Portable bit functions

src/bits.h

@@ -0,0 +1,107 @@
+/*
+ * Copyright 2015 WebAssembly Community Group participants
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef wasm_bits_h
+#define wasm_bits_h
+
+/*
+ * Portable bit functions.
+ *
+ * Not all platforms offer fast intrinsics for these functions, and some
+ * compilers require checking CPUID at runtime before using the intrinsic.
+ *
+ * We instead use portable and reasonably-fast implementations, while
+ * avoiding implementations with large lookup tables.
+ */
+
+namespace wasm {
+
+// Only the specialized templates should be instantiated, getting
+// a linker error with these functions means an unsupported type was used.
+
+template<typename T> inline int PopCount(T /* v */);
+template<typename T> inline T BitReverse(T /* v */);
+template<typename T> inline int CountTrailingZeroes(T /* v */);
+template<typename T> inline int CountLeadingZeroes(T /* v */);
+
+// Implementations for the above templates.
+
+template<> inline int PopCount<uint8_t>(uint8_t v) {
+  // Small table lookup.
+  static const uint8_t tbl[32] = {
+    0, 1, 1, 2, 1, 2, 2, 3, 1, 2, 2, 3, 2, 3, 3, 4,
+    1, 2, 2, 3, 2, 3, 3, 4, 2, 3, 3, 4, 3, 4, 4, 5
+  };
+  return tbl[v & 0xf] + tbl[v >> 4];
+}
+template<> inline int PopCount<uint16_t>(uint16_t v) {
+  return PopCount<uint8_t>(v & 0xff) + PopCount<uint8_t>(v >> 8);
+}
+template<> inline int PopCount<uint32_t>(uint32_t v) {
+  // See Stanford bithacks, counting bits set in parallel, "best method":
+  // http://graphics.stanford.edu/~seander/bithacks.html#CountBitsSetParallel
+  v = v - ((v >> 1) & 0x55555555);
+  v = (v & 0x33333333) + ((v >> 2) & 0x33333333);
+  return (((v + (v >> 4)) & 0xF0F0F0F) * 0x1010101) >> 24;
+}
+template<> inline int PopCount<uint64_t>(uint64_t v) {
+  return PopCount<uint32_t>((uint32_t)v) + PopCount<uint32_t>(v >> 32);
+}
+
+template<> inline uint32_t BitReverse<uint32_t>(uint32_t v) {
+  // See Hacker's Delight, first edition, figure 7-1.
+  v = ((v & 0x55555555) << 1) | ((v >> 1) & 0x55555555);
+  v = ((v & 0x33333333) << 2) | ((v >> 2) & 0x33333333);
+  v = ((v & 0x0F0F0F0F) << 4) | ((v >> 4) & 0x0F0F0F0F);
+  v = (v << 24) | ((v & 0xFF00) << 8) |
+      ((v >> 8) & 0xFF00) | (v >> 24);
+  return v;
+}
+
+template<> inline int CountTrailingZeroes<uint32_t>(uint32_t v) {
+  // See Stanford bithacks, count the consecutive zero bits (trailing) on the
+  // right with multiply and lookup:
+  // http://graphics.stanford.edu/~seander/bithacks.html#ZerosOnRightMultLookup
+  static const uint8_t tbl[32] = {
+    0,   1, 28,  2, 29, 14, 24, 3, 30, 22, 20, 15, 25, 17,  4, 8,
+    31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18,  6, 11,  5, 10, 9
+  };
+  return v ?
+      (int)tbl[((uint32_t)((v & -(int32_t)v) * 0x077CB531U)) >> 27] :
+      -1;
+}
+
+template<> inline int CountLeadingZeroes<uint32_t>(uint32_t v) {
+  // See Stanford bithacks, find the log base 2 of an N-bit integer in
+  // O(lg(N)) operations with multiply and lookup:
+  // http://graphics.stanford.edu/~seander/bithacks.html#IntegerLogDeBruijn
+  static const uint8_t tbl[32] = {
+    31, 22, 30, 21, 18, 10, 29,  2, 20, 17, 15, 13, 9,  6, 28, 1,
+    23, 19, 11,  3, 16, 14,  7, 24, 12,  4,  8, 25, 5, 26, 27, 0
+  };
+  v = v | (v >>  1);
+  v = v | (v >>  2);
+  v = v | (v >>  4);
+  v = v | (v >>  8);
+  v = v | (v >> 16);
+  return v ?
+      (int)tbl[((uint32_t)(v * 0x07C4ACDDU)) >> 27] :
+      -1;
+}
+
+} // namespace wasm
+
+#endif // wasm_bits_h