decode: faster huffman code loading on 32-bit Arm (#703)

* platform: add macro for using the 'aligned' attribute

* decode: add accessor macros for HuffmanCode fields

Adds a constructor function for building HuffmanCode values
so they can be accessed quickly on different architectures.

Also adds macros for marking a HuffmanCode table pointer
that can be accessed quickly (BROTLI_HC_MARK_TABLE_FOR_FAST_LOAD),
adjusting the index into that table (BROTLI_HC_ADJUST_TABLE_INDEX),
and getting the .bits or .value fields out of the table at the
current index (BROTLI_HC_GET_BITS/VALUE).

For example, assuming |table| contains a HuffmanCode pointer:

  BROTLI_HC_MARK_TABLE_FOR_FAST_LOAD(table);
  BROTLI_HC_ADJUST_TABLE_INDEX(table, index_into_table);
  *bits = BROTLI_HC_GET_BITS(table);
  *value = BROTLI_HC_GET_VALUE(table);
  BROTLI_HC_ADJUST_TABLE_INDEX(table, offset);
  *bits2 = BROTLI_HC_GET_BITS(table);
  *value2 = BROTLI_HC_GET_VALUE(table);

All uses of the HuffmanCode have been updated appropriately.

* decode: add alternative accessors for HuffmanCode on Arm AArch32

4 files changed, 139 insertions, 84 deletions

diff --git a/c/common/platform.h b/c/common/platform.h
index 0d84b19..3945a27 100755
--- a/c/common/platform.h
+++ b/c/common/platform.h
@@ -180,6 +180,12 @@ OR:
 #define BROTLI_UNUSED_FUNCTION static BROTLI_INLINE
 #endif
 
+#if BROTLI_GNUC_HAS_ATTRIBUTE(aligned, 2, 7, 0)
+#define BROTLI_ALIGNED(N) __attribute__((aligned(N)))
+#else
+#define BROTLI_ALIGNED(N)
+#endif
+
 #if (defined(__ARM_ARCH) && (__ARM_ARCH == 7)) || \
     (defined(M_ARM) && (M_ARM == 7))
 #define BROTLI_TARGET_ARMV7
@@ -343,7 +349,7 @@ static BROTLI_INLINE void BrotliUnalignedWrite64(void* p, uint64_t v) {
 /* If __attribute__(aligned) is available, use that. Otherwise, memcpy. */
 
 #if BROTLI_GNUC_HAS_ATTRIBUTE(aligned, 2, 7, 0)
-typedef  __attribute__((aligned(1))) uint64_t brotli_unaligned_uint64_t;
+typedef BROTLI_ALIGNED(1) uint64_t brotli_unaligned_uint64_t;
 
 static BROTLI_INLINE uint64_t BrotliUnalignedRead64(const void* p) {
   return (uint64_t) ((brotli_unaligned_uint64_t*) p)[0];
diff --git a/c/dec/decode.c b/c/dec/decode.c
index 674b829..1fde329 100644
--- a/c/dec/decode.c
+++ b/c/dec/decode.c
@@ -347,15 +347,17 @@ static BrotliDecoderErrorCode BROTLI_NOINLINE DecodeMetaBlockLength(
 static BROTLI_INLINE uint32_t DecodeSymbol(uint32_t bits,
                                            const HuffmanCode* table,
                                            BrotliBitReader* br) {
-  table += bits & HUFFMAN_TABLE_MASK;
-  if (table->bits > HUFFMAN_TABLE_BITS) {
-    uint32_t nbits = table->bits - HUFFMAN_TABLE_BITS;
+  BROTLI_HC_MARK_TABLE_FOR_FAST_LOAD(table);
+  BROTLI_HC_ADJUST_TABLE_INDEX(table, bits & HUFFMAN_TABLE_MASK);
+  if (BROTLI_HC_FAST_LOAD_BITS(table) > HUFFMAN_TABLE_BITS) {
+    uint32_t nbits = BROTLI_HC_FAST_LOAD_BITS(table) - HUFFMAN_TABLE_BITS;
     BrotliDropBits(br, HUFFMAN_TABLE_BITS);
-    table += table->value;
-    table += (bits >> HUFFMAN_TABLE_BITS) & BitMask(nbits);
+    BROTLI_HC_ADJUST_TABLE_INDEX(table,
+        BROTLI_HC_FAST_LOAD_VALUE(table) +
+        ((bits >> HUFFMAN_TABLE_BITS) & BitMask(nbits)));
   }
-  BrotliDropBits(br, table->bits);
-  return table->value;
+  BrotliDropBits(br, BROTLI_HC_FAST_LOAD_BITS(table));
+  return BROTLI_HC_FAST_LOAD_VALUE(table);
 }
 
