From 843bf9488b441c69195ae5fc37c1a76ecd35e32c Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Wed, 12 Aug 2026 15:48:58 -0400
Subject: [PATCH v1 4/4] Micro-optimize appendStringInfo[VA].

In the loop in appendStringInfo, avoid a useless assignment to
errno during the first (and usually only) iteration.  This
might make a noticeable difference depending on how efficiently
the platform deals with thread-local variables.

Try to make the compiler inline appendStringInfoVA into appendStringInfo.

Instead of having appendStringInfoVA go through pvsnprintf, make
it call vsnprintf directly.  pvsnprintf adds little except an
int-versus-size_t impedance mismatch.  We do have to duplicate its
error handling for the nprinted < 0 case, but we don't need to
duplicate its check for MaxAllocSize overrun, because
enlargeStringInfo can handle that just as easily.  Also, its
insistence on adding one to nprinted is not needed here, since
enlargeStringInfo expects the number of data bytes to add.

In combination, these changes seem to about halve the penalty
for going through appendStringInfo rather than directly to
snprintf.c.  This seems likely to be worth doing whether or
not we accept the preceding patches.
---
 src/common/stringinfo.c | 39 ++++++++++++++++++++++++++-------------
 1 file changed, 26 insertions(+), 13 deletions(-)

diff --git a/src/common/stringinfo.c b/src/common/stringinfo.c
index 468954e484e..28ab6c7df83 100644
--- a/src/common/stringinfo.c
+++ b/src/common/stringinfo.c
@@ -152,16 +152,16 @@ appendStringInfo(StringInfo str, const char *fmt, ...)
 		int			needed;
 
 		/* Try to format the data. */
-		errno = save_errno;
 		va_start(args, fmt);
 		needed = appendStringInfoVA(str, fmt, args);
 		va_end(args);
 
-		if (needed == 0)
+		if (likely(needed == 0))
 			break;				/* success */
 
 		/* Increase the buffer size and try again. */
 		enlargeStringInfo(str, needed);
+		errno = save_errno;
 	}
 }
 
@@ -183,13 +183,11 @@ appendStringInfo(StringInfo str, const char *fmt, ...)
  * to redo va_start before you can rescan the argument list, and we can't do
  * that from here.
  */
-int
+inline int
 appendStringInfoVA(StringInfo str, const char *fmt, va_list args)
 {
 	int			avail;
-	size_t		nprinted;
-
-	Assert(str != NULL);
+	int			nprinted;
 
 	/*
 	 * If there's hardly any space, don't bother trying, just fail to make the
@@ -200,12 +198,24 @@ appendStringInfoVA(StringInfo str, const char *fmt, va_list args)
 	if (avail < 16)
 		return 32;
 
-	nprinted = pvsnprintf(str->data + str->len, (size_t) avail, fmt, args);
+	nprinted = vsnprintf(str->data + str->len, (size_t) avail, fmt, args);
+
+	/* We assume failure means the fmt is bogus, hence hard failure is OK */
+	if (unlikely(nprinted < 0))
+	{
+#ifndef FRONTEND
+		elog(ERROR, "vsnprintf failed: %m with format string \"%s\"", fmt);
+#else
+		fprintf(stderr, "vsnprintf failed: %m with format string \"%s\"\n",
+				fmt);
+		exit(EXIT_FAILURE);
+#endif
+	}
 
-	if (nprinted < (size_t) avail)
+	if (likely(nprinted < avail))
 	{
 		/* Success.  Note nprinted does not include trailing null. */
-		str->len += (int) nprinted;
+		str->len += nprinted;
 		return 0;
 	}
 
@@ -213,11 +223,14 @@ appendStringInfoVA(StringInfo str, const char *fmt, va_list args)
 	str->data[str->len] = '\0';
 
 	/*
-	 * Return pvsnprintf's estimate of the space needed.  (Although this is
-	 * given as a size_t, we know it will fit in int because it's not more
-	 * than MaxAllocSize.)
+	 * We assume a C99-compliant vsnprintf, so believe its estimate of the
+	 * required space.  (If it's wrong, the logic will still work, but we may
+	 * loop multiple times.)
+	 *
+	 * Unlike pvsnprintf(), we don't check for overrunning MaxAllocSize,
+	 * preferring to leave that to enlargeStringInfo().
 	 */
-	return (int) nprinted;
+	return nprinted;
 }
 
 /*
-- 
2.52.0