Add more efficient functions to pqformat API.

There's three prongs to achieve greater efficiency here:

1) Allow reusing a stringbuffer across pq_beginmessage/endmessage,
   with the new pq_beginmessage_reuse/endmessage_reuse. This can be
   beneficial both because it avoids allocating the initial buffer,
   and because it's more likely to already have an correctly sized
   buffer.

2) Replacing pq_sendint() with pq_sendint$width() inline
   functions. Previously unnecessary and unpredictable branches in
   pq_sendint() were needed. Additionally the replacement functions
   are implemented more efficiently.  pq_sendint is now deprecated, a
   separate commit will convert all in-tree callers.

3) Add pq_writeint$width(), pq_writestring(). These rely on sufficient
   space in the StringInfo's buffer, avoiding individual space checks
   & potential individual resizing.  To allow this to be used for
   strings, expose mbutil.c's MAX_CONVERSION_GROWTH.

Followup commits will make use of these facilities.

Author: Andres Freund
Discussion: https://postgr.es/m/20170914063418.sckdzgjfrsbekae4@alap3.anarazel.de

src/backend/libpq/pqformat.c

@@ -97,13 +97,24 @@ pq_beginmessage(StringInfo buf, char msgtype)
 }
 
 /* --------------------------------
- *		pq_sendbyte		- append a raw byte to a StringInfo buffer
+
+ *		pq_beginmessage_reuse - initialize for sending a message, reuse buffer
+ *
+ * This requires the buffer to be allocated in an sufficiently long-lived
+ * memory context.
  * --------------------------------
  */
 void
-pq_sendbyte(StringInfo buf, int byt)
+pq_beginmessage_reuse(StringInfo buf, char msgtype)
 {
-	appendStringInfoCharMacro(buf, byt);
+	resetStringInfo(buf);
+
+	/*
+	 * We stash the message type into the buffer's cursor field, expecting
+	 * that the pq_sendXXX routines won't touch it.  We could alternatively
+	 * make it the first byte of the buffer contents, but this seems easier.
+	 */
+	buf->cursor = msgtype;
 }
 
 /* --------------------------------
@@ -113,6 +124,7 @@ pq_sendbyte(StringInfo buf, int byt)
 void
 pq_sendbytes(StringInfo buf, const char *data, int datalen)
 {
+	/* use variant that maintains a trailing null-byte, out of caution */
 	appendBinaryStringInfo(buf, data, datalen);
 }
 
@@ -137,13 +149,13 @@ pq_sendcountedtext(StringInfo buf, const char *str, int slen,
 	if (p != str)				/* actual conversion has been done? */
 	{
 		slen = strlen(p);
-		pq_sendint(buf, slen + extra, 4);
+		pq_sendint32(buf, slen + extra);
 		appendBinaryStringInfoNT(buf, p, slen);
 		pfree(p);
 	}
 	else
 	{
-		pq_sendint(buf, slen + extra, 4);
+		pq_sendint32(buf, slen + extra);
 		appendBinaryStringInfoNT(buf, str, slen);
 	}
 }
@@ -227,53 +239,6 @@ pq_send_ascii_string(StringInfo buf, const char *str)
 	appendStringInfoChar(buf, '\0');
 }
 
-/* --------------------------------
- *		pq_sendint		- append a binary integer to a StringInfo buffer
- * --------------------------------
- */
-void
-pq_sendint(StringInfo buf, int i, int b)
-{
-	unsigned char n8;
-	uint16		n16;
-	uint32		n32;
-
-	switch (b)
-	{
-		case 1:
-			n8 = (unsigned char) i;
-			appendBinaryStringInfoNT(buf, (char *) &n8, 1);
-			break;
-		case 2:
-			n16 = pg_hton16((uint16) i);
-			appendBinaryStringInfoNT(buf, (char *) &n16, 2);
-			break;
-		case 4:
-			n32 = pg_hton32((uint32) i);
-			appendBinaryStringInfoNT(buf, (char *) &n32, 4);
-			break;
-		default:
-			elog(ERROR, "unsupported integer size %d", b);
-			break;
-	}
-}
-
-/* --------------------------------
- *		pq_sendint64	- append a binary 8-byte int to a StringInfo buffer
- *
- * It is tempting to merge this with pq_sendint, but we'd have to make the
- * argument int64 for all data widths --- that could be a big performance
- * hit on machines where int64 isn't efficient.
- * --------------------------------
- */
-void
-pq_sendint64(StringInfo buf, int64 i)
-{
-	uint64		n64 = pg_hton64(i);
-
-	appendBinaryStringInfoNT(buf, (char *) &n64, sizeof(n64));
-}
-
 /* --------------------------------
  *		pq_sendfloat4	- append a float4 to a StringInfo buffer
  *
@@ -295,9 +260,7 @@ pq_sendfloat4(StringInfo buf, float4 f)
 	}			swap;
 
 	swap.f = f;
-	swap.i = pg_hton32(swap.i);
-
-	appendBinaryStringInfoNT(buf, (char *) &swap.i, 4);
+	pq_sendint32(buf, swap.i);
 }
 
