Introduce helper SIMD functions for small byte arrays

vector8_min - helper for emulating ">=" semantics

vector8_highbit_mask - used to turn the result of a vector
comparison into a bitmask

Masahiko Sawada

Reviewed by Nathan Bossart, with additional adjustments by me
Discussion: https://postgr.es/m/CAFBsxsHbBm_M22gLBO%2BAZT4mfMq3L_oX3wdKZxjeNnT7fHsYMQ%40mail.gmail.com

src/include/port/simd.h

@@ -79,6 +79,7 @@ static inline bool vector8_has_le(const Vector8 v, const uint8 c);
 static inline bool vector8_is_highbit_set(const Vector8 v);
 #ifndef USE_NO_SIMD
 static inline bool vector32_is_highbit_set(const Vector32 v);
+static inline uint32 vector8_highbit_mask(const Vector8 v);
 #endif
 
 /* arithmetic operations */
@@ -96,6 +97,7 @@ static inline Vector8 vector8_ssub(const Vector8 v1, const Vector8 v2);
  */
 #ifndef USE_NO_SIMD
 static inline Vector8 vector8_eq(const Vector8 v1, const Vector8 v2);
+static inline Vector8 vector8_min(const Vector8 v1, const Vector8 v2);
 static inline Vector32 vector32_eq(const Vector32 v1, const Vector32 v2);
 #endif
 
@@ -299,6 +301,36 @@ vector32_is_highbit_set(const Vector32 v)
 }
 #endif							/* ! USE_NO_SIMD */
 
+/*
+ * Return a bitmask formed from the high-bit of each element.
+ */
+#ifndef USE_NO_SIMD
+static inline uint32
+vector8_highbit_mask(const Vector8 v)
+{
+#ifdef USE_SSE2
+	return (uint32) _mm_movemask_epi8(v);
+#elif defined(USE_NEON)
+	/*
+	 * Note: It would be faster to use vget_lane_u64 and vshrn_n_u16, but that
+	 * returns a uint64, making it inconvenient to combine mask values from
+	 * multiple vectors.
+	 */
+	static const uint8 mask[16] = {
+		1 << 0, 1 << 1, 1 << 2, 1 << 3,
+		1 << 4, 1 << 5, 1 << 6, 1 << 7,
+		1 << 0, 1 << 1, 1 << 2, 1 << 3,
+		1 << 4, 1 << 5, 1 << 6, 1 << 7,
+	};
+
+	uint8x16_t	masked = vandq_u8(vld1q_u8(mask), (uint8x16_t) vshrq_n_s8(v, 7));
+	uint8x16_t	maskedhi = vextq_u8(masked, masked, 8);
+
+	return (uint32) vaddvq_u16((uint16x8_t) vzip1q_u8(masked, maskedhi));
+#endif
+}
+#endif							/* ! USE_NO_SIMD */
+
 /*
  * Return the bitwise OR of the inputs
  */
@@ -372,4 +404,19 @@ vector32_eq(const Vector32 v1, const Vector32 v2)
 }
 #endif							/* ! USE_NO_SIMD */
 
+/*
+ * Given two vectors, return a vector with the minimum element of each.
+ */
+#ifndef USE_NO_SIMD
+static inline Vector8
+vector8_min(const Vector8 v1, const Vector8 v2)
+{
+#ifdef USE_SSE2
+	return _mm_min_epu8(v1, v2);
+#elif defined(USE_NEON)
+	return vminq_u8(v1, v2);
+#endif
+}
+#endif							/* ! USE_NO_SIMD */
+
 #endif							/* SIMD_H */