Date: Mon, 18 Mar 2024 12:30:04 -0500
From: Nathan Bossart <nathandbossart@gmail.com>
To: "Amonson, Paul D" <paul.d.amonson@intel.com>
Cc: David Rowley <dgrowleyml@gmail.com>, Andres Freund <andres@anarazel.de>,
	Alvaro Herrera <alvherre@alvh.no-ip.org>,
	"Shankaran, Akash" <akash.shankaran@intel.com>,
	Noah Misch <noah@leadboat.com>, Tom Lane <tgl@sss.pgh.pa.us>,
	Matthias van de Meent <boekewurm+postgres@gmail.com>,
	"pgsql-hackers@lists.postgresql.org" <pgsql-hackers@lists.postgresql.org>
Subject: Re: Popcount optimization using AVX512
Message-ID: <20240318173004.GA361138@nathanxps13>
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On Mon, Mar 18, 2024 at 11:20:18AM -0500, Nathan Bossart wrote:
> I don't think David was suggesting that we need to remove the runtime
> checks for AVX512.  IIUC he was pointing out that most of the performance
> gain is from removing the function call overhead, which your v8-0002 patch
> already does for the proposed AVX512 code.  We can apply a similar
> optimization for systems without AVX512 by inlining the code for
> pg_popcount64() and pg_popcount32().

Here is a more fleshed-out version of what I believe David is proposing.
On my machine, the gains aren't quite as impressive (~8.8s to ~5.2s for the
test_popcount benchmark).  I assume this is because this patch turns
pg_popcount() into a function pointer, which is what the AVX512 patches do,
too.  I left out the 32-bit section from pg_popcount_fast(), but I'll admit
that I'm not yet 100% sure that we can assume we're on a 64-bit system
there.

IMHO this work is arguably a prerequisite for the AVX512 work, as turning
pg_popcount() into a function pointer will likely regress performance for
folks on systems without AVX512 otherwise.

-- 
Nathan Bossart
Amazon Web Services: https://aws.amazon.com

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	filename="0001-inline-function-calls-in-pg_popcount-when-possible.patch"

From 1d33c803feb7428f798b13fd643a16c73628f8a9 Mon Sep 17 00:00:00 2001
From: Nathan Bossart <nathan@postgresql.org>
Date: Mon, 18 Mar 2024 12:18:15 -0500
Subject: [PATCH 1/1] inline function calls in pg_popcount() when possible

---
 src/include/port/pg_bitutils.h |   5 +-
 src/port/pg_bitutils.c         | 123 +++++++++++++++++++++++++--------
 2 files changed, 97 insertions(+), 31 deletions(-)

diff --git a/src/include/port/pg_bitutils.h b/src/include/port/pg_bitutils.h
index 46bf4f0103..53e5239717 100644
--- a/src/include/port/pg_bitutils.h
+++ b/src/include/port/pg_bitutils.h
@@ -302,17 +302,16 @@ pg_ceil_log2_64(uint64 num)
 /* Attempt to use the POPCNT instruction, but perform a runtime check first */
 extern PGDLLIMPORT int (*pg_popcount32) (uint32 word);
 extern PGDLLIMPORT int (*pg_popcount64) (uint64 word);
+extern PGDLLIMPORT uint64 (*pg_popcount) (const char *buf, int bytes);
 
 #else
 /* Use a portable implementation -- no need for a function pointer. */
 extern int	pg_popcount32(uint32 word);
 extern int	pg_popcount64(uint64 word);
+extern uint64 pg_popcount(const char *buf, int bytes);
 
 #endif							/* TRY_POPCNT_FAST */
 
-/* Count the number of one-bits in a byte array */
-extern uint64 pg_popcount(const char *buf, int bytes);
-
 /*
  * Rotate the bits of "word" to the right/left by n bits.
  */
diff --git a/src/port/pg_bitutils.c b/src/port/pg_bitutils.c
index 640a89561a..e374e753d7 100644
--- a/src/port/pg_bitutils.c
+++ b/src/port/pg_bitutils.c
@@ -105,16 +105,20 @@ const uint8 pg_number_of_ones[256] = {
 
 static int	pg_popcount32_slow(uint32 word);
 static int	pg_popcount64_slow(uint64 word);
+static uint64 pg_popcount_slow(const char *buf, int bytes);
 
 #ifdef TRY_POPCNT_FAST
 static bool pg_popcount_available(void);
 static int	pg_popcount32_choose(uint32 word);
 static int	pg_popcount64_choose(uint64 word);
+static uint64 pg_popcount_choose(const char *buf, int bytes);
 static int	pg_popcount32_fast(uint32 word);
-static int	pg_popcount64_fast(uint64 word);
+static inline int pg_popcount64_fast(uint64 word);
+static uint64 pg_popcount_fast(const char *buf, int bytes);
 
 int			(*pg_popcount32) (uint32 word) = pg_popcount32_choose;
 int			(*pg_popcount64) (uint64 word) = pg_popcount64_choose;
+uint64		(*pg_popcount) (const char *buf, int bytes) = pg_popcount_choose;
 #endif							/* TRY_POPCNT_FAST */
 
