Add optimized functions for linear search within byte arrays

In similar vein to b6ef167, add pg_lfind8() and pg_lfind8_le()
to search for bytes equal or less-than-or-equal to a given byte,
respectively. To abstract away platform details, add helper functions
and typedefs to simd.h.

John Naylor and Nathan Bossart, per suggestion from Andres Freund

Discussion: https://www.postgresql.org/message-id/CAFBsxsGzaaGLF%3DNuq61iRXTyspbO9rOjhSqFN%3DV6ozzmta5mXg%40mail.gmail.com

Author: j-naylor

src/include/port/pg_lfind.h

@@ -1,7 +1,8 @@
 /*-------------------------------------------------------------------------
  *
  * pg_lfind.h
- *	  Optimized linear search routines.
+ *	  Optimized linear search routines using SIMD intrinsics where
+ *	  available.
  *
  * Copyright (c) 2022, PostgreSQL Global Development Group
  *
@@ -15,6 +16,70 @@
 
 #include "port/simd.h"
 
+/*
+ * pg_lfind8
+ *
+ * Return true if there is an element in 'base' that equals 'key', otherwise
+ * return false.
+ */
+static inline bool
+pg_lfind8(uint8 key, uint8 *base, uint32 nelem)
+{
+	uint32		i;
+
+	/* round down to multiple of vector length */
+	uint32		tail_idx = nelem & ~(sizeof(Vector8) - 1);
+	Vector8		chunk;
+
+	for (i = 0; i < tail_idx; i += sizeof(Vector8))
+	{
+		vector8_load(&chunk, &base[i]);
+		if (vector8_has(chunk, key))
+			return true;
+	}
+
+	/* Process the remaining elements one at a time. */
+	for (; i < nelem; i++)
+	{
+		if (key == base[i])
+			return true;
+	}
+
+	return false;
+}
+
+/*
+ * pg_lfind8_le
+ *
+ * Return true if there is an element in 'base' that is less than or equal to
+ * 'key', otherwise return false.
+ */
+static inline bool
+pg_lfind8_le(uint8 key, uint8 *base, uint32 nelem)
+{
+	uint32		i;
+
+	/* round down to multiple of vector length */
+	uint32		tail_idx = nelem & ~(sizeof(Vector8) - 1);
+	Vector8		chunk;
+
+	for (i = 0; i < tail_idx; i += sizeof(Vector8))
+	{
+		vector8_load(&chunk, &base[i]);
+		if (vector8_has_le(chunk, key))
+			return true;
+	}
+
+	/* Process the remaining elements one at a time. */
+	for (; i < nelem; i++)
+	{
+		if (base[i] <= key)
+			return true;
+	}
+
+	return false;
+}
+
 /*
  * pg_lfind32
  *
@@ -26,7 +91,6 @@ pg_lfind32(uint32 key, uint32 *base, uint32 nelem)
 {
 	uint32		i = 0;
 
-	/* Use SIMD intrinsics where available. */
 #ifdef USE_SSE2
 
 	/*

src/include/port/simd.h

@@ -8,11 +8,17 @@
  *
  * src/include/port/simd.h
  *
+ * NOTES
+ * - VectorN in this file refers to a register where the element operands
+ * are N bits wide. The vector width is platform-specific, so users that care
+ * about that will need to inspect "sizeof(VectorN)".
+ *
  *-------------------------------------------------------------------------
  */
 #ifndef SIMD_H
 #define SIMD_H
 