 /* Reads and decodes the next Huffman code from bit-stream.
@@ -371,19 +373,20 @@ static BROTLI_NOINLINE BROTLI_BOOL SafeDecodeSymbol(
     const HuffmanCode* table, BrotliBitReader* br, uint32_t* result) {
   uint32_t val;
   uint32_t available_bits = BrotliGetAvailableBits(br);
+  BROTLI_HC_MARK_TABLE_FOR_FAST_LOAD(table);
   if (available_bits == 0) {
-    if (table->bits == 0) {
-      *result = table->value;
+    if (BROTLI_HC_FAST_LOAD_BITS(table) == 0) {
+      *result = BROTLI_HC_FAST_LOAD_VALUE(table);
       return BROTLI_TRUE;
     }
     return BROTLI_FALSE;  /* No valid bits at all. */
   }
   val = (uint32_t)BrotliGetBitsUnmasked(br);
-  table += val & HUFFMAN_TABLE_MASK;
-  if (table->bits <= HUFFMAN_TABLE_BITS) {
-    if (table->bits <= available_bits) {
-      BrotliDropBits(br, table->bits);
-      *result = table->value;
+  BROTLI_HC_ADJUST_TABLE_INDEX(table, val & HUFFMAN_TABLE_MASK);
+  if (BROTLI_HC_FAST_LOAD_BITS(table) <= HUFFMAN_TABLE_BITS) {
+    if (BROTLI_HC_FAST_LOAD_BITS(table) <= available_bits) {
+      BrotliDropBits(br, BROTLI_HC_FAST_LOAD_BITS(table));
+      *result = BROTLI_HC_FAST_LOAD_VALUE(table);
       return BROTLI_TRUE;
     } else {
       return BROTLI_FALSE;  /* Not enough bits for the first level. */
@@ -394,15 +397,15 @@ static BROTLI_NOINLINE BROTLI_BOOL SafeDecodeSymbol(
   }
 
   /* Speculatively drop HUFFMAN_TABLE_BITS. */
-  val = (val & BitMask(table->bits)) >> HUFFMAN_TABLE_BITS;
+  val = (val & BitMask(BROTLI_HC_FAST_LOAD_BITS(table))) >> HUFFMAN_TABLE_BITS;
   available_bits -= HUFFMAN_TABLE_BITS;
-  table += table->value + val;
-  if (available_bits < table->bits) {
+  BROTLI_HC_ADJUST_TABLE_INDEX(table, BROTLI_HC_FAST_LOAD_VALUE(table) + val);
+  if (available_bits < BROTLI_HC_FAST_LOAD_BITS(table)) {
     return BROTLI_FALSE;  /* Not enough bits for the second level. */
   }
 
-  BrotliDropBits(br, HUFFMAN_TABLE_BITS + table->bits);
-  *result = table->value;
+  BrotliDropBits(br, HUFFMAN_TABLE_BITS + BROTLI_HC_FAST_LOAD_BITS(table));
+  *result = BROTLI_HC_FAST_LOAD_VALUE(table);
   return BROTLI_TRUE;
 }
 
@@ -425,9 +428,10 @@ static BROTLI_INLINE void PreloadSymbol(int safe,
   if (safe) {
     return;
   }
-  table += BrotliGetBits(br, HUFFMAN_TABLE_BITS);
-  *bits = table->bits;
-  *value = table->value;
+  BROTLI_HC_MARK_TABLE_FOR_FAST_LOAD(table);
+  BROTLI_HC_ADJUST_TABLE_INDEX(table, BrotliGetBits(br, HUFFMAN_TABLE_BITS));
+  *bits = BROTLI_HC_FAST_LOAD_BITS(table);
+  *value = BROTLI_HC_FAST_LOAD_VALUE(table);
 }
 
