src = new byte[100];

			// It looks like this should allow listing any key that isn't "default-test-key", however
			// the policy engine matches all Deny statements first, without regard to Resources (for KMS).
			// This is for backwards compatibility where historically KMS statements ignored Resources.
			policy: `{
						"Effect": "Allow",
						"Action": ["kms:ListKeys"]
					},{
						"Effect": "Deny",
						"Action": ["kms:ListKeys"],

	// 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

      // Both a and b are odd. Assume a > b; then gcd(a - b, b) = gcd(a, b).
      // But in gcd(a - b, b), a - b is even and b is odd, so we can divide out powers of two.

      // We bend over backwards to avoid branching, adapting a technique from
      // http://graphics.stanford.edu/~seander/bithacks.html#IntegerMinOrMax

      long delta = a - b; // can't overflow, since a and b are nonnegative

		last := n - 1
		i := last
		fails := 0
		for i < len(s) {
			if s[i] != b[last] {
				o := IndexByte(s[i+1:], b[last])
				if o < 0 {
					return -1
				}
				i += o + 1
			}
			// Step backwards comparing bytes.
			for j := 1; j < n; j++ {
				if s[i-j] != b[last-j] {
					goto next
				}
			}
			return i - last
		next:
			fails++
			i++