net/http: optimize StatusText implementation

The current implementation, although more succinct, relies on a runtime
lookup to a "constant" unexported map (which also needs to be
initialized at runtime).

The proposed implementation is able to be optimized by the compiler at
build-time, resulting in *much* more efficient instructions.
Additionally, unused string literals may even be removed altogether
from the generated binary in some cases.

This change is fully backwards-compatible behavior-wise with the
existing implementation.

Change-Id: I36450320aacff5b322195820552f2831d4fecd52
GitHub-Last-Rev: e2058f132ef7a193529d4b0e84329ac93e5d1dcb
GitHub-Pull-Request: golang/go#49811
Reviewed-on: https://go-review.googlesource.com/c/go/+/367201
Run-TryBot: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

3 files changed, 132 insertions, 75 deletions

diff --git a/src/net/http/response.go b/src/net/http/response.go
index eb4cd9b0ad..755c696557 100644
--- a/src/net/http/response.go
+++ b/src/net/http/response.go
@@ -246,9 +246,8 @@ func (r *Response) Write(w io.Writer) error {
 	// Status line
 	text := r.Status
 	if text == "" {
-		var ok bool
-		text, ok = statusText[r.StatusCode]
-		if !ok {
+		text = StatusText(r.StatusCode)
+		if text == "" {
 			text = "status code " + strconv.Itoa(r.StatusCode)
 		}
 	} else {
diff --git a/src/net/http/server.go b/src/net/http/server.go
index 62bdf16959..d44b0fb256 100644
--- a/src/net/http/server.go
+++ b/src/net/http/server.go
@@ -1516,7 +1516,7 @@ func writeStatusLine(bw *bufio.Writer, is11 bool, code int, scratch []byte) {
 	} else {
 		bw.WriteString("HTTP/1.0 ")
 	}
-	if text, ok := statusText[code]; ok {
+	if text := StatusText(code); text != "" {
 		bw.Write(strconv.AppendInt(scratch[:0], int64(code), 10))
 		bw.WriteByte(' ')
 		bw.WriteString(text)
@@ -2192,7 +2192,7 @@ func Redirect(w ResponseWriter, r *Request, url string, code int) {
 
 	// Shouldn't send the body for POST or HEAD; that leaves GET.
 	if !hadCT && r.Method == "GET" {
-		body := "<a href=\"" + htmlEscape(url) + "\">" + statusText[code] + "</a>.\n"
+		body := "<a href=\"" + htmlEscape(url) + "\">" + StatusText(code) + "</a>.\n"
 		fmt.Fprintln(w, body)
 	}
 }
diff --git a/src/net/http/status.go b/src/net/http/status.go
index 286315f639..75fea0ca35 100644
--- a/src/net/http/status.go
+++ b/src/net/http/status.go
@@ -76,77 +76,135 @@ const (
 	StatusNetworkAuthenticationRequired = 511 // RFC 6585, 6
 )
 