 /* Decodes the next Huffman code using data prepared by PreloadSymbol.
@@ -441,10 +445,11 @@ static BROTLI_INLINE uint32_t ReadPreloadedSymbol(const HuffmanCode* table,
     uint32_t val = BrotliGet16BitsUnmasked(br);
     const HuffmanCode* ext = table + (val & HUFFMAN_TABLE_MASK) + *value;
     uint32_t mask = BitMask((*bits - HUFFMAN_TABLE_BITS));
+    BROTLI_HC_MARK_TABLE_FOR_FAST_LOAD(ext);
     BrotliDropBits(br, HUFFMAN_TABLE_BITS);
-    ext += (val >> HUFFMAN_TABLE_BITS) & mask;
-    BrotliDropBits(br, ext->bits);
-    result = ext->value;
+    BROTLI_HC_ADJUST_TABLE_INDEX(ext, (val >> HUFFMAN_TABLE_BITS) & mask);
+    BrotliDropBits(br, BROTLI_HC_FAST_LOAD_BITS(ext));
+    result = BROTLI_HC_FAST_LOAD_VALUE(ext);
   } else {
     BrotliDropBits(br, *bits);
   }
@@ -597,6 +602,7 @@ static BrotliDecoderErrorCode ReadSymbolCodeLengths(
   while (symbol < alphabet_size && space > 0) {
     const HuffmanCode* p = s->table;
     uint32_t code_len;
+    BROTLI_HC_MARK_TABLE_FOR_FAST_LOAD(p);
     if (!BrotliCheckInputAmount(br, BROTLI_SHORT_FILL_BIT_WINDOW_READ)) {
       s->symbol = symbol;
       s->repeat = repeat;
@@ -606,10 +612,10 @@ static BrotliDecoderErrorCode ReadSymbolCodeLengths(
       return BROTLI_DECODER_NEEDS_MORE_INPUT;
     }
     BrotliFillBitWindow16(br);
-    p += BrotliGetBitsUnmasked(br) &
-        BitMask(BROTLI_HUFFMAN_MAX_CODE_LENGTH_CODE_LENGTH);
-    BrotliDropBits(br, p->bits);  /* Use 1..5 bits. */
-    code_len = p->value;  /* code_len == 0..17 */
+    BROTLI_HC_ADJUST_TABLE_INDEX(p, BrotliGetBitsUnmasked(br) &
+        BitMask(BROTLI_HUFFMAN_MAX_CODE_LENGTH_CODE_LENGTH));
+    BrotliDropBits(br, BROTLI_HC_FAST_LOAD_BITS(p));  /* Use 1..5 bits. */
+    code_len = BROTLI_HC_FAST_LOAD_VALUE(p);  /* code_len == 0..17 */
     if (code_len < BROTLI_REPEAT_PREVIOUS_CODE_LENGTH) {
       ProcessSingleCodeLength(code_len, &symbol, &repeat, &space,
           &prev_code_len, symbol_lists, code_length_histo, next_symbol);
@@ -637,31 +643,34 @@ static BrotliDecoderErrorCode SafeReadSymbolCodeLengths(
     uint32_t code_len;
     uint32_t available_bits;
     uint32_t bits = 0;
+    BROTLI_HC_MARK_TABLE_FOR_FAST_LOAD(p);
     if (get_byte && !BrotliPullByte(br)) return BROTLI_DECODER_NEEDS_MORE_INPUT;
     get_byte = BROTLI_FALSE;
     available_bits = BrotliGetAvailableBits(br);
     if (available_bits != 0) {
       bits = (uint32_t)BrotliGetBitsUnmasked(br);
     }
-    p += bits & BitMask(BROTLI_HUFFMAN_MAX_CODE_LENGTH_CODE_LENGTH);
-    if (p->bits > available_bits) {
+    BROTLI_HC_ADJUST_TABLE_INDEX(p,
+        bits & BitMask(BROTLI_HUFFMAN_MAX_CODE_LENGTH_CODE_LENGTH));
+    if (BROTLI_HC_FAST_LOAD_BITS(p) > available_bits) {
       get_byte = BROTLI_TRUE;
       continue;
     }
-    code_len = p->value;  /* code_len == 0..17 */
+    code_len = BROTLI_HC_FAST_LOAD_VALUE(p);  /* code_len == 0..17 */
     if (code_len < BROTLI_REPEAT_PREVIOUS_CODE_LENGTH) {
-      BrotliDropBits(br, p->bits);
+      BrotliDropBits(br, BROTLI_HC_FAST_LOAD_BITS(p));
       ProcessSingleCodeLength(code_len, &s->symbol, &s->repeat, &s->space,
           &s->prev_code_len, s->symbol_lists, s->code_length_histo,
           s->next_symbol);
     } else {  /* code_len == 16..17, extra_bits == 2..3 */
       uint32_t extra_bits = code_len - 14U;
-      uint32_t repeat_delta = (bits >> p->bits) & BitMask(extra_bits);
-      if (available_bits < p->bits + extra_bits) {
+      uint32_t repeat_delta = (bits >> BROTLI_HC_FAST_LOAD_BITS(p)) &
+          BitMask(extra_bits);
+      if (available_bits < BROTLI_HC_FAST_LOAD_BITS(p) + extra_bits) {
         get_byte = BROTLI_TRUE;
         continue;
       }
-      BrotliDropBits(br, p->bits + extra_bits);
+      BrotliDropBits(br, BROTLI_HC_FAST_LOAD_BITS(p) + extra_bits);
       ProcessRepeatedCodeLength(code_len, repeat_delta, alphabet_size,
           &s->symbol, &s->repeat, &s->space, &s->prev_code_len,
           &s->repeat_code_len, s->symbol_lists, s->code_length_histo,
diff --git a/c/dec/huffman.c b/c/dec/huffman.c
index c5eafad..9deb42c 100644
--- a/c/dec/huffman.c
+++ b/c/dec/huffman.c
@@ -142,8 +142,7 @@ void BrotliBuildCodeLengthsHuffmanTable(HuffmanCode* table,
 
