Subject: [PATCH v8 1/3] Remove some unnecessary optimizations in popcount code.

From: Nathan Bossart <[email-masked]>
Date: Thu, 22 Jan 2026 11:33:56 -0600

Over the past few releases, we've added a huge amount of complexity
to our popcount implementations.  Commits fbe327e5b4, 79e232ca01,
8c6653516c, and 25dc485074 did some preliminary refactoring, but
many opportunities remain.  In particular, if we disclaim interest
in micro-optimizing this code for 32-bit builds and in unproven
alignment checks, we can remove a decent chunk of code.

This commit does the following:

* Removes the code in pg_popcount() and pg_popcount_masked() that
sets the function pointer threshold based on SIDEOF_VOID_P.
Consequently, 32-bit builds should follow the inline path for
inputs between 4-8 bytes instead of calling pg_popcount_optimized()
(which is probably just calling pg_popcount_portable(), anyway).
While it is possible that this results in a small regression for
those inputs on 32-bit builds, it seems unlikely to produce
noticeable performance differences on those machines.  Furthermore,
I found no evidence of benchmarks for this area of code for 32-bit
builds.

* Removes the 32-bit optimizations in pg_popcount_portable() and
pg_popcount_masked_portable().  This means that 32-bit builds
instead use a simple while loop.  As above, we are not too
concerned about regressions on 32-bit machines.

* Removes 32-bit optimizations in pg_popcount_x86.c.  This is dead
code because everything in this file is only compiled when
HAVE_X86_64_POPCNTQ is defined, and that macro is only defined for
x86-64.

* Removes alignment checks in pg_popcount_sse42() and
pg_popcount_masked_sse42().  These are unnecessary for x86, and
it's unclear whether they make any meaningful performance
difference.  Since we allow misaligned accesses now, this commit
also adds pg_attribute_no_sanitize_alignment() to these functions.

Suggested-by: John Naylor <[email-masked]>
Reviewed-by: John Naylor <[email-masked]>
Discussion: https://postgr.es/m/CANWCAZY7R%2Biy%2Br9YM_sySNydHzNqUirx1xk0tB3ej5HO62GdgQ%40mail.gmail.com
---
 src/include/port/pg_bitutils.h | 24 +-----------
 src/port/pg_bitutils.c         | 30 ---------------
 src/port/pg_popcount_x86.c     | 67 ++++++----------------------------
 3 files changed, 14 insertions(+), 107 deletions(-)

diff --git a/src/include/port/pg_bitutils.h b/src/include/port/pg_bitutils.h
index 35761f509ec..c3049d71894 100644
--- a/src/include/port/pg_bitutils.h
+++ b/src/include/port/pg_bitutils.h
@@ -329,17 +329,7 @@ extern uint64 pg_popcount_masked_optimized(const char *buf, int bytes, bits8 mas
 static inline uint64
 pg_popcount(const char *buf, int bytes)
 {
-	/*
-	 * We set the threshold to the point at which we'll first use special
-	 * instructions in the optimized version.
-	 */
-#if SIZEOF_VOID_P >= 8
-	int			threshold = 8;
-#else
-	int			threshold = 4;
-#endif
-
-	if (bytes < threshold)
+	if (bytes < 8)
 	{
 		uint64		popcnt = 0;
 
@@ -360,17 +350,7 @@ pg_popcount(const char *buf, int bytes)
 static inline uint64
 pg_popcount_masked(const char *buf, int bytes, bits8 mask)
 {
-	/*
-	 * We set the threshold to the point at which we'll first use special
-	 * instructions in the optimized version.
-	 */
-#if SIZEOF_VOID_P >= 8
-	int			threshold = 8;
-#else
-	int			threshold = 4;
-#endif
-
-	if (bytes < threshold)
+	if (bytes < 8)
 	{
 		uint64		popcnt = 0;
 
diff --git a/src/port/pg_bitutils.c b/src/port/pg_bitutils.c
index ffda75825e5..bec06c06fc3 100644
--- a/src/port/pg_bitutils.c
+++ b/src/port/pg_bitutils.c
@@ -167,20 +167,6 @@ pg_popcount_portable(const char *buf, int bytes)
 			bytes -= 8;
 		}
 