-var statusText = map[int]string{
-	StatusContinue:           "Continue",
-	StatusSwitchingProtocols: "Switching Protocols",
-	StatusProcessing:         "Processing",
-	StatusEarlyHints:         "Early Hints",
-
-	StatusOK:                   "OK",
-	StatusCreated:              "Created",
-	StatusAccepted:             "Accepted",
-	StatusNonAuthoritativeInfo: "Non-Authoritative Information",
-	StatusNoContent:            "No Content",
-	StatusResetContent:         "Reset Content",
-	StatusPartialContent:       "Partial Content",
-	StatusMultiStatus:          "Multi-Status",
-	StatusAlreadyReported:      "Already Reported",
-	StatusIMUsed:               "IM Used",
-
-	StatusMultipleChoices:   "Multiple Choices",
-	StatusMovedPermanently:  "Moved Permanently",
-	StatusFound:             "Found",
-	StatusSeeOther:          "See Other",
-	StatusNotModified:       "Not Modified",
-	StatusUseProxy:          "Use Proxy",
-	StatusTemporaryRedirect: "Temporary Redirect",
-	StatusPermanentRedirect: "Permanent Redirect",
-
-	StatusBadRequest:                   "Bad Request",
-	StatusUnauthorized:                 "Unauthorized",
-	StatusPaymentRequired:              "Payment Required",
-	StatusForbidden:                    "Forbidden",
-	StatusNotFound:                     "Not Found",
-	StatusMethodNotAllowed:             "Method Not Allowed",
-	StatusNotAcceptable:                "Not Acceptable",
-	StatusProxyAuthRequired:            "Proxy Authentication Required",
-	StatusRequestTimeout:               "Request Timeout",
-	StatusConflict:                     "Conflict",
-	StatusGone:                         "Gone",
-	StatusLengthRequired:               "Length Required",
-	StatusPreconditionFailed:           "Precondition Failed",
-	StatusRequestEntityTooLarge:        "Request Entity Too Large",
-	StatusRequestURITooLong:            "Request URI Too Long",
-	StatusUnsupportedMediaType:         "Unsupported Media Type",
-	StatusRequestedRangeNotSatisfiable: "Requested Range Not Satisfiable",
-	StatusExpectationFailed:            "Expectation Failed",
-	StatusTeapot:                       "I'm a teapot",
-	StatusMisdirectedRequest:           "Misdirected Request",
-	StatusUnprocessableEntity:          "Unprocessable Entity",
-	StatusLocked:                       "Locked",
-	StatusFailedDependency:             "Failed Dependency",
-	StatusTooEarly:                     "Too Early",
-	StatusUpgradeRequired:              "Upgrade Required",
-	StatusPreconditionRequired:         "Precondition Required",
-	StatusTooManyRequests:              "Too Many Requests",
-	StatusRequestHeaderFieldsTooLarge:  "Request Header Fields Too Large",
-	StatusUnavailableForLegalReasons:   "Unavailable For Legal Reasons",
-
-	StatusInternalServerError:           "Internal Server Error",
-	StatusNotImplemented:                "Not Implemented",
-	StatusBadGateway:                    "Bad Gateway",
-	StatusServiceUnavailable:            "Service Unavailable",
-	StatusGatewayTimeout:                "Gateway Timeout",
-	StatusHTTPVersionNotSupported:       "HTTP Version Not Supported",
-	StatusVariantAlsoNegotiates:         "Variant Also Negotiates",
-	StatusInsufficientStorage:           "Insufficient Storage",
-	StatusLoopDetected:                  "Loop Detected",
-	StatusNotExtended:                   "Not Extended",
-	StatusNetworkAuthenticationRequired: "Network Authentication Required",
-}
-
 // StatusText returns a text for the HTTP status code. It returns the empty
 // string if the code is unknown.
 func StatusText(code int) string {
-	return statusText[code]
+	switch code {
+	case StatusContinue:
+		return "Continue"
+	case StatusSwitchingProtocols:
+		return "Switching Protocols"
+	case StatusProcessing:
+		return "Processing"
+	case StatusEarlyHints:
+		return "Early Hints"
+	case StatusOK:
+		return "OK"
+	case StatusCreated:
+		return "Created"
+	case StatusAccepted:
+		return "Accepted"
+	case StatusNonAuthoritativeInfo:
+		return "Non-Authoritative Information"
+	case StatusNoContent:
+		return "No Content"
+	case StatusResetContent:
+		return "Reset Content"
+	case StatusPartialContent:
+		return "Partial Content"
+	case StatusMultiStatus:
+		return "Multi-Status"
+	case StatusAlreadyReported:
+		return "Already Reported"
+	case StatusIMUsed:
+		return "IM Used"
+	case StatusMultipleChoices:
+		return "Multiple Choices"
+	case StatusMovedPermanently:
+		return "Moved Permanently"
+	case StatusFound:
+		return "Found"
+	case StatusSeeOther:
+		return "See Other"
+	case StatusNotModified:
+		return "Not Modified"
+	case StatusUseProxy:
+		return "Use Proxy"
+	case StatusTemporaryRedirect:
+		return "Temporary Redirect"
+	case StatusPermanentRedirect:
+		return "Permanent Redirect"
+	case StatusBadRequest:
+		return "Bad Request"
+	case StatusUnauthorized:
+		return "Unauthorized"
+	case StatusPaymentRequired:
+		return "Payment Required"
+	case StatusForbidden:
+		return "Forbidden"
+	case StatusNotFound:
+		return "Not Found"
+	case StatusMethodNotAllowed:
+		return "Method Not Allowed"
+	case StatusNotAcceptable:
+		return "Not Acceptable"
+	case StatusProxyAuthRequired:
+		return "Proxy Authentication Required"
+	case StatusRequestTimeout:
+		return "Request Timeout"
+	case StatusConflict:
+		return "Conflict"
+	case StatusGone:
+		return "Gone"
+	case StatusLengthRequired:
+		return "Length Required"
+	case StatusPreconditionFailed:
+		return "Precondition Failed"
+	case StatusRequestEntityTooLarge:
+		return "Request Entity Too Large"
+	case StatusRequestURITooLong:
+		return "Request URI Too Long"
+	case StatusUnsupportedMediaType:
+		return "Unsupported Media Type"
+	case StatusRequestedRangeNotSatisfiable:
+		return "Requested Range Not Satisfiable"
+	case StatusExpectationFailed:
+		return "Expectation Failed"
+	case StatusTeapot:
+		return "I'm a teapot"
+	case StatusMisdirectedRequest:
+		return "Misdirected Request"
+	case StatusUnprocessableEntity:
+		return "Unprocessable Entity"
+	case StatusLocked:
+		return "Locked"
+	case StatusFailedDependency:
+		return "Failed Dependency"
+	case StatusTooEarly:
+		return "Too Early"
+	case StatusUpgradeRequired:
+		return "Upgrade Required"
+	case StatusPreconditionRequired:
+		return "Precondition Required"
+	case StatusTooManyRequests:
+		return "Too Many Requests"
+	case StatusRequestHeaderFieldsTooLarge:
+		return "Request Header Fields Too Large"
+	case StatusUnavailableForLegalReasons:
+		return "Unavailable For Legal Reasons"
+	case StatusInternalServerError:
+		return "Internal Server Error"
+	case StatusNotImplemented:
+		return "Not Implemented"
+	case StatusBadGateway:
+		return "Bad Gateway"
+	case StatusServiceUnavailable:
+		return "Service Unavailable"
+	case StatusGatewayTimeout:
+		return "Gateway Timeout"
+	case StatusHTTPVersionNotSupported:
+		return "HTTP Version Not Supported"
+	case StatusVariantAlsoNegotiates:
+		return "Variant Also Negotiates"
+	case StatusInsufficientStorage:
+		return "Insufficient Storage"
+	case StatusLoopDetected:
+		return "Loop Detected"
+	case StatusNotExtended:
+		return "Not Extended"
+	case StatusNetworkAuthenticationRequired:
+		return "Network Authentication Required"
+	default:
+		return ""
+	}
 }