   /* Special case: all symbols but one have 0 code length. */
   if (offset[0] == 0) {
-    code.bits = 0;
-    code.value = (uint16_t)sorted[0];
+    code = ConstructHuffmanCode(0, (uint16_t)sorted[0]);
     for (key = 0; key < (brotli_reg_t)table_size; ++key) {
       table[key] = code;
     }
@@ -157,9 +156,9 @@ void BrotliBuildCodeLengthsHuffmanTable(HuffmanCode* table,
   bits = 1;
   step = 2;
   do {
-    code.bits = (uint8_t)bits;
     for (bits_count = count[bits]; bits_count != 0; --bits_count) {
-      code.value = (uint16_t)sorted[symbol++];
+      code = ConstructHuffmanCode((uint8_t)bits,
+          (uint16_t)sorted[symbol++]);
       ReplicateValue(&table[BrotliReverseBits(key)], step, table_size, code);
       key += key_step;
     }
@@ -211,11 +210,10 @@ uint32_t BrotliBuildHuffmanTable(HuffmanCode* root_table,
   bits = 1;
   step = 2;
   do {
-    code.bits = (uint8_t)bits;
     symbol = bits - (BROTLI_HUFFMAN_MAX_CODE_LENGTH + 1);
     for (bits_count = count[bits]; bits_count != 0; --bits_count) {
       symbol = symbol_lists[symbol];
-      code.value = (uint16_t)symbol;
+      code = ConstructHuffmanCode((uint8_t)bits, (uint16_t)symbol);
       ReplicateValue(&table[BrotliReverseBits(key)], step, table_size, code);
       key += key_step;
     }
@@ -244,14 +242,14 @@ uint32_t BrotliBuildHuffmanTable(HuffmanCode* root_table,
         total_size += table_size;
         sub_key = BrotliReverseBits(key);
         key += key_step;
-        root_table[sub_key].bits = (uint8_t)(table_bits + root_bits);
-        root_table[sub_key].value =
-            (uint16_t)(((size_t)(table - root_table)) - sub_key);
+        root_table[sub_key] = ConstructHuffmanCode(
+            (uint8_t)(table_bits + root_bits),
+            (uint16_t)(((size_t)(table - root_table)) - sub_key));
         sub_key = 0;
       }
-      code.bits = (uint8_t)(len - root_bits);
       symbol = symbol_lists[symbol];
-      code.value = (uint16_t)symbol;
+      code = ConstructHuffmanCode((uint8_t)(len - root_bits),
+          (uint16_t)symbol);
       ReplicateValue(
           &table[BrotliReverseBits(sub_key)], step, table_size, code);
       sub_key += sub_key_step;
@@ -270,35 +268,28 @@ uint32_t BrotliBuildSimpleHuffmanTable(HuffmanCode* table,
   const uint32_t goal_size = 1U << root_bits;
   switch (num_symbols) {
     case 0:
-      table[0].bits = 0;
-      table[0].value = val[0];
+      table[0] = ConstructHuffmanCode(0, val[0]);
       break;
     case 1:
-      table[0].bits = 1;
-      table[1].bits = 1;
       if (val[1] > val[0]) {
-        table[0].value = val[0];
-        table[1].value = val[1];
+        table[0] = ConstructHuffmanCode(1, val[0]);
