cmd/internal/obj/x86: clean up asm buffer

c2go translated writing and advancing a pointer using slices.
Switch to something more idiomatic.
It is also more efficient, but not enough to matter.

Change-Id: I67709632ac53253615a35365824ae97bbe5458d5
Reviewed-on: https://go-review.googlesource.com/20767
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/internal/obj/link.go

@@ -635,8 +635,7 @@ type Link struct {
 	Repn          int
 	Lock          int
 	Asmode        int
-	Andptr        []byte
+	AsmBuf        AsmBuf // instruction buffer for x86
-	And           [100]uint8
 	Instoffset    int64
 	Autosize      int32
 	Armsize       int32
@@ -737,3 +736,96 @@ func Linknewplist(ctxt *Link) *Plist {
 	ctxt.Plast = pl
 	return pl
 }
+
+// AsmBuf is a simple buffer to assemble variable-length x86 instructions into.
+type AsmBuf struct {
+	buf [100]byte
+	off int
+}
+
+// Put1 appends one byte to the end of the buffer.
+func (a *AsmBuf) Put1(x byte) {
+	a.buf[a.off] = x
+	a.off++
+}
+
+// Put2 appends two bytes to the end of the buffer.
+func (a *AsmBuf) Put2(x, y byte) {
+	a.buf[a.off+0] = x
+	a.buf[a.off+1] = y
+	a.off += 2
+}
+
+// Put3 appends three bytes to the end of the buffer.
+func (a *AsmBuf) Put3(x, y, z byte) {
+	a.buf[a.off+0] = x
+	a.buf[a.off+1] = y
+	a.buf[a.off+2] = z
+	a.off += 3
+}
+
+// Put4 appends four bytes to the end of the buffer.
+func (a *AsmBuf) Put4(x, y, z, w byte) {
+	a.buf[a.off+0] = x
+	a.buf[a.off+1] = y
+	a.buf[a.off+2] = z
+	a.buf[a.off+3] = w
+	a.off += 4
+}
+
+// PutInt16 writes v into the buffer using little-endian encoding.
+func (a *AsmBuf) PutInt16(v int16) {
+	a.buf[a.off+0] = byte(v)
+	a.buf[a.off+1] = byte(v >> 8)
+	a.off += 2
+}
+
+// PutInt32 writes v into the buffer using little-endian encoding.
+func (a *AsmBuf) PutInt32(v int32) {
+	a.buf[a.off+0] = byte(v)
+	a.buf[a.off+1] = byte(v >> 8)
+	a.buf[a.off+2] = byte(v >> 16)
+	a.buf[a.off+3] = byte(v >> 24)
+	a.off += 4
+}
+
+// PutInt64 writes v into the buffer using little-endian encoding.
+func (a *AsmBuf) PutInt64(v int64) {
+	a.buf[a.off+0] = byte(v)
+	a.buf[a.off+1] = byte(v >> 8)
+	a.buf[a.off+2] = byte(v >> 16)
+	a.buf[a.off+3] = byte(v >> 24)
+	a.buf[a.off+4] = byte(v >> 32)
+	a.buf[a.off+5] = byte(v >> 40)
+	a.buf[a.off+6] = byte(v >> 48)
+	a.buf[a.off+7] = byte(v >> 56)
+	a.off += 8
+}
+
+// Put copies b into the buffer.
+func (a *AsmBuf) Put(b []byte) {
+	copy(a.buf[a.off:], b)
+	a.off += len(b)
+}
+
+// Insert inserts b at offset i.
+func (a *AsmBuf) Insert(i int, b byte) {
+	a.off++
+	copy(a.buf[i+1:a.off], a.buf[i:a.off-1])
+	a.buf[i] = b
+}
+
+// Last returns the byte at the end of the buffer.
+func (a *AsmBuf) Last() byte { return a.buf[a.off-1] }
+
+// Len returns the length of the buffer.
+func (a *AsmBuf) Len() int { return a.off }
+
+// Bytes returns the contents of the buffer.
+func (a *AsmBuf) Bytes() []byte { return a.buf[:a.off] }
+
+// Reset empties the buffer.
+func (a *AsmBuf) Reset() { a.off = 0 }
+
+// Peek returns the byte at offset i.
+func (a *AsmBuf) Peek(i int) byte { return a.buf[i] }