 /* --------------------------------
@@ -341,6 +304,21 @@ pq_endmessage(StringInfo buf)
 	buf->data = NULL;
 }
 
+/* --------------------------------
+ *		pq_endmessage_reuse	- send the completed message to the frontend
+ *
+ * The data buffer is *not* freed, allowing to reuse the buffer with
+ * pg_beginmessage_reuse.
+ --------------------------------
+ */
+
+void
+pq_endmessage_reuse(StringInfo buf)
+{
+	/* msgtype was saved in cursor field */
+	(void) pq_putmessage(buf->cursor, buf->data, buf->len);
+}
+
 
 /* --------------------------------
  *		pq_begintypsend		- initialize for constructing a bytea result

src/backend/utils/mb/mbutils.c

@@ -41,17 +41,6 @@
 #include "utils/memutils.h"
 #include "utils/syscache.h"
 
-/*
- * When converting strings between different encodings, we assume that space
- * for converted result is 4-to-1 growth in the worst case. The rate for
- * currently supported encoding pairs are within 3 (SJIS JIS X0201 half width
- * kanna -> UTF8 is the worst case).  So "4" should be enough for the moment.
- *
- * Note that this is not the same as the maximum character width in any
- * particular encoding.
- */
-#define MAX_CONVERSION_GROWTH  4
-
 /*
  * We maintain a simple linked list caching the fmgr lookup info for the
  * currently selected conversion functions, as well as any that have been

src/include/libpq/pqformat.h

@@ -14,20 +14,180 @@
 #define PQFORMAT_H
 
 #include "lib/stringinfo.h"
+#include "mb/pg_wchar.h"
+#include "port/pg_bswap.h"
 
