parser: optimize memory allocation for command word parsing

This could be optimized further to reuse buffers between
commands but that would require chaning signature for parser
dispatch function.

Signed-off-by: Tonis Tiigi <[email protected]>

Author: tonistiigi

frontend/dockerfile/parser/line_parsers.go

@@ -59,11 +59,11 @@ func parseWords(rest string, d *directives) []string {
 
 	words := []string{}
 	phase := inSpaces
-	word := ""
 	quote := '\000'
 	blankOK := false
 	var ch rune
 	var chWidth int
+	var sbuilder strings.Builder
 
 	for pos := 0; pos <= len(rest); pos += chWidth {
 		if pos != len(rest) {
@@ -80,18 +80,18 @@ func parseWords(rest string, d *directives) []string {
 			phase = inWord // found it, fall through
 		}
 		if (phase == inWord || phase == inQuote) && (pos == len(rest)) {
-			if blankOK || len(word) > 0 {
-				words = append(words, word)
+			if blankOK || sbuilder.Len() > 0 {
+				words = append(words, sbuilder.String())
 			}
 			break
 		}
 		if phase == inWord {
 			if unicode.IsSpace(ch) {
 				phase = inSpaces
-				if blankOK || len(word) > 0 {
-					words = append(words, word)
+				if blankOK || sbuilder.Len() > 0 {
+					words = append(words, sbuilder.String())
 				}
-				word = ""
+				sbuilder.Reset()
 				blankOK = false
 				continue
 			}
@@ -107,11 +107,11 @@ func parseWords(rest string, d *directives) []string {
 				// If we're not quoted and we see an escape token, then always just
 				// add the escape token plus the char to the word, even if the char
 				// is a quote.
-				word += string(ch)
+				sbuilder.WriteRune(ch)
 				pos += chWidth
 				ch, chWidth = utf8.DecodeRuneInString(rest[pos:])
 			}
-			word += string(ch)
+			sbuilder.WriteRune(ch)
 			continue
 		}
 		if phase == inQuote {
@@ -125,10 +125,10 @@ func parseWords(rest string, d *directives) []string {
 					continue // just skip the escape token at end
 				}
 				pos += chWidth
-				word += string(ch)
+				sbuilder.WriteRune(ch)
 				ch, chWidth = utf8.DecodeRuneInString(rest[pos:])
 			}
-			word += string(ch)
+			sbuilder.WriteRune(ch)
 		}
 	}