assertion failure on darwin_x86_64, have to investigage

PiperOrigin-RevId: 303346402

snappy-internal.h

@@ -89,18 +89,12 @@ 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,
-                                                      uint64* data) {
+                                                      const char* s2_limit) {
   assert(s2_limit >= s2);
   size_t matched = 0;
 
@@ -109,28 +103,12 @@ 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 - 16)) {
+  if (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 8)) {
     uint64 a1 = UNALIGNED_LOAD64(s1);
     uint64 a2 = UNALIGNED_LOAD64(s2);
-    if (SNAPPY_PREDICT_TRUE(a1 != a2)) {
-      uint64 xorval = a1 ^ a2;
-      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);
+    if (a1 != a2) {
+      return std::pair<size_t, bool>(Bits::FindLSBSetNonZero64(a1 ^ a2) >> 3,
+                                     true);
     } else {
       matched = 8;
       s2 += 8;
@@ -141,25 +119,14 @@ 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 - 16)) {
-    uint64 a1 = UNALIGNED_LOAD64(s1 + matched);
-    uint64 a2 = UNALIGNED_LOAD64(s2);
-    if (a1 == a2) {
+  while (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 8)) {
+    if (UNALIGNED_LOAD64(s2) == UNALIGNED_LOAD64(s1 + matched)) {
       s2 += 8;
       matched += 8;
     } else {
-      uint64 xorval = a1 ^ a2;
-      int shift = Bits::FindLSBSetNonZero64(xorval);
-      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;
+      uint64 x = UNALIGNED_LOAD64(s2) ^ UNALIGNED_LOAD64(s1 + matched);
+      int matching_bits = Bits::FindLSBSetNonZero64(x);
+      matched += matching_bits >> 3;
       assert(matched >= 8);
       return std::pair<size_t, bool>(matched, false);
     }
@@ -169,9 +136,6 @@ 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);
     }
   }
@@ -180,8 +144,7 @@ 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,
-                                                      uint64* data) {
+                                                      const char* s2_limit) {
   // Implementation based on the x86-64 version, above.
   assert(s2_limit >= s2);
   int matched = 0;
@@ -201,7 +164,6 @@ static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
       ++matched;
     }
   }
-  *data = UNALIGNED_LOAD64(s2 + matched);
   return std::pair<size_t, bool>(matched, matched < 8);
 }
 #endif

snappy.cc

@@ -385,18 +385,11 @@ 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) {
-    uint32 u = (len << 2) + (offset << 8);
-    uint32 copy1 = COPY_1_BYTE_OFFSET - (4 << 2) + ((offset >> 3) & 0xe0);
-    uint32 copy2 = COPY_2_BYTE_OFFSET - (1 << 2);
-    // 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;
+  if (len_less_than_12 && SNAPPY_PREDICT_TRUE(offset < 2048)) {
+    // offset fits in 11 bits.  The 3 highest go in the top of the first byte,
+    // and the rest go in the second byte.
+    *op++ = COPY_1_BYTE_OFFSET + ((len - 4) << 2) + ((offset >> 3) & 0xe0);
+    *op++ = offset & 0xff;
   } 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.
@@ -622,7 +615,7 @@ 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, &data);
+            FindMatchLength(candidate + 4, ip + 4, ip_end);
         size_t matched = 4 + p.first;
         ip += matched;
         size_t offset = base - candidate;
@@ -636,12 +629,12 @@ 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)].
-        table[HashBytes(LittleEndian::Load32(ip - 1), shift)] =
-            ip - base_ip - 1;
+        data = LittleEndian::Load64(ip - 1);
+        table[HashBytes(data, shift)] = ip - base_ip - 1;
+        data >>= 8;
         uint32 hash = HashBytes(data, shift);
         candidate = base_ip + table[hash];
         table[hash] = ip - base_ip;

snappy_unittest.cc

@@ -957,9 +957,8 @@ 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, &data);
+      snappy::internal::FindMatchLength(s1, s2, s2 + length);
   CHECK_EQ(p.first < 8, p.second);
   return p.first;
 }