Reduce stack pressure due to array copies

Ports golang/go#18625

README.md

@@ -12,6 +12,7 @@ It offers slightly better compression at lower compression settings, and up to 3
 [![Build Status](https://travis-ci.org/klauspost/compress.svg?branch=master)](https://travis-ci.org/klauspost/compress)
 
 # changelog
+* Jan 14, 2017: Reduce stack pressure due to array copies. See [Issue #18625(https://github.com/golang/go/issues/18625).
 * Oct 25, 2016: Level 2-4 have been rewritten and now offers significantly better performance than before.
 * Oct 20, 2016: Port zlib changes from Go 1.7 to fix zlib writer issue. Please update.
 * Oct 16, 2016: Go 1.7 changes merged. Apples to apples this package is a few percent faster, but has a significantly better balance between speed and compression per level. 

flate/deflate.go

@@ -136,14 +136,16 @@ func (d *compressor) fillDeflate(b []byte) int {
 			delta := d.hashOffset - 1
 			d.hashOffset -= delta
 			d.chainHead -= delta
-			for i, v := range d.hashPrev {
+			// Iterate over slices instead of arrays to avoid copying
+			// the entire table onto the stack (Issue #18625).
+			for i, v := range d.hashPrev[:] {
 				if int(v) > delta {
 					d.hashPrev[i] = uint32(int(v) - delta)
 				} else {
 					d.hashPrev[i] = 0
 				}
 			}
-			for i, v := range d.hashHead {
+			for i, v := range d.hashHead[:] {
 				if int(v) > delta {
 					d.hashHead[i] = uint32(int(v) - delta)
 				} else {

flate/snappy.go

@@ -255,7 +255,7 @@ func (e *snappyL2) Encode(dst *tokens, src []byte) {
 
 	// Protect against e.cur wraparound.
 	if e.cur > 1<<30 {
-		for i := range e.table {
+		for i := range e.table[:] {
 			e.table[i] = tableEntry{}
 		}
 		e.cur = maxStoreBlockSize
@@ -416,7 +416,7 @@ func (e *snappyL3) Encode(dst *tokens, src []byte) {
 
 	// Protect against e.cur wraparound.
 	if e.cur > 1<<30 {
-		for i := range e.table {
+		for i := range e.table[:] {
 			e.table[i] = tableEntryPrev{}
 		}
 		e.snappyGen = snappyGen{cur: maxStoreBlockSize, prev: e.prev[:0]}
@@ -625,7 +625,7 @@ func (e *snappyL4) Encode(dst *tokens, src []byte) {
 
 	// Protect against e.cur wraparound.
 	if e.cur > 1<<30 {
-		for i := range e.table {
+		for i := range e.table[:] {
 			e.table[i] = tableEntryPrev{}
 		}
 		e.snappyGen = snappyGen{cur: maxStoreBlockSize, prev: e.prev[:0]}