 #ifdef TRY_POPCNT_FAST
@@ -151,11 +155,13 @@ pg_popcount32_choose(uint32 word)
 	{
 		pg_popcount32 = pg_popcount32_fast;
 		pg_popcount64 = pg_popcount64_fast;
+		pg_popcount = pg_popcount_fast;
 	}
 	else
 	{
 		pg_popcount32 = pg_popcount32_slow;
 		pg_popcount64 = pg_popcount64_slow;
+		pg_popcount = pg_popcount_slow;
 	}
 
 	return pg_popcount32(word);
@@ -168,16 +174,37 @@ pg_popcount64_choose(uint64 word)
 	{
 		pg_popcount32 = pg_popcount32_fast;
 		pg_popcount64 = pg_popcount64_fast;
+		pg_popcount = pg_popcount_fast;
 	}
 	else
 	{
 		pg_popcount32 = pg_popcount32_slow;
 		pg_popcount64 = pg_popcount64_slow;
+		pg_popcount = pg_popcount_slow;
 	}
 
 	return pg_popcount64(word);
 }
 
+static uint64
+pg_popcount_choose(const char *buf, int bytes)
+{
+	if (pg_popcount_available())
+	{
+		pg_popcount32 = pg_popcount32_fast;
+		pg_popcount64 = pg_popcount64_fast;
+		pg_popcount = pg_popcount_fast;
+	}
+	else
+	{
+		pg_popcount32 = pg_popcount32_slow;
+		pg_popcount64 = pg_popcount64_slow;
+		pg_popcount = pg_popcount_slow;
+	}
+
+	return pg_popcount(buf, bytes);
+}
+
 /*
  * pg_popcount32_fast
  *		Return the number of 1 bits set in word
@@ -199,7 +226,7 @@ __asm__ __volatile__(" popcntl %1,%0\n":"=q"(res):"rm"(word):"cc");
  * pg_popcount64_fast
  *		Return the number of 1 bits set in word
  */
-static int
+static inline int
 pg_popcount64_fast(uint64 word)
 {
 #ifdef _MSC_VER
@@ -212,6 +239,36 @@ __asm__ __volatile__(" popcntq %1,%0\n":"=q"(res):"rm"(word):"cc");
 #endif
 }
 
+/*
+ * pg_popcount_fast
+ *		Returns the number of 1-bits in buf
+ */
+static uint64
+pg_popcount_fast(const char *buf, int bytes)
+{
+	uint64		popcnt = 0;
+
+	/* Process in 64-bit chunks if the buffer is aligned. */
+	if (buf == (const char *) TYPEALIGN(8, buf))
+	{
+		const uint64 *words = (const uint64 *) buf;
+
+		while (bytes >= 8)
+		{
+			popcnt += pg_popcount64_fast(*words++);
+			bytes -= 8;
+		}
+
+		buf = (const char *) words;
+	}
+
+	/* Process any remaining bytes */
+	while (bytes--)
+		popcnt += pg_number_of_ones[(unsigned char) *buf++];
+
+	return popcnt;
+}
+
 #endif							/* TRY_POPCNT_FAST */
 
 
@@ -265,35 +322,12 @@ pg_popcount64_slow(uint64 word)
 #endif							/* HAVE__BUILTIN_POPCOUNT */
 }
 
-#ifndef TRY_POPCNT_FAST
-
 /*
- * When the POPCNT instruction is not available, there's no point in using
- * function pointers to vary the implementation between the fast and slow
- * method.  We instead just make these actual external functions when
- * TRY_POPCNT_FAST is not defined.  The compiler should be able to inline
- * the slow versions here.
- */
-int
-pg_popcount32(uint32 word)
-{
-	return pg_popcount32_slow(word);
-}
-
-int
-pg_popcount64(uint64 word)
-{
-	return pg_popcount64_slow(word);
-}
-
-#endif							/* !TRY_POPCNT_FAST */
-
-/*
- * pg_popcount
+ * pg_popcount_slow
  *		Returns the number of 1-bits in buf
  */
-uint64
-pg_popcount(const char *buf, int bytes)
+static uint64
+pg_popcount_slow(const char *buf, int bytes)
 {
 	uint64		popcnt = 0;
 
@@ -333,3 +367,36 @@ pg_popcount(const char *buf, int bytes)
 
 	return popcnt;
 }
+
+#ifndef TRY_POPCNT_FAST
+
+/*
+ * When the POPCNT instruction is not available, there's no point in using
+ * function pointers to vary the implementation between the fast and slow
+ * method.  We instead just make these actual external functions when
+ * TRY_POPCNT_FAST is not defined.  The compiler should be able to inline
+ * the slow versions here.
+ */
+int
+pg_popcount32(uint32 word)
+{
+	return pg_popcount32_slow(word);
+}
+
+int
+pg_popcount64(uint64 word)
+{
+	return pg_popcount64_slow(word);
+}
+
+/*
+ * pg_popcount
+ *		Returns the number of 1-bits in buf
+ */
+uint64
+pg_popcount(const char *buf, int bytes)
+{
+	return pg_popcount_slow(buf, bytes);
+}
+
+#endif							/* !TRY_POPCNT_FAST */
-- 
2.25.1


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