 extern void pq_beginmessage(StringInfo buf, char msgtype);
-extern void pq_sendbyte(StringInfo buf, int byt);
+extern void pq_beginmessage_reuse(StringInfo buf, char msgtype);
+extern void pq_endmessage(StringInfo buf);
+extern void pq_endmessage_reuse(StringInfo buf);
+
 extern void pq_sendbytes(StringInfo buf, const char *data, int datalen);
 extern void pq_sendcountedtext(StringInfo buf, const char *str, int slen,
 				   bool countincludesself);
 extern void pq_sendtext(StringInfo buf, const char *str, int slen);
 extern void pq_sendstring(StringInfo buf, const char *str);
 extern void pq_send_ascii_string(StringInfo buf, const char *str);
-extern void pq_sendint(StringInfo buf, int i, int b);
-extern void pq_sendint64(StringInfo buf, int64 i);
 extern void pq_sendfloat4(StringInfo buf, float4 f);
 extern void pq_sendfloat8(StringInfo buf, float8 f);
-extern void pq_endmessage(StringInfo buf);
+
+extern void pq_sendfloat4(StringInfo buf, float4 f);
+extern void pq_sendfloat8(StringInfo buf, float8 f);
+
+/*
+ * Append a int8 to a StringInfo buffer, which already has enough space
+ * preallocated.
+ *
+ * The use of restrict allows the compiler to optimize the code based on the
+ * assumption that buf, buf->len, buf->data and *buf->data don't
+ * overlap. Without the annotation buf->len etc cannot be kept in a register
+ * over subsequent pq_writeint* calls.
+ */
+static inline void
+pq_writeint8(StringInfo restrict buf, int8 i)
+{
+	int8		ni = i;
+
+	Assert(buf->len + sizeof(i) <= buf->maxlen);
+	memcpy((char *restrict) (buf->data + buf->len), &ni, sizeof(ni));
+	buf->len += sizeof(i);
+}
+
+/*
+ * Append a int16 to a StringInfo buffer, which already has enough space
+ * preallocated.
+ */
+static inline void
+pq_writeint16(StringInfo restrict buf, int16 i)
+{
+	int16		ni = pg_hton16(i);
+
+	Assert(buf->len + sizeof(ni) <= buf->maxlen);
+	memcpy((char *restrict) (buf->data + buf->len), &ni, sizeof(i));
+	buf->len += sizeof(i);
+}
+
+/*
+ * Append a int32 to a StringInfo buffer, which already has enough space
+ * preallocated.
+ */
+static inline void
+pq_writeint32(StringInfo restrict buf, int32 i)
+{
+	int32		ni = pg_hton32(i);
+
+	Assert(buf->len + sizeof(i) <= buf->maxlen);
+	memcpy((char *restrict) (buf->data + buf->len), &ni, sizeof(i));
+	buf->len += sizeof(i);
+}
+
+/*
+ * Append a int64 to a StringInfo buffer, which already has enough space
+ * preallocated.
+ */
+static inline void
+pq_writeint64(StringInfo restrict buf, int64 i)
+{
+	int64		ni = pg_hton64(i);
+
+	Assert(buf->len + sizeof(i) <= buf->maxlen);
+	memcpy((char *restrict) (buf->data + buf->len), &ni, sizeof(i));
+	buf->len += sizeof(i);
+}
+
+/*
+ * Append a null-terminated text string (with conversion) to a buffer with
+ * preallocated space.
+ *
+ * NB: The pre-allocated space needs to be sufficient for the string after
+ * converting to client encoding.
+ *
+ * NB: passed text string must be null-terminated, and so is the data
+ * sent to the frontend.
+ */
+static inline void
+pq_writestring(StringInfo restrict buf, const char *restrict str)
+{
+	int			slen = strlen(str);
+	char	   *p;
+
+	p = pg_server_to_client(str, slen);
+	if (p != str)				/* actual conversion has been done? */
+		slen = strlen(p);
+
+	Assert(buf->len + slen + 1 <= buf->maxlen);
+
+	memcpy(((char *restrict) buf->data + buf->len), p, slen + 1);
+	buf->len += slen + 1;
+
+	if (p != str)
+		pfree(p);
+}
+
+/* append a binary int8 to a StringInfo buffer */
+static inline void
+pq_sendint8(StringInfo buf, int8 i)
+{
+	enlargeStringInfo(buf, sizeof(i));
+	pq_writeint8(buf, i);
+}
+
+/* append a binary int16 to a StringInfo buffer */
+static inline void
+pq_sendint16(StringInfo buf, int16 i)
+{
+	enlargeStringInfo(buf, sizeof(i));
+	pq_writeint16(buf, i);
+}
+
+/* append a binary int32 to a StringInfo buffer */
+static inline void
+pq_sendint32(StringInfo buf, int32 i)
+{
+	enlargeStringInfo(buf, sizeof(i));
+	pq_writeint32(buf, i);
+}
+
+/* append a binary int64 to a StringInfo buffer */
+static inline void
+pq_sendint64(StringInfo buf, int64 i)
+{
+	enlargeStringInfo(buf, sizeof(i));
+	pq_writeint64(buf, i);
+}
+
+/* append a binary byte to a StringInfo buffer */
+static inline void
+pq_sendbyte(StringInfo buf, int8 byt)
+{
+	pq_sendint8(buf, byt);
+}
+
+/*
+ * Append a binary integer to a StringInfo buffer
+ *
+ * This function is deprecated.
+ */
+static inline void
+pq_sendint(StringInfo buf, int i, int b)
+{
+	switch (b)
+	{
+		case 1:
+			pq_sendint8(buf, (int8) i);
+			break;
+		case 2:
+			pq_sendint16(buf, (int16) i);
+			break;
+		case 4:
+			pq_sendint32(buf, (int32) i);
+			break;
+		default:
+			elog(ERROR, "unsupported integer size %d", b);
+			break;
+	}
+}
+
 
 extern void pq_begintypsend(StringInfo buf);
 extern bytea *pq_endtypsend(StringInfo buf);

src/include/mb/pg_wchar.h

@@ -304,6 +304,17 @@ typedef enum pg_enc
 /* On FE are possible all encodings */
 #define PG_VALID_FE_ENCODING(_enc)	PG_VALID_ENCODING(_enc)
 
+/*
+ * When converting strings between different encodings, we assume that space
+ * for converted result is 4-to-1 growth in the worst case. The rate for
+ * currently supported encoding pairs are within 3 (SJIS JIS X0201 half width
+ * kanna -> UTF8 is the worst case).  So "4" should be enough for the moment.
+ *
+ * Note that this is not the same as the maximum character width in any
+ * particular encoding.
+ */
+#define MAX_CONVERSION_GROWTH  4
+
 /*
  * Table for mapping an encoding number to official encoding name and
  * possibly other subsidiary data.  Be careful to check encoding number