[runtime/metrics](/pkg/runtime/metrics/) counter
named `/godebug/non-default-behavior/<name>:events`
that counts the number of times a particular program's
behavior has changed based on a non-default value
for that setting.
For example, when `GODEBUG=http2client=0` is set,
`/godebug/non-default-behavior/http2client:events`
counts the number of HTTP transports that the program
has configured without HTTP/2 support.

## Default GODEBUG Values {#default}

pkg go/doc/comment, type List struct, Items []*ListItem #51082
pkg go/doc/comment, type ListItem struct #51082
pkg go/doc/comment, type ListItem struct, Content []Block #51082
pkg go/doc/comment, type ListItem struct, Number string #51082
pkg go/doc/comment, type Paragraph struct #51082
pkg go/doc/comment, type Paragraph struct, Text []Text #51082
pkg go/doc/comment, type Parser struct #51082

	&& !config.noDuffDevice && logLargeCopy(v, s)  =>
	(DUFFCOPY [16 * (128 - s/8)] dst src mem)
// 16 and 128 are magic constants.  16 is the number of bytes to encode:
//	MOVV	(R1), R23
//	ADDV	$8, R1
//	MOVV	R23, (R2)
//	ADDV	$8, R2
// and 128 is the number of such blocks. See runtime/duff_mips64.s:duffcopy.

// large or unaligned move uses a loop
(Move [s] {t} dst src mem)

	&& !config.noDuffDevice && logLargeCopy(v, s)  =>
	(DUFFCOPY [16 * (128 - s/8)] dst src mem)
// 16 and 128 are magic constants.  16 is the number of bytes to encode:
//	MOVV	(R1), R23
//	ADDV	$8, R1
//	MOVV	R23, (R2)
//	ADDV	$8, R2
// and 128 is the number of such blocks. See runtime/duff_mips64.s:duffcopy.

// large or unaligned move uses a loop
(Move [s] {t} dst src mem)

// Strip off any fractional word zeroing.
(Zero [s] destptr mem) && s%8 != 0 && s > 8 && s < 32 =>
	(Zero [s-s%8] (OffPtr <destptr.Type> destptr [s%8])
		(I64Store destptr (I64Const [0]) mem))

// Zero small numbers of words directly.
(Zero [16] destptr mem) =>
	(I64Store [8] destptr (I64Const [0])
		(I64Store destptr (I64Const [0]) mem))
(Zero [24] destptr mem) =>
	(I64Store [16] destptr (I64Const [0])