Remove ARCH_* guards around Bits::FindLSBSetNonZero64().

Bits::FindLSBSetNonZero64() is now available unconditionally, and it
should be easier to reason about the code included in each build
configuration.

This reduces the amount of conditional compiling going on, which makes
it easier to reason about what stubs are a used in each build
configuration.

The guards were added to work around the fact that MSVC has a
_BitScanForward64() intrinsic, but the intrinsic is only implemented on
64-bit targets, and causes a compilation error on 32-bit targets errors.
By contrast, Clang and GCC support __builtin_ctzll() everywhere, and
implement it with varying efficiency.

This CL reworks the conditional compilation directives so that
Bits::FindLSBSetNonZero64() uses the _BitScanForward64() intrinsic on
MSVC when available, and the portable implementation otherwise.
PiperOrigin-RevId: 310007748

Author: pwnall

snappy-stubs-internal.h

@@ -264,17 +264,15 @@ class Bits {
   // that it's 0-indexed.
   static int FindLSBSetNonZero(uint32_t n);
 
-#if defined(ARCH_K8) || defined(ARCH_PPC) || defined(ARCH_ARM)
   static int FindLSBSetNonZero64(uint64_t n);
-#endif  // defined(ARCH_K8) || defined(ARCH_PPC) || defined(ARCH_ARM)
 
  private:
   // No copying
   Bits(const Bits&);
   void operator=(const Bits&);
 };
 
-#ifdef HAVE_BUILTIN_CTZ
+#if defined(HAVE_BUILTIN_CTZ)
 
 inline int Bits::Log2FloorNonZero(uint32_t n) {
   assert(n != 0);
@@ -296,23 +294,18 @@ inline int Bits::FindLSBSetNonZero(uint32_t n) {
   return __builtin_ctz(n);
 }
 
-#if defined(ARCH_K8) || defined(ARCH_PPC) || defined(ARCH_ARM)
-inline int Bits::FindLSBSetNonZero64(uint64_t n) {
-  assert(n != 0);
-  return __builtin_ctzll(n);
-}
-#endif  // defined(ARCH_K8) || defined(ARCH_PPC) || defined(ARCH_ARM)
-
 #elif defined(_MSC_VER)
 
 inline int Bits::Log2FloorNonZero(uint32_t n) {
   assert(n != 0);
+  // NOLINTNEXTLINE(runtime/int): The MSVC intrinsic demands unsigned long.
   unsigned long where;
   _BitScanReverse(&where, n);
   return static_cast<int>(where);
 }
 
 inline int Bits::Log2Floor(uint32_t n) {
+  // NOLINTNEXTLINE(runtime/int): The MSVC intrinsic demands unsigned long.
   unsigned long where;
   if (_BitScanReverse(&where, n))
     return static_cast<int>(where);
@@ -321,22 +314,13 @@ inline int Bits::Log2Floor(uint32_t n) {
 
 inline int Bits::FindLSBSetNonZero(uint32_t n) {
   assert(n != 0);
+  // NOLINTNEXTLINE(runtime/int): The MSVC intrinsic demands unsigned long.
   unsigned long where;
   if (_BitScanForward(&where, n))
     return static_cast<int>(where);
   return 32;
 }
 
-#if defined(ARCH_K8) || defined(ARCH_PPC) || defined(ARCH_ARM)
-inline int Bits::FindLSBSetNonZero64(uint64_t n) {
-  assert(n != 0);
-  unsigned long where;
-  if (_BitScanForward64(&where, n))
-    return static_cast<int>(where);
-  return 64;
-}
-#endif  // defined(ARCH_K8) || defined(ARCH_PPC) || defined(ARCH_ARM)
-
 #else  // Portable versions.
 
 inline int Bits::Log2FloorNonZero(uint32_t n) {
@@ -375,22 +359,43 @@ inline int Bits::FindLSBSetNonZero(uint32_t n) {
   return rc;
 }
 
-#if defined(ARCH_K8) || defined(ARCH_PPC) || defined(ARCH_ARM)
+#endif  // End portable versions.
+
+#if defined(HAVE_BUILTIN_CTZ)
+
+inline int Bits::FindLSBSetNonZero64(uint64_t n) {
+  assert(n != 0);
+  return __builtin_ctzll(n);
+}
+
+#elif defined(_MSC_VER) && (defined(_M_X64) || defined(_M_ARM64))
+// _BitScanForward64() is only available on x64 and ARM64.
+
+inline int Bits::FindLSBSetNonZero64(uint64_t n) {
+  assert(n != 0);
+  // NOLINTNEXTLINE(runtime/int): The MSVC intrinsic demands unsigned long.
+  unsigned long where;
+  if (_BitScanForward64(&where, n))
+    return static_cast<int>(where);
+  return 64;
+}
+
+#else  // Portable version.
+
 // FindLSBSetNonZero64() is defined in terms of FindLSBSetNonZero().
 inline int Bits::FindLSBSetNonZero64(uint64_t n) {
   assert(n != 0);
 
   const uint32_t bottombits = static_cast<uint32_t>(n);
   if (bottombits == 0) {
-    // Bottom bits are zero, so scan in top bits
+    // Bottom bits are zero, so scan the top bits.
     return 32 + FindLSBSetNonZero(static_cast<uint32_t>(n >> 32));
   } else {
     return FindLSBSetNonZero(bottombits);
   }
 }
-#endif  // defined(ARCH_K8) || defined(ARCH_PPC) || defined(ARCH_ARM)
 
-#endif  // End portable versions.
+#endif  // End portable version.
 
 // Variable-length integer encoding.
 class Varint {