+        table[1] = ConstructHuffmanCode(1, val[1]);
       } else {
-        table[0].value = val[1];
-        table[1].value = val[0];
+        table[0] = ConstructHuffmanCode(1, val[1]);
+        table[1] = ConstructHuffmanCode(1, val[0]);
       }
       table_size = 2;
       break;
     case 2:
-      table[0].bits = 1;
-      table[0].value = val[0];
-      table[2].bits = 1;
-      table[2].value = val[0];
+      table[0] = ConstructHuffmanCode(1, val[0]);
+      table[2] = ConstructHuffmanCode(1, val[0]);
       if (val[2] > val[1]) {
-        table[1].value = val[1];
-        table[3].value = val[2];
+        table[1] = ConstructHuffmanCode(2, val[1]);
+        table[3] = ConstructHuffmanCode(2, val[2]);
       } else {
-        table[1].value = val[2];
-        table[3].value = val[1];
+        table[1] = ConstructHuffmanCode(2, val[2]);
+        table[3] = ConstructHuffmanCode(2, val[1]);
       }
-      table[1].bits = 2;
-      table[3].bits = 2;
       table_size = 4;
       break;
     case 3: {
@@ -312,33 +303,27 @@ uint32_t BrotliBuildSimpleHuffmanTable(HuffmanCode* table,
           }
         }
       }
-      for (i = 0; i < 4; ++i) {
-        table[i].bits = 2;
-      }
-      table[0].value = val[0];
-      table[2].value = val[1];
-      table[1].value = val[2];
-      table[3].value = val[3];
+      table[0] = ConstructHuffmanCode(2, val[0]);
+      table[2] = ConstructHuffmanCode(2, val[1]);
+      table[1] = ConstructHuffmanCode(2, val[2]);
+      table[3] = ConstructHuffmanCode(2, val[3]);
       table_size = 4;
       break;
     }
     case 4: {
-      int i;
       if (val[3] < val[2]) {
         uint16_t t = val[3];
         val[3] = val[2];
         val[2] = t;
       }
-      for (i = 0; i < 7; ++i) {
-        table[i].value = val[0];
-        table[i].bits = (uint8_t)(1 + (i & 1));
-      }
-      table[1].value = val[1];
-      table[3].value = val[2];
-      table[5].value = val[1];
-      table[7].value = val[3];
-      table[3].bits = 3;
-      table[7].bits = 3;
+      table[0] = ConstructHuffmanCode(1, val[0]);
+      table[1] = ConstructHuffmanCode(2, val[1]);
+      table[2] = ConstructHuffmanCode(1, val[0]);
+      table[3] = ConstructHuffmanCode(3, val[2]);
+      table[4] = ConstructHuffmanCode(1, val[0]);
+      table[5] = ConstructHuffmanCode(2, val[1]);
+      table[6] = ConstructHuffmanCode(1, val[0]);
+      table[7] = ConstructHuffmanCode(3, val[3]);
       table_size = 8;
       break;
     }
diff --git a/c/dec/huffman.h b/c/dec/huffman.h
index 521ec6e..b9f0716 100644
--- a/c/dec/huffman.h
+++ b/c/dec/huffman.h
@@ -33,11 +33,66 @@ static const uint16_t kMaxHuffmanTableSize[] = {
 
