assertion failure on darwin_x86_64, have to investigage

PiperOrigin-RevId: 303428229

snappy-internal.h

@@ -89,12 +89,18 @@ char* CompressFragment(const char* input,
 // Does not read *(s1 + (s2_limit - s2)) or beyond.
 // Requires that s2_limit >= s2.
 //
+// In addition populate *data with the next 8 bytes from the end of the match.
+// This is only done if 8 bytes are available (s2_limit - s2 >= 8). The point is
+// that on some arch's this can be done faster in this routine than subsequent
+// loading from s2 + n.
+//
 // Separate implementation for 64-bit, little-endian cpus.
 #if !defined(SNAPPY_IS_BIG_ENDIAN) && \
     (defined(ARCH_K8) || defined(ARCH_PPC) || defined(ARCH_ARM))
 static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
                                                       const char* s2,
-                                                      const char* s2_limit) {
+                                                      const char* s2_limit,
+                                                      uint64* data) {
   assert(s2_limit >= s2);
   size_t matched = 0;
 
@@ -103,12 +109,28 @@ static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
   // uncommon code paths that determine, without extra effort, whether the match
   // length is less than 8.  In short, we are hoping to avoid a conditional
   // branch, and perhaps get better code layout from the C++ compiler.
-  if (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 8)) {
+  if (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 16)) {
     uint64 a1 = UNALIGNED_LOAD64(s1);
     uint64 a2 = UNALIGNED_LOAD64(s2);
-    if (a1 != a2) {
+    if (SNAPPY_PREDICT_TRUE(a1 != a2)) {
-      return std::pair<size_t, bool>(Bits::FindLSBSetNonZero64(a1 ^ a2) >> 3,
+      uint64 xorval = a1 ^ a2;
-                                     true);
+      int shift = Bits::FindLSBSetNonZero64(xorval);
+      size_t matched_bytes = shift >> 3;
+#ifndef __x86_64__
+      *data = UNALIGNED_LOAD64(s2 + matched_bytes);
+#else
+      // Unfortunately the compiler cannot find this using the obvious c++ code
+      // *data = shift == 0 ? a2 : (a2 >> shift) | (a3 << (64 - shift);
+      // the reason is that the above needs the conditional clause to guard
+      // against UB when shift == 0. The compiler doesn't realize the full
+      // expression can be lowered into a single "shrd" instruction and in
+      // effect the conditional can be ignored.
+      uint64 a3 = UNALIGNED_LOAD64(s2 + 8);
+      asm ("shrdq %%cl, %1, %0\n\t" : "+r"(a2) : "r"(a3), "c"(shift & -8));
+      *data = a2;
+#endif
+      assert(*data == UNALIGNED_LOAD64(s2 + matched_bytes));
+      return std::pair<size_t, bool>(matched_bytes, true);
     } else {
       matched = 8;
       s2 += 8;
@@ -119,14 +141,25 @@ static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
   // time until we find a 64-bit block that doesn't match; then we find
   // the first non-matching bit and use that to calculate the total
   // length of the match.
-  while (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 8)) {
+  while (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 16)) {
-    if (UNALIGNED_LOAD64(s2) == UNALIGNED_LOAD64(s1 + matched)) {
+    uint64 a1 = UNALIGNED_LOAD64(s1 + matched);
+    uint64 a2 = UNALIGNED_LOAD64(s2);
+    if (a1 == a2) {
       s2 += 8;
       matched += 8;
     } else {
-      uint64 x = UNALIGNED_LOAD64(s2) ^ UNALIGNED_LOAD64(s1 + matched);
+      uint64 xorval = a1 ^ a2;
-      int matching_bits = Bits::FindLSBSetNonZero64(x);
+      int shift = Bits::FindLSBSetNonZero64(xorval);
-      matched += matching_bits >> 3;
+      size_t matched_bytes = shift >> 3;
+#ifndef __x86_64__
+      *data = UNALIGNED_LOAD64(s2 + matched_bytes);
+#else
+      uint64 a3 = UNALIGNED_LOAD64(s2 + 8);
+      asm("shrdq %%cl, %1, %0\n\t" : "+r"(a2) : "r"(a3), "c"(shift & -8));
+      *data = a2;
+#endif
+      assert(*data == UNALIGNED_LOAD64(s2 + matched_bytes));
+      matched += matched_bytes;
       assert(matched >= 8);
       return std::pair<size_t, bool>(matched, false);
     }
@@ -136,6 +169,9 @@ static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
       ++s2;
       ++matched;
     } else {
+      if (s2 <= s2_limit - 8) {
+        *data = UNALIGNED_LOAD64(s2);
+      }
       return std::pair<size_t, bool>(matched, matched < 8);
     }
   }
@@ -144,7 +180,8 @@ static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
 #else
 static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
                                                       const char* s2,
-                                                      const char* s2_limit) {
+                                                      const char* s2_limit,
+                                                      uint64* data) {
   // Implementation based on the x86-64 version, above.
   assert(s2_limit >= s2);
   int matched = 0;
@@ -158,12 +195,14 @@ static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
     uint32 x = UNALIGNED_LOAD32(s2) ^ UNALIGNED_LOAD32(s1 + matched);
     int matching_bits = Bits::FindLSBSetNonZero(x);
     matched += matching_bits >> 3;
+    s2 += matching_bits >> 3;
   } else {
     while ((s2 < s2_limit) && (s1[matched] == *s2)) {
       ++s2;
       ++matched;
     }
   }
+  if (s2 <= s2_limit - 8) *data = LittleEndian::Load64(s2);
   return std::pair<size_t, bool>(matched, matched < 8);
 }
 #endif

