input to the linker.
If you want to see what the instructions look like in assembly for a given architecture, say amd64, there
are many examples in the sources of the standard library, in packages such as
<a href="/pkg/runtime/"><code>runtime</code></a> and
<a href="/pkg/math/big/"><code>math/big</code></a>.
You can also examine what the compiler emits as assembly code
(the actual output may differ from what you see here):
</p>

<pre>
$ cat x.go

	for len(s) > 0 && s[0] == c {
		s = s[1:]
	}
	if len(s) == 0 {
		// This is what we've historically done.
		return nil
	}
	return s
}

func trimLeftASCII(s []byte, as *asciiSet) []byte {
	for len(s) > 0 {
		if !as.contains(s[0]) {
			break
		}
		s = s[1:]
	}
	if len(s) == 0 {
		// This is what we've historically done.
		return nil
	}
	return s
}

// scanners.
func (s *Scanner) Scan() bool {
	if s.done {
		return false
	}
	s.scanCalled = true
	// Loop until we have a token.
	for {
		// See if we can get a token with what we already have.
		// If we've run out of data but have an error, give the split function
		// a chance to recover any remaining, possibly empty token.
		if s.end > s.start || s.err != nil {

package asm

import (
	"internal/buildcfg"
	"strings"
	"testing"

	"cmd/asm/internal/arch"
	"cmd/asm/internal/lex"
	"cmd/internal/obj"
)

// A simple in-out test: Do we print what we parse?

func setArch(goarch string) (*arch.Arch, *obj.Link) {
	buildcfg.GOOS = "linux" // obj can handle this OS for all architectures.
	buildcfg.GOARCH = goarch
	architecture := arch.Set(goarch, false)

	// compiled by gcc, into foo.cgo2.o

	void
	_cgo_gcc_Cfunc_sin(void *v)
	{
		struct {
			double p0;
			double r;
		} __attribute__((__packed__)) *a = v;
		a->r = sin(a->p0);
	}

What happens at link time depends on whether the final binary is linked
using the internal or external mode. If other packages are compiled in
"external only" mode, then the final link will be an external one.

				} else {
					err = err1
				}
			} else if r.hash.Sum32() != r.f.CRC32 {
				err = ErrChecksum
			}
		} else {
			// If there's not a data descriptor, we still compare
			// the CRC32 of what we've read against the file header
			// or TOC's CRC32, if it seems like it was set.
			if r.f.CRC32 != 0 && r.hash.Sum32() != r.f.CRC32 {
				err = ErrChecksum
			}
		}
	}
	r.err = err
	return
}

		m = b.grow(len(s))
	}
	return copy(b.buf[m:], s), nil
}

// MinRead is the minimum slice size passed to a Read call by
// [Buffer.ReadFrom]. As long as the [Buffer] has at least MinRead bytes beyond
// what is required to hold the contents of r, ReadFrom will not grow the
// underlying buffer.
const MinRead = 512

// ReadFrom reads data from r until EOF and appends it to the buffer, growing

//
// Calling Write on special types like [TypeLink], [TypeSymlink], [TypeChar],
// [TypeBlock], [TypeDir], and [TypeFifo] returns (0, [ErrWriteTooLong]) regardless
// of what the [Header.Size] claims.
func (tw *Writer) Write(b []byte) (int, error) {
	if tw.err != nil {
		return 0, tw.err
	}
	n, err := tw.curr.Write(b)
	if err != nil && err != ErrWriteTooLong {
		tw.err = err
	}

			hdr.Format = format
			ustar := tr.blk.toUSTAR()
			prefix = p.parseString(ustar.prefix())

			// For Format detection, check if block is properly formatted since
			// the parser is more liberal than what USTAR actually permits.
			notASCII := func(r rune) bool { return r >= 0x80 }
			if bytes.IndexFunc(tr.blk[:], notASCII) >= 0 {
				hdr.Format = FormatUnknown // Non-ASCII characters in block.
			}

	}
	// General address (with a few exceptions) looks like
	//	$sym±offset(SB)(reg)(index*scale)
	// Exceptions are:
	//
	//	R1
	//	offset
	//	$offset
	// Every piece is optional, so we scan left to right and what
	// we discover tells us where we are.

	// Prefix: $.
	var prefix rune
	switch tok := p.peek(); tok {
	case '$', '*':
		prefix = rune(tok)
		p.next()
	}

	// Symbol: sym±offset(SB)