Don't interfere with the __builtin_foo namespace under MSVC

This patch follows up on feedback received in the review for
D32988. Specifically that libc++ should not mess with the
__builtin namespace, and that libc++ should use __popcnt to implement
__pop_count under MSVC.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@302731 91177308-0d34-0410-b5e6-96231b3b80d8

include/algorithm

@@ -645,7 +645,7 @@ template <class BidirectionalIterator, class Compare>
 #include "support/ibm/support.h"
 #endif
 #if defined(_LIBCPP_COMPILER_MSVC)
-#include "support/win32/msvc_builtin_support.h"
+#include <intrin.h>
 #endif
 
 #include <__undef_min_max>
@@ -783,51 +783,132 @@ struct __debug_less
 
 // Precondition:  __x != 0
 inline _LIBCPP_INLINE_VISIBILITY
-unsigned
-__ctz(unsigned __x)
-{
+unsigned __ctz(unsigned __x) {
+#ifndef _LIBCPP_COMPILER_MSVC
     return static_cast<unsigned>(__builtin_ctz(__x));
+#else
+  static_assert(sizeof(unsigned) == sizeof(unsigned long), "");
+  static_assert(sizeof(unsigned long) == 4, "");
+  unsigned long where;
+  // Search from LSB to MSB for first set bit.
+  // Returns zero if no set bit is found.
+  if (_BitScanForward(&where, mask))
+    return where;
+  return 32;
+#endif
 }
 
 inline _LIBCPP_INLINE_VISIBILITY
-unsigned long
-__ctz(unsigned long __x)
-{
+unsigned long __ctz(unsigned long __x) {
+#ifndef _LIBCPP_COMPILER_MSVC
     return static_cast<unsigned long>(__builtin_ctzl(__x));
+#else
+    static_assert(sizeof(unsigned long) == sizeof(unsigned), "");
+    return __ctz(static_cast<unsigned>(__x));
+#endif
 }
 
 inline _LIBCPP_INLINE_VISIBILITY
-unsigned long long
-__ctz(unsigned long long __x)
-{
+unsigned long long __ctz(unsigned long long __x) {
+#ifndef _LIBCPP_COMPILER_MSVC
     return static_cast<unsigned long long>(__builtin_ctzll(__x));
+#else
+    unsigned long where;
+// Search from LSB to MSB for first set bit.
+// Returns zero if no set bit is found.
+#if defined(_LIBCPP_HAS_BITSCAN64)
+    (defined(_M_AMD64) || defined(__x86_64__))
+  if (_BitScanForward64(&where, mask))
+    return static_cast<int>(where);
+#else
+  // Win32 doesn't have _BitScanForward64 so emulate it with two 32 bit calls.
+  // Scan the Low Word.
+  if (_BitScanForward(&where, static_cast<unsigned long>(mask)))
+    return where;
+  // Scan the High Word.
+  if (_BitScanForward(&where, static_cast<unsigned long>(mask >> 32)))
+    return where + 32; // Create a bit offset from the LSB.
+#endif
+  return 64;
+#endif // _LIBCPP_COMPILER_MSVC
 }
 
 // Precondition:  __x != 0
 inline _LIBCPP_INLINE_VISIBILITY
-unsigned
-__clz(unsigned __x)
-{
+unsigned __clz(unsigned __x) {
+#ifndef _LIBCPP_COMPILER_MSVC
     return static_cast<unsigned>(__builtin_clz(__x));
+#else
+  static_assert(sizeof(unsigned) == sizeof(unsigned long), "");
+  static_assert(sizeof(unsigned long) == 4, "");
+  unsigned long where;
+  // Search from LSB to MSB for first set bit.
+  // Returns zero if no set bit is found.
+  if (_BitScanReverse(&where, mask))
+    return 31 - where;
+  return 32; // Undefined Behavior.
+#endif
 }
 
 inline _LIBCPP_INLINE_VISIBILITY
-unsigned long
-__clz(unsigned long __x)
-{
+unsigned long __clz(unsigned long __x) {
+#ifndef _LIBCPP_COMPILER_MSVC
     return static_cast<unsigned long>(__builtin_clzl (__x));
+#else
+    static_assert(sizeof(unsigned) == sizeof(unsigned long), "");
+    return __clz(static_cast<unsigned>(__x));
+#endif
 }
 
 inline _LIBCPP_INLINE_VISIBILITY
-unsigned long long
-__clz(unsigned long long __x)
-{
+unsigned long long __clz(unsigned long long __x) {
+#ifndef _LIBCPP_COMPILER_MSVC
     return static_cast<unsigned long long>(__builtin_clzll(__x));
+#else
+  unsigned long where;
+// BitScanReverse scans from MSB to LSB for first set bit.
+// Returns 0 if no set bit is found.
+#if defined(_LIBCPP_HAS_BITSCAN64)
+  if (_BitScanReverse64(&where, mask))
+    return static_cast<int>(63 - where);
+#else
+  // Scan the high 32 bits.
+  if (_BitScanReverse(&where, static_cast<unsigned long>(mask >> 32)))
+    return 63 - (where + 32); // Create a bit offset from the MSB.
+  // Scan the low 32 bits.
+  if (_BitScanReverse(&where, static_cast<unsigned long>(mask)))
+    return 63 - where;
+#endif
+  return 64; // Undefined Behavior.
+#endif // _LIBCPP_COMPILER_MSVC
 }
 
-inline _LIBCPP_INLINE_VISIBILITY int __pop_count(unsigned           __x) {return __builtin_popcount  (__x);}
-inline _LIBCPP_INLINE_VISIBILITY int __pop_count(unsigned      long __x) {return __builtin_popcountl (__x);}
-inline _LIBCPP_INLINE_VISIBILITY int __pop_count(unsigned long long __x) {return __builtin_popcountll(__x);}
+inline _LIBCPP_INLINE_VISIBILITY int __pop_count(unsigned __x) {
+#ifndef _LIBCPP_COMPILER_MSVC
+  return __builtin_popcount  (__x);
+#else
+  static_assert(sizeof(unsigned) == 4, "");
+  return __popcnt(__x);
+#endif
+}
+
+inline _LIBCPP_INLINE_VISIBILITY int __pop_count(unsigned long __x) {
+#ifndef _LIBCPP_COMPILER_MSVC
+  return __builtin_popcountl (__x);
+#else
+  static_assert(sizeof(unsigned long) == 4, "");
+  return __popcnt(__x);
+#endif
+}
+
+inline _LIBCPP_INLINE_VISIBILITY int __pop_count(unsigned long long __x) {
+#ifndef _LIBCPP_COMPILER_MSVC
+  return __builtin_popcountll(__x);
+#else
+  static_assert(sizeof(unsigned long long) == 8, "");
+  return __popcnt64(__x);
+#endif
+}
 
 // all_of