[PATCH] sysdeps/x86_64/multiarch/memmem-avx2.c: add memmem-avx2.c

[James Tirta Halim]

Find the rarest byte in NE. Do a naive loop until HS is aligned. Once aligned, find
the parts of HS that matches the rare byte and the byte after it, shift
back to the position of HS that should match NE and do a memcmp.

Average timings (Core i5 8400):
__memmem_avx2   basic_memmem    twoway_memmem   memmem
1342.942864     19100.87074     3335.335377     2745.971856

---
 sysdeps/x86_64/multiarch/memmem-avx2.c | 72 ++++++++++++++++++++++++++
 1 file changed, 72 insertions(+)
 create mode 100644 sysdeps/x86_64/multiarch/memmem-avx2.c

diff --git a/sysdeps/x86_64/multiarch/memmem-avx2.c b/sysdeps/x86_64/multiarch/memmem-avx2.c
new file mode 100644
index 0000000000..524d0fe45f
--- /dev/null
+++ b/sysdeps/x86_64/multiarch/memmem-avx2.c
@@ -0,0 +1,72 @@
+#include <immintrin.h>
+#include <string.h>
+#include <inttypes.h>
+#include <libc-pointer-arith.h>
+
+static inline void *
+__find_rarest_byte (const void *ne,
+                      size_t n)
+{
+  static const unsigned char rarebyte_table[256] = { 0, 1, 13, 56, 59, 60, 61, 62, 63, 232, 248, 2, 158, 4, 5, 6, 7, 8, 9, 10, 14, 20, 26, 29, 37, 46, 52, 53, 54, 55, 57, 58, 255, 172, 242, 193, 162, 174, 178, 182, 218, 219, 212, 180, 249, 197, 221, 210, 253, 231, 230, 224, 225, 226, 227, 223, 222, 220, 176, 213, 184, 229, 188, 164, 159, 209, 181, 203, 189, 216, 196, 192, 185, 205, 161, 168, 215, 187, 211, 194, 195, 165, 206, 204, 214, 198, 173, 179, 175, 183, 167, 202, 239, 201, 160, 241, 163, 246, 233, 238, 240, 254, 237, 208, 234, 250, 169, 186, 236, 217, 245, 243, 228, 170, 247, 244, 251, 235, 199, 200, 252, 207, 177, 191, 171, 190, 166, 3, 140, 134, 124, 126, 86, 128, 95, 117, 114, 93, 81, 87, 132, 96, 112, 97, 103, 82, 139, 89, 98, 88, 119, 74, 156, 115, 104, 75, 120, 106, 76, 155, 90, 122, 107, 125, 152, 145, 136, 137, 101, 116, 102, 108, 99, 141, 77, 78, 118, 79, 109, 100, 150, 73, 94, 72, 121, 151, 113, 135, 110, 105, 83, 91, 11, 12, 64, 149, 146, 111, 65, 69, 66, 15, 16, 17, 18, 19, 130, 92, 144, 123, 21, 22, 23, 24, 131, 133, 127, 142, 25, 70, 129, 27, 28, 67, 153, 84, 143, 138, 147, 157, 148, 68, 71, 30, 31, 32, 33, 34, 35, 36, 154, 38, 39, 40, 41, 42, 80, 43, 44, 45, 47, 48, 85, 49, 50, 51 };
+  const unsigned char *rare = (const unsigned char *)  ne;
+  const unsigned char *p = (const unsigned char *)  ne;
+  int c_rare = rarebyte_table[*rare];
+  int c;
+  for (; n--; ++p)
+    {
+    c = rarebyte_table[*p];
+    if (c < c_rare) {
+      rare = p;
+      c_rare = c;
+    }
+    }
+  return (void *) rare;
+}
+
+void *
+__memmem_avx2 (const void *hs,
+              size_t hs_len,
+              const void *ne,
+              size_t ne_len)
+{
+  if (ne_len == 1)
+    return (void *) memchr (hs, *(unsigned char *) ne, hs_len);
+  if (__glibc_unlikely (ne_len == 0))
+    return (void *) hs;
+  if (__glibc_unlikely (hs_len < ne_len))
+    return NULL;
+  const unsigned char *h = (const unsigned char *) hs;
+  const unsigned char *const end = h + hs_len - ne_len;
+  size_t shift = PTR_DIFF (__find_rarest_byte (ne, ne_len), ne);
+  if (shift == ne_len - 1)
+    --shift;
+  h += shift;
+  for (; !PTR_IS_ALIGNED (h, sizeof (__m256i)); ++h)
+    {
+    if (__glibc_unlikely (h - shift > end))
+      return NULL;
+    if (*h == *((unsigned char *) ne + shift) && !memcmp (h - shift, ne, ne_len))
+      return (void *) (h - shift);
+    }
+  const __m256i nv = _mm256_set1_epi8 (*((char *) ne + shift));
+  const __m256i nv1 = _mm256_set1_epi8 (*((char *) ne + shift + 1));
+  __m256i hv, hv1;
+  uint32_t i, hm0, hm1, m;
+  for (; h - shift <= end; h += sizeof (__m256i)) {
+    hv = _mm256_load_si256 ((const __m256i *) h);
+    hv1 = _mm256_loadu_si256 ((const __m256i *) (h + 1));
+    hm0 = (uint32_t) _mm256_movemask_epi8 (_mm256_cmpeq_epi8 (hv, nv));
+    hm1 = (uint32_t) _mm256_movemask_epi8 (_mm256_cmpeq_epi8 (hv1, nv1));
+    m = hm0 & hm1;
+    while (m)
+      {
+      i = _tzcnt_u32 (m);
+      m = _blsr_u32 (m);
+      if (__glibc_unlikely (h + i - shift > end))
+        return NULL;
+      if (!memcmp (h + i - shift, ne, ne_len))
+        return (char *) h + i - shift;
+      }
+  }
+  return NULL;
+}
-- 
2.43.0