[LLVM][Support][NFC] Make multiple bitwise MathExtras functions constexpr

[gAlfonso-bit]

Summary

Submitted as an upstream fix

MathExtras has many useful tools. However, they aren't available to use in constexpr equations or functions. Originally this was a patch for swift llvm but it works upstream and as such I have put this PR upstream.

[gAlfonso-bit]

Made even more functions constexpr

[gAlfonso-bit]

Fix formatting

[gAlfonso-bit]

Fix compilation errors

[craig.topper]

Please upload patches with full context using -U999999 per the instructions here https://llvm.org/docs/Phabricator.html#requesting-a-review-via-the-web-interface

[gAlfonso-bit]

In D107240#2926914, @craig.topper wrote:

Please upload patches with full context using -U999999 per the instructions here https://llvm.org/docs/Phabricator.html#requesting-a-review-via-the-web-interface

Alright, I did.

[craig.topper]

llvm/include/llvm/Support/MathExtras.h

215

Does _BitScanReverse work in a constexpr in MSVC? The implementation in clang-cl doesn't work in constexpr, but clang-cl would use __builtin_clz.

[gAlfonso-bit]

llvm/include/llvm/Support/MathExtras.h

215

Yes it does. This is one of those exceptions where you can pass a reference because you don't read Index, but only write to it, and Index is in a constexpr anyway

[craig.topper]

llvm/include/llvm/Support/MathExtras.h

215

I tried it on godbolt and it didn't seem to work. Am I doing something wrong? https://godbolt.org/z/MnEY763E4

[gAlfonso-bit]

llvm/include/llvm/Support/MathExtras.h

215

Not sure, but so far LLVM compiles on Windows without a hitch, and the testbot is not having issues either

[gAlfonso-bit]

clang-tidy is complaining about function names, but changing every function instance would modify too many files

[craig.topper]

llvm/include/llvm/Support/MathExtras.h

215

Maybe that just means llvm never uses it in a context where it needs to be evaluated as a constexpr since your patch didn't add any new usages. So it might fail if someone tries to use the constexpr capability?

[gAlfonso-bit]

llvm/include/llvm/Support/MathExtras.h

215

Well, the other functions I made constexpr calls them, so that isn't it.

[gAlfonso-bit]

@craig.topper Only clang-tidy is having issues

[craig.topper]

I copied a chunk of this patch to godbolt and added two different constexpr usages of countLeadingZeros that both fail with MSVC https://godbolt.org/z/Maen4bac4 I'm guessing that because everything is a template, MSVC doesn't diagnose anything until you try to use it in constexpr context.

[craig.topper]

llvm/include/llvm/Support/MathExtras.h

112–113

Doesn't this mean that users of this that want to use it in constexpr can't compile that code with MSVC and would need a different solution just for MSVC? Is that useful for the project?

[gAlfonso-bit]

llvm/include/llvm/Support/MathExtras.h

112–113

No, since people who want to use it with constexpr but can't already exist, but this group gets smaller. Hopefully Microsoft finds a way, but there are downstream projects that would benefit from this already, like Swift.

[gAlfonso-bit]

By the way, it is not windows that gives clang tidy errors, but Linux.

[gAlfonso-bit]

Unfortunately it can't be constexpr for Windows at this time.

[gAlfonso-bit]

No build errors, only clang-tidy ones!

[gAlfonso-bit]

Remove contexpr from functions that call _BitScan Microsoft functions

[gAlfonso-bit]

This comment was removed by gAlfonso-bit.

llvm/include/llvm/Support/MathExtras.h

215

So I think I figured it out. Since I made the FIRST count function constexpr where the second value in the <> braces isn't constant, the compiler is using that instead of the non constexpr ones in these cases.

[craig.topper]

Is it possible to add unittests showing the count functions being used in constexpr contexts to prove that this will work with MSVC?

[gAlfonso-bit]

Unfortunately, as long as we are using c++14, there is actually no easy solution to this

[gAlfonso-bit]

@craig.topper I could use the libcpp_clz, which is constexpr, but that seems to be c++ 20 only.

[gAlfonso-bit]

I don't see how to progress here, so I am closing this for now

llvm/include/llvm/Support/MathExtras.h

 #include <limits>
 #include <type_traits>
 
 #ifdef __ANDROID_NDK__
 #include <android/api-level.h>
 #endif
 
 #ifdef _MSC_VER
-// Declare these intrinsics manually rather including intrin.h. It's very
-// expensive, and MathExtras.h is popular.
-// #include <intrin.h>
-extern "C" {
-unsigned char _BitScanForward(unsigned long *_Index, unsigned long _Mask);
-unsigned char _BitScanForward64(unsigned long *_Index, unsigned __int64 _Mask);
-unsigned char _BitScanReverse(unsigned long *_Index, unsigned long _Mask);
-unsigned char _BitScanReverse64(unsigned long *_Index, unsigned __int64 _Mask);
-}
+#include <intrin.h>
+#pragma intrinsic(_BitScanForward)
+#pragma intrinsic(_BitScanReverse)
+
+#ifdef _M_X64
+#pragma intrinsic(_BitScanForward64)
+#pragma intrinsic(_BitScanReverse64)
+#endif
 #endif
 