+#if (defined(__x86_64__) || defined(_M_AMD64))
 /*
  * SSE2 instructions are part of the spec for the 64-bit x86 ISA. We assume
  * that compilers targeting this architecture understand SSE2 intrinsics.
@@ -22,9 +28,169 @@
  * will allow the use of intrinsics that haven't been enabled at compile
  * time.
  */
-#if (defined(__x86_64__) || defined(_M_AMD64))
 #include <emmintrin.h>
 #define USE_SSE2
+typedef __m128i Vector8;
+
+#else
+/*
+ * If no SIMD instructions are available, we can in some cases emulate vector
+ * operations using bitwise operations on unsigned integers.
+ */
+#define USE_NO_SIMD
+typedef uint64 Vector8;
+#endif
+
+
+/* load/store operations */
+static inline void vector8_load(Vector8 *v, const uint8 *s);
+
+/* assignment operations */
+static inline Vector8 vector8_broadcast(const uint8 c);
+
+/* element-wise comparisons to a scalar */
+static inline bool vector8_has(const Vector8 v, const uint8 c);
+static inline bool vector8_has_zero(const Vector8 v);
+static inline bool vector8_has_le(const Vector8 v, const uint8 c);
+
+
+/*
+ * Load a chunk of memory into the given vector.
+ */
+static inline void
+vector8_load(Vector8 *v, const uint8 *s)
+{
+#if defined(USE_SSE2)
+	*v = _mm_loadu_si128((const __m128i *) s);
+#else
+	memcpy(v, s, sizeof(Vector8));
 #endif
+}
+
+
+/*
+ * Create a vector with all elements set to the same value.
+ */
+static inline Vector8
+vector8_broadcast(const uint8 c)
+{
+#if defined(USE_SSE2)
+	return _mm_set1_epi8(c);
+#else
+	return ~UINT64CONST(0) / 0xFF * c;
+#endif
+}
+
+/*
+ * Return true if any elements in the vector are equal to the given scalar.
+ */
+static inline bool
+vector8_has(const Vector8 v, const uint8 c)
+{
+	bool		result;
+
+	/* pre-compute the result for assert checking */
+#ifdef USE_ASSERT_CHECKING
+	bool		assert_result = false;
+
+	for (int i = 0; i < sizeof(Vector8); i++)
+	{
+		if (((const uint8 *) &v)[i] == c)
+		{
+			assert_result = true;
+			break;
+		}
+	}
+#endif							/* USE_ASSERT_CHECKING */
+
+#if defined(USE_NO_SIMD)
+	/* any bytes in v equal to c will evaluate to zero via XOR */
+	result = vector8_has_zero(v ^ vector8_broadcast(c));
+#elif defined(USE_SSE2)
+	result = _mm_movemask_epi8(_mm_cmpeq_epi8(v, vector8_broadcast(c)));
+#endif
+
+	Assert(assert_result == result);
+	return result;
+}
+
+/*
+ * Convenience function equivalent to vector8_has(v, 0)
+ */
+static inline bool
+vector8_has_zero(const Vector8 v)
+{
+#if defined(USE_NO_SIMD)
+	/*
+	 * We cannot call vector8_has() here, because that would lead to a circular
+	 * definition.
+	 */
+	return vector8_has_le(v, 0);
+#elif defined(USE_SSE2)
+	return vector8_has(v, 0);
+#endif
+}
+
+/*
+ * Return true if any elements in the vector are less than or equal to the
+ * given scalar.
+ */
+static inline bool
+vector8_has_le(const Vector8 v, const uint8 c)
+{
+	bool		result = false;
+#if defined(USE_SSE2)
+	__m128i		sub;
+#endif
+
+	/* pre-compute the result for assert checking */
+#ifdef USE_ASSERT_CHECKING
+	bool		assert_result = false;
+
+	for (int i = 0; i < sizeof(Vector8); i++)
+	{
+		if (((const uint8 *) &v)[i] <= c)
+		{
+			assert_result = true;
+			break;
+		}
+	}
+#endif							/* USE_ASSERT_CHECKING */
+
+#if defined(USE_NO_SIMD)
+
+	/*
+	 * To find bytes <= c, we can use bitwise operations to find bytes < c+1,
+	 * but it only works if c+1 <= 128 and if the highest bit in v is not set.
+	 * Adapted from
+	 * https://graphics.stanford.edu/~seander/bithacks.html#HasLessInWord
+	 */
+	if ((int64) v >= 0 && c < 0x80)
+		result = (v - vector8_broadcast(c + 1)) & ~v & vector8_broadcast(0x80);
+	else
+	{
+		/* one byte at a time */
+		for (int i = 0; i < sizeof(Vector8); i++)
+		{
+			if (((const uint8 *) &v)[i] <= c)
+			{
+				result = true;
+				break;
+			}
+		}
+	}
+#elif defined(USE_SSE2)
+
+	/*
+	 * Use saturating subtraction to find bytes <= c, which will present as
+	 * NUL bytes in 'sub'.
+	 */
+	sub = _mm_subs_epu8(v, vector8_broadcast(c));
+	result = vector8_has_zero(sub);
+#endif
+
+	Assert(assert_result == result);
+	return result;
+}
 
 #endif							/* SIMD_H */

src/test/modules/test_lfind/expected/test_lfind.out

@@ -4,9 +4,21 @@ CREATE EXTENSION test_lfind;
 -- the operations complete without crashing or hanging and that none of their
 -- internal sanity tests fail.
 --
-SELECT test_lfind();
- test_lfind 
-------------
+SELECT test_lfind8();
+ test_lfind8 
+-------------
+ 
+(1 row)
+
+SELECT test_lfind8_le();
+ test_lfind8_le 
+----------------
+ 
+(1 row)
+
+SELECT test_lfind32();
+ test_lfind32 
+--------------
 
 (1 row)
 

src/test/modules/test_lfind/sql/test_lfind.sql

@@ -5,4 +5,6 @@ CREATE EXTENSION test_lfind;
 -- the operations complete without crashing or hanging and that none of their
 -- internal sanity tests fail.
 --
-SELECT test_lfind();
+SELECT test_lfind8();
+SELECT test_lfind8_le();
+SELECT test_lfind32();

src/test/modules/test_lfind/test_lfind--1.0.sql

@@ -3,6 +3,14 @@
 -- complain if script is sourced in psql, rather than via CREATE EXTENSION
 \echo Use "CREATE EXTENSION test_lfind" to load this file. \quit
 
-CREATE FUNCTION test_lfind()
+CREATE FUNCTION test_lfind32()
+	RETURNS pg_catalog.void
+	AS 'MODULE_PATHNAME' LANGUAGE C;
+
+CREATE FUNCTION test_lfind8()
+	RETURNS pg_catalog.void
+	AS 'MODULE_PATHNAME' LANGUAGE C;
+
+CREATE FUNCTION test_lfind8_le()
 	RETURNS pg_catalog.void
 	AS 'MODULE_PATHNAME' LANGUAGE C;