internal/gen: work around long string concatenation.

The compiler has trouble compiling the concatenation of many strings, s0 + s1 +
s2 + ... + sN, for large N. We insert redundant, explicit parentheses to work
around that, lowering the N at any given step.

Some before/after's on GOOS=linux GOARCH=amd64 from
/usr/bin/time -v go install golang.org/x/text/unicode/runenames

Elapsed (wall clock) time (h:mm:ss or m:ss): 0:02.21
Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.33

Maximum resident set size (kbytes): 4414556
Maximum resident set size (kbytes): 135040

Minor (reclaiming a frame) page faults: 1071428
Minor (reclaiming a frame) page faults: 40242

Fixes golang/go#18078

Change-Id: I6f0bfcc26ae4f1c8fea010a2ee3d815567426b2d
Reviewed-on: https://go-review.googlesource.com/33598
Reviewed-by: Matthew Dempsky <[email protected]>
Run-TryBot: Brad Fitzpatrick <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: nigeltao

internal/gen/code.go

@@ -197,16 +197,27 @@ func (w *CodeWriter) WriteString(s string) {
 	// When starting on its own line, go fmt indents line 2+ an extra level.
 	n, max := maxWidth, maxWidth-4
 
+	// As per https://golang.org/issue/18078, the compiler has trouble
+	// compiling the concatenation of many strings, s0 + s1 + s2 + ... + sN,
+	// for large N. We insert redundant, explicit parentheses to work around
+	// that, lowering the N at any given step: (s0 + s1 + ... + s63) + (s64 +
+	// ... + s127) + etc + (etc + ... + sN).
+	explicitParens, extraComment := len(s) > 128*1024, ""
+	if explicitParens {
+		w.printf(`(`)
+		extraComment = "; the redundant, explicit parens are for https://golang.org/issue/18078"
+	}
+
 	// Print "" +\n, if a string does not start on its own line.
 	b := w.buf.Bytes()
 	if p := len(bytes.TrimRight(b, " \t")); p > 0 && b[p-1] != '\n' {
-		w.printf("\"\" + // Size: %d bytes\n", len(s))
+		w.printf("\"\" + // Size: %d bytes%s\n", len(s), extraComment)
 		n, max = maxWidth, maxWidth
 	}
 
 	w.printf(`"`)
 
-	for sz, p := 0, 0; p < len(s); {
+	for sz, p, nLines := 0, 0, 0; p < len(s); {
 		var r rune
 		r, sz = utf8.DecodeRuneInString(s[p:])
 		out := s[p : p+sz]
@@ -223,13 +234,20 @@ func (w *CodeWriter) WriteString(s string) {
 			chars = len(out)
 		}
 		if n -= chars; n < 0 {
+			nLines++
+			if explicitParens && nLines&63 == 63 {
+				w.printf("\") + (\"")
+			}
 			w.printf("\" +\n\"")
 			n = max - len(out)
 		}
 		w.printf("%s", out)
 		p += sz
 	}
 	w.printf(`"`)
+	if explicitParens {
+		w.printf(`)`)
+	}
 }
 
 // WriteSlice writes a slice value.