Revert "Remove support for MSVC"

sky/engine/wtf/BitwiseOperations.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 24 matching lines @@
 // countLeadingZeros() is a bitwise operation that counts the number of leading
 // zeros in a binary value, starting with the most significant bit. C does not
 // have an operator to do this, but fortunately the various compilers have
 // built-ins that map to fast underlying processor instructions.
 
 #include "wtf/CPU.h"
 #include "wtf/Compiler.h"
 
 #include <stdint.h>
 
+#if COMPILER(MSVC)
+#include <intrin.h>
+#endif
+
 namespace WTF {
 
-#if COMPILER(GCC)
+#if COMPILER(MSVC)
+
+ALWAYS_INLINE uint32_t countLeadingZeros32(uint32_t x)
+{
+    unsigned long index;
+    return LIKELY(_BitScanReverse(&index, x)) ? (31 - index) : 32;
+}
+
+#if CPU(64BIT)
+
+// MSVC only supplies _BitScanForward64 when building for a 64-bit target.
+ALWAYS_INLINE uint64_t countLeadingZeros64(uint64_t x)
+{
+    unsigned long index;
+    return LIKELY(_BitScanReverse64(&index, x)) ? (63 - index) : 64;
+}
+
+#endif
+
+#elif COMPILER(GCC)
 
 // This is very annoying. __builtin_clz has undefined behaviour for an input of
 // 0, even though these's clearly a return value that makes sense, and even
 // though nascent processor clz instructions have defined behaviour for 0.
 // We could drop to raw __asm__ to do better, but we'll avoid doing that unless
 // we see proof that we need to.
 ALWAYS_INLINE uint32_t countLeadingZeros32(uint32_t x)
 {
     return LIKELY(x) ? __builtin_clz(x) : 32;
 }
@@ skipping 11 matching lines @@
 
 #else
 
 ALWAYS_INLINE size_t countLeadingZerosSizet(size_t x) { return countLeadingZeros32(x); }
 
 #endif
 
 } // namespace WTF
 
 #endif // WTF_BitwiseOperations_h