rmuir commented on code in PR #13572:
URL: https://github.com/apache/lucene/pull/13572#discussion_r1699259519
##########
lucene/core/src/c/dotProduct.c:
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@@ -0,0 +1,143 @@
+// dotProduct.c
+
+#include <stdio.h>
+#include <arm_neon.h>
+
+#ifdef __ARM_ACLE
+#include <arm_acle.h>
+#endif
+
+#if (defined(__ARM_FEATURE_SVE) && !defined(__APPLE__))
+#include <arm_sve.h>
+/*
+ * Unrolled and vectorized int8 dotProduct implementation using SVE
instructions
+ * NOTE: Clang 15.0 compiler on Apple M3 Max compiles the code below
sucessfully
+ * with '-march=native+sve' option but throws "Illegal Hardware Instruction"
error
+ * Looks like Apple M3 does not implement SVE and Apple's official
documentation
+ * is not explicit about this or at least I could not find it.
+ *
+ */
+int32_t vdot8s_sve(int8_t *vec1, int8_t *vec2, int32_t limit) {
+ int32_t result = 0;
+ int32_t i = 0;
+ // Vectors of 8-bit signed integers
+ svint8_t va1, va2, va3, va4;
+ svint8_t vb1, vb2, vb3, vb4;
+ // Init accumulators
+ svint32_t acc1 = svdup_n_s32(0);
+ svint32_t acc2 = svdup_n_s32(0);
+ svint32_t acc3 = svdup_n_s32(0);
+ svint32_t acc4 = svdup_n_s32(0);
+
+ // Number of 8-bits elements in the SVE vector
+ int32_t vec_length = svcntb();
+
+ // Manually unroll the loop
+ for (i = 0; i + 4 * vec_length <= limit; i += 4 * vec_length) {
+ // Load vectors into the Z registers which can range from 128-bit to
2048-bit wide
+ // The predicate register - P determines which bytes are active
+ // svptrue_b8() returns a predictae in which every element is true
+ //
+ va1 = svld1_s8(svptrue_b8(), vec1 + i);
+ vb1 = svld1_s8(svptrue_b8(), vec2 + i);
+
+ va2 = svld1_s8(svptrue_b8(), vec1 + i + vec_length);
+ vb2 = svld1_s8(svptrue_b8(), vec2 + i + vec_length);
+
+ va3 = svld1_s8(svptrue_b8(), vec1 + i + 2 * vec_length);
+ vb3 = svld1_s8(svptrue_b8(), vec2 + i + 2 * vec_length);
+
+ va4 = svld1_s8(svptrue_b8(), vec1 + i + 3 * vec_length);
+ vb4 = svld1_s8(svptrue_b8(), vec2 + i + 3 * vec_length);
+
+ // Dot product using SDOT instruction on Z vectors
+ acc1 = svdot_s32(acc1, va1, vb1);
+ acc2 = svdot_s32(acc2, va2, vb2);
+ acc3 = svdot_s32(acc3, va3, vb3);
+ acc4 = svdot_s32(acc4, va4, vb4);
+ }
+ // Add correspponding active elements in each of the vectors
+ acc1 = svadd_s32_x(svptrue_b8() , acc1, acc2);
+ acc3 = svadd_s32_x(svptrue_b8() , acc3, acc4);
+ acc1 = svadd_s32_x(svptrue_b8(), acc1, acc3);
+
+ // REDUCE: Add every vector element in target and write result to scalar
+ result = svaddv_s32(svptrue_b8(), acc1);
+
+ // Scalar tail. TODO: Use FMA
+ for (; i < limit; i++) {
+ result += vec1[i] * vec2[i];
+ }
Review Comment:
for any "tails" like this where we manually unroll and vectorize the main
loop, we can add pragmas to prevent GCC/LLVM from trying to unroll and
vectorize the tail. It is not strictly necessary but will lead to tighter code.
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