 #define BROTLI_HUFFMAN_MAX_CODE_LENGTH_CODE_LENGTH 5
 
+#if ((defined(BROTLI_TARGET_ARMV7) || defined(BROTLI_TARGET_ARMV8_32)) && \
+  BROTLI_GNUC_HAS_ATTRIBUTE(aligned, 2, 7, 0))
+#define BROTLI_HUFFMAN_CODE_FAST_LOAD
+#endif
+
+#if !defined(BROTLI_HUFFMAN_CODE_FAST_LOAD)
+/* Do not create this struct directly - use the ConstructHuffmanCode
+ * constructor below! */
 typedef struct {
   uint8_t bits;    /* number of bits used for this symbol */
   uint16_t value;  /* symbol value or table offset */
 } HuffmanCode;
 
+static BROTLI_INLINE HuffmanCode ConstructHuffmanCode(const uint8_t bits,
+    const uint16_t value) {
+  HuffmanCode h;
+  h.bits = bits;
+  h.value = value;
+  return h;
+}
+
+/* Please use the following macros to optimize HuffmanCode accesses in hot
+ * paths.
+ *
+ * For example, assuming |table| contains a HuffmanCode pointer:
+ *
+ *   BROTLI_HC_MARK_TABLE_FOR_FAST_LOAD(table);
+ *   BROTLI_HC_ADJUST_TABLE_INDEX(table, index_into_table);
+ *   *bits = BROTLI_HC_GET_BITS(table);
+ *   *value = BROTLI_HC_GET_VALUE(table);
+ *   BROTLI_HC_ADJUST_TABLE_INDEX(table, offset);
+ *   *bits2 = BROTLI_HC_GET_BITS(table);
+ *   *value2 = BROTLI_HC_GET_VALUE(table);
+ *
+ */
+
+#define BROTLI_HC_MARK_TABLE_FOR_FAST_LOAD(H)
+#define BROTLI_HC_ADJUST_TABLE_INDEX(H, V) H += (V)
+
+/* These must be given a HuffmanCode pointer! */
+#define BROTLI_HC_FAST_LOAD_BITS(H) (H->bits)
+#define BROTLI_HC_FAST_LOAD_VALUE(H) (H->value)
+
+#else /* BROTLI_HUFFMAN_CODE_FAST_LOAD */
+
+typedef BROTLI_ALIGNED(4) uint32_t HuffmanCode;
+
+static BROTLI_INLINE HuffmanCode ConstructHuffmanCode(const uint8_t bits,
+    const uint16_t value) {
+  return ((value & 0xFFFF) << 16) | (bits & 0xFF);
+}
+
+#define BROTLI_HC_MARK_TABLE_FOR_FAST_LOAD(H) uint32_t __fastload_##H = (*H)
+#define BROTLI_HC_ADJUST_TABLE_INDEX(H, V) H += (V); __fastload_##H = (*H)
+
+/* These must be given a HuffmanCode pointer! */
+#define BROTLI_HC_FAST_LOAD_BITS(H) ((__fastload_##H) & 0xFF)
+#define BROTLI_HC_FAST_LOAD_VALUE(H) ((__fastload_##H) >> 16)
+#endif /* BROTLI_HUFFMAN_CODE_FAST_LOAD */
+
 /* Builds Huffman lookup table assuming code lengths are in symbol order. */
 BROTLI_INTERNAL void BrotliBuildCodeLengthsHuffmanTable(HuffmanCode* root_table,
     const uint8_t* const code_lengths, uint16_t* count);