snappy.cc

@@ -385,11 +385,18 @@ static inline char* EmitCopyAtMost64(char* op, size_t offset, size_t len) {
   assert(offset < 65536);
   assert(len_less_than_12 == (len < 12));
 
-  if (len_less_than_12 && SNAPPY_PREDICT_TRUE(offset < 2048)) {
+  if (len_less_than_12) {
-    // offset fits in 11 bits.  The 3 highest go in the top of the first byte,
+    uint32 u = (len << 2) + (offset << 8);
-    // and the rest go in the second byte.
+    uint32 copy1 = COPY_1_BYTE_OFFSET - (4 << 2) + ((offset >> 3) & 0xe0);
-    *op++ = COPY_1_BYTE_OFFSET + ((len - 4) << 2) + ((offset >> 3) & 0xe0);
+    uint32 copy2 = COPY_2_BYTE_OFFSET - (1 << 2);
-    *op++ = offset & 0xff;
+    // It turns out that offset < 2048 is a difficult to predict branch.
+    // `perf record` shows this is the highest percentage of branch misses in
+    // benchmarks. This code produces branch free code, the data dependency
+    // chain that bottlenecks the throughput is so long that a few extra
+    // instructions are completely free (IPC << 6 because of data deps).
+    u += offset < 2048 ? copy1 : copy2;
+    LittleEndian::Store32(op, u);
+    op += offset < 2048 ? 2 : 3;
   } else {
     // Write 4 bytes, though we only care about 3 of them.  The output buffer
     // is required to have some slack, so the extra byte won't overrun it.
@@ -615,7 +622,7 @@ char* CompressFragment(const char* input,
         // "literal bytes" prior to ip.
         const char* base = ip;
         std::pair<size_t, bool> p =
-            FindMatchLength(candidate + 4, ip + 4, ip_end);
+            FindMatchLength(candidate + 4, ip + 4, ip_end, &data);
         size_t matched = 4 + p.first;
         ip += matched;
         size_t offset = base - candidate;
@@ -629,12 +636,12 @@ char* CompressFragment(const char* input,
         if (SNAPPY_PREDICT_FALSE(ip >= ip_limit)) {
           goto emit_remainder;
         }
+        assert(LittleEndian::Load64(ip) == data);
         // We are now looking for a 4-byte match again.  We read
         // table[Hash(ip, shift)] for that.  To improve compression,
         // we also update table[Hash(ip - 1, shift)] and table[Hash(ip, shift)].
-        data = LittleEndian::Load64(ip - 1);
+        table[HashBytes(LittleEndian::Load32(ip - 1), shift)] =
-        table[HashBytes(data, shift)] = ip - base_ip - 1;
+            ip - base_ip - 1;
-        data >>= 8;
         uint32 hash = HashBytes(data, shift);
         candidate = base_ip + table[hash];
         table[hash] = ip - base_ip;

snappy_unittest.cc

@@ -957,8 +957,9 @@ TEST(Snappy, ZeroOffsetCopyValidation) {
 namespace {
 
 int TestFindMatchLength(const char* s1, const char *s2, unsigned length) {
+  uint64 data;
   std::pair<size_t, bool> p =
-      snappy::internal::FindMatchLength(s1, s2, s2 + length);
+      snappy::internal::FindMatchLength(s1, s2, s2 + length, &data);
   CHECK_EQ(p.first < 8, p.second);
   return p.first;
 }