FastRegexMatcher: use stack memory for lowercase copy of string

Up to 32-byte values this saves garbage, runs faster.
For prefixes, only `toLower` the part we need for the map lookup.

Split toNormalisedLower into fast and slow paths, to avoid a penalty
for the `copy` call in the case where no allocations are done.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

model/labels/regexp.go

@@ -802,7 +802,7 @@ type equalMultiStringMapMatcher struct {
 
 func (m *equalMultiStringMapMatcher) add(s string) {
 	if !m.caseSensitive {
-		s = toNormalisedLower(s)
+		s = toNormalisedLower(s, nil) // Don't pass a stack buffer here - it will always escape to heap.
 	}
 
 	m.values[s] = struct{}{}
@@ -840,15 +840,24 @@ func (m *equalMultiStringMapMatcher) setMatches() []string {
 }
 
 func (m *equalMultiStringMapMatcher) Matches(s string) bool {
-	if !m.caseSensitive {
-		s = toNormalisedLower(s)
+	if len(m.values) > 0 {
+		sNorm := s
+		var a [32]byte
+		if !m.caseSensitive {
+			sNorm = toNormalisedLower(s, a[:])
+		}
+		if _, ok := m.values[sNorm]; ok {
+			return true
+		}
 	}
 
-	if _, ok := m.values[s]; ok {
-		return true
-	}
 	if m.minPrefixLen > 0 && len(s) >= m.minPrefixLen {
-		for _, matcher := range m.prefixes[s[:m.minPrefixLen]] {
+		prefix := s[:m.minPrefixLen]
+		var a [32]byte
+		if !m.caseSensitive {
+			prefix = toNormalisedLower(s[:m.minPrefixLen], a[:])
+		}
+		for _, matcher := range m.prefixes[prefix] {
 			if matcher.Matches(s) {
 				return true
 			}
@@ -859,22 +868,37 @@ func (m *equalMultiStringMapMatcher) Matches(s string) bool {
 
 // toNormalisedLower normalise the input string using "Unicode Normalization Form D" and then convert
 // it to lower case.
-func toNormalisedLower(s string) string {
-	var buf []byte
+func toNormalisedLower(s string, a []byte) string {
 	for i := 0; i < len(s); i++ {
 		c := s[i]
 		if c >= utf8.RuneSelf {
 			return strings.Map(unicode.ToLower, norm.NFKD.String(s))
 		}
 		if 'A' <= c && c <= 'Z' {
-			if buf == nil {
-				buf = []byte(s)
-			}
-			buf[i] = c + 'a' - 'A'
+			return toNormalisedLowerSlow(s, i, a)
 		}
 	}
-	if buf == nil {
-		return s
+	return s
+}
+
+// toNormalisedLowerSlow is split from toNormalisedLower because having a call
+// to `copy` slows it down even when it is not called.
+func toNormalisedLowerSlow(s string, i int, a []byte) string {
+	var buf []byte
+	if cap(a) > len(s) {
+		buf = a[:len(s)]
+		copy(buf, s)
+	} else {
+		buf = []byte(s)
+	}
+	for ; i < len(s); i++ {
+		c := s[i]
+		if c >= utf8.RuneSelf {
+			return strings.Map(unicode.ToLower, norm.NFKD.String(s))
+		}
+		if 'A' <= c && c <= 'Z' {
+			buf[i] = c + 'a' - 'A'
+		}
 	}
 	return yoloString(buf)
 }

model/labels/regexp_test.go

@@ -333,7 +333,8 @@ func BenchmarkToNormalizedLower(b *testing.B) {
 							}
 							b.ResetTimer()
 							for n := 0; n < b.N; n++ {
-								toNormalisedLower(inputs[n%len(inputs)])
+								var a [256]byte
+								toNormalisedLower(inputs[n%len(inputs)], a[:])
 							}
 						})
 					}
@@ -1390,6 +1391,6 @@ func TestToNormalisedLower(t *testing.T) {
 		"ſſAſſa": "ssassa",
 	}
 	for input, expectedOutput := range testCases {
-		require.Equal(t, expectedOutput, toNormalisedLower(input))
+		require.Equal(t, expectedOutput, toNormalisedLower(input, nil))
 	}
 }