-		buf = (const char *) words;
-	}
-#else
-	/* Process in 32-bit chunks if the buffer is aligned. */
-	if (buf == (const char *) TYPEALIGN(4, buf))
-	{
-		const uint32 *words = (const uint32 *) buf;
-
-		while (bytes >= 4)
-		{
-			popcnt += pg_popcount32_portable(*words++);
-			bytes -= 4;
-		}
-
 		buf = (const char *) words;
 	}
 #endif
@@ -215,22 +201,6 @@ pg_popcount_masked_portable(const char *buf, int bytes, bits8 mask)
 			bytes -= 8;
 		}
 
-		buf = (const char *) words;
-	}
-#else
-	/* Process in 32-bit chunks if the buffer is aligned. */
-	uint32		maskv = ~((uint32) 0) / 0xFF * mask;
-
-	if (buf == (const char *) TYPEALIGN(4, buf))
-	{
-		const uint32 *words = (const uint32 *) buf;
-
-		while (bytes >= 4)
-		{
-			popcnt += pg_popcount32_portable(*words++ & maskv);
-			bytes -= 4;
-		}
-
 		buf = (const char *) words;
 	}
 #endif
diff --git a/src/port/pg_popcount_x86.c b/src/port/pg_popcount_x86.c
index 245f0167d00..7aebf69898b 100644
--- a/src/port/pg_popcount_x86.c
+++ b/src/port/pg_popcount_x86.c
@@ -376,40 +376,20 @@ __asm__ __volatile__(" popcntq %1,%0\n":"=q"(res):"rm"(word):"cc");
  * pg_popcount_sse42
  *		Returns the number of 1-bits in buf
  */
+pg_attribute_no_sanitize_alignment()
 static uint64
 pg_popcount_sse42(const char *buf, int bytes)
 {
 	uint64		popcnt = 0;
+	const uint64 *words = (const uint64 *) buf;
 
-#if SIZEOF_VOID_P >= 8
-	/* Process in 64-bit chunks if the buffer is aligned. */
-	if (buf == (const char *) TYPEALIGN(8, buf))
+	while (bytes >= 8)
 	{
-		const uint64 *words = (const uint64 *) buf;
-
-		while (bytes >= 8)
-		{
-			popcnt += pg_popcount64_sse42(*words++);
-			bytes -= 8;
-		}
-
-		buf = (const char *) words;
+		popcnt += pg_popcount64_sse42(*words++);
+		bytes -= 8;
 	}
-#else
-	/* Process in 32-bit chunks if the buffer is aligned. */
-	if (buf == (const char *) TYPEALIGN(4, buf))
-	{
-		const uint32 *words = (const uint32 *) buf;
 
-		while (bytes >= 4)
-		{
-			popcnt += pg_popcount32_sse42(*words++);
-			bytes -= 4;
-		}
-
-		buf = (const char *) words;
-	}
-#endif
+	buf = (const char *) words;
 
 	/* Process any remaining bytes */
 	while (bytes--)
@@ -422,44 +402,21 @@ pg_popcount_sse42(const char *buf, int bytes)
  * pg_popcount_masked_sse42
  *		Returns the number of 1-bits in buf after applying the mask to each byte
  */
+pg_attribute_no_sanitize_alignment()
 static uint64
 pg_popcount_masked_sse42(const char *buf, int bytes, bits8 mask)
 {
 	uint64		popcnt = 0;
-
-#if SIZEOF_VOID_P >= 8
-	/* Process in 64-bit chunks if the buffer is aligned */
 	uint64		maskv = ~UINT64CONST(0) / 0xFF * mask;
+	const uint64 *words = (const uint64 *) buf;
 
-	if (buf == (const char *) TYPEALIGN(8, buf))
+	while (bytes >= 8)
 	{
-		const uint64 *words = (const uint64 *) buf;
-
-		while (bytes >= 8)
-		{
-			popcnt += pg_popcount64_sse42(*words++ & maskv);
-			bytes -= 8;
-		}
-
-		buf = (const char *) words;
+		popcnt += pg_popcount64_sse42(*words++ & maskv);
+		bytes -= 8;
 	}
-#else
-	/* Process in 32-bit chunks if the buffer is aligned. */
-	uint32		maskv = ~((uint32) 0) / 0xFF * mask;
-
-	if (buf == (const char *) TYPEALIGN(4, buf))
-	{
-		const uint32 *words = (const uint32 *) buf;
-
-		while (bytes >= 4)
-		{
-			popcnt += pg_popcount32_sse42(*words++ & maskv);
-			bytes -= 4;
-		}
 
-		buf = (const char *) words;
-	}
-#endif
+	buf = (const char *) words;
 
 	/* Process any remaining bytes */
 	while (bytes--)
-- 
2.50.1 (Apple Git-155)

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