 namespace llvm {
 
 /// The behavior an operation has on an input of 0.
 enum ZeroBehavior {
   /// The returned value is undefined.
   ZB_Undefined,
constexpr float ef          = 2.71828183F, // (0x1.5bf0a8P+1) https://oeis.org/A001113
                 inv_sqrt2f  = .707106781F, // (0x1.6a09e6P-1)
                 sqrt3f      = 1.73205081F, // (0x1.bb67aeP+0) https://oeis.org/A002194
                 inv_sqrt3f  = .577350269F, // (0x1.279a74P-1)
                 phif        = 1.61803399F; // (0x1.9e377aP+0) https://oeis.org/A001622
 } // namespace numbers
 
 namespace detail {
 template <typename T, std::size_t SizeOfT> struct TrailingZerosCounter {
-  static unsigned count(T Val, ZeroBehavior) {
+  static constexpr unsigned count(T Val, ZeroBehavior) {
     if (!Val)
       return std::numeric_limits<T>::digits;
     if (Val & 0x1)
       return 0;
 
     // Bisection method.
     unsigned ZeroBits = 0;
     T Shift = std::numeric_limits<T>::digits >> 1;
     T Mask = std::numeric_limits<T>::max() >> Shift;
     while (Shift) {
       if ((Val & Mask) == 0) {
         Val >>= Shift;
         ZeroBits |= Shift;
       }
       Shift >>= 1;
       Mask >>= Shift;
     }
     return ZeroBits;
   }
 };
 
-#if defined(__GNUC__) || defined(_MSC_VER)
+#if defined(_MSC_VER) || __has_builtin(__builtin_ctz) || defined(__GNUC__)
+#if defined(_MSC_VER)
 template <typename T> struct TrailingZerosCounter<T, 4> {
   static unsigned count(T Val, ZeroBehavior ZB) {
+#else
+template <typename T> struct TrailingZerosCounter<T, 4> {
+  static constexpr unsigned count(T Val, ZeroBehavior ZB) {
+#endif
     if (ZB != ZB_Undefined && Val == 0)
       return 32;
 
 #if __has_builtin(__builtin_ctz) || defined(__GNUC__)
     return __builtin_ctz(Val);
-#elif defined(_MSC_VER)
+#else // defined(_MSC_VER)
     unsigned long Index;
     _BitScanForward(&Index, Val);
     return Index;
 #endif
   }
 };
+#endif
 
-#if !defined(_MSC_VER) || defined(_M_X64)
+#if (defined(_MSC_VER) && defined(_M_X64)) || __has_builtin(__builtin_ctz) ||  \
+    defined(__GNUC__)
+#if defined(_MSC_VER) && defined(_M_X64)
 template <typename T> struct TrailingZerosCounter<T, 8> {
   static unsigned count(T Val, ZeroBehavior ZB) {
+#else
+template <typename T> struct TrailingZerosCounter<T, 8> {
+  static constexpr unsigned count(T Val, ZeroBehavior ZB) {
+#endif
     if (ZB != ZB_Undefined && Val == 0)
       return 64;
 
 #if __has_builtin(__builtin_ctzll) || defined(__GNUC__)
     return __builtin_ctzll(Val);
-#elif defined(_MSC_VER)
+#else // defined(_MSC_VER) && defined(_M_X64)
     unsigned long Index;
     _BitScanForward64(&Index, Val);
     return Index;
 #endif
   }
 };
 #endif
-#endif
 } // namespace detail
 
 /// Count number of 0's from the least significant bit to the most
 ///   stopping at the first 1.
 ///
 /// Only unsigned integral types are allowed.
 ///
 /// \param ZB the behavior on an input of 0. Only ZB_Width and ZB_Undefined are
 ///   valid arguments.
+
+#if defined(_MSC_VER) || __has_builtin(__builtin_ctz) || defined(__GNUC__)
+#if defined(_MSC_VER)
 template <typename T>
 unsigned countTrailingZeros(T Val, ZeroBehavior ZB = ZB_Width) {
+#else
+template <typename T>
+constexpr unsigned countTrailingZeros(T Val, ZeroBehavior ZB = ZB_Width) {
+#endif
   static_assert(std::numeric_limits<T>::is_integer &&
                     !std::numeric_limits<T>::is_signed,
                 "Only unsigned integral types are allowed.");
   return llvm::detail::TrailingZerosCounter<T, sizeof(T)>::count(Val, ZB);
 }
+#endif
 
 namespace detail {
 template <typename T, std::size_t SizeOfT> struct LeadingZerosCounter {
-  static unsigned count(T Val, ZeroBehavior) {
+  static constexpr unsigned count(T Val, ZeroBehavior) {
     if (!Val)
       return std::numeric_limits<T>::digits;
 
     // Bisection method.
     unsigned ZeroBits = 0;
     for (T Shift = std::numeric_limits<T>::digits >> 1; Shift; Shift >>= 1) {
       T Tmp = Val >> Shift;
       if (Tmp)
         Val = Tmp;
       else
         ZeroBits |= Shift;
     }
     return ZeroBits;
   }
 };
 
-#if defined(__GNUC__) || defined(_MSC_VER)
+#if defined(_MSC_VER) || __has_builtin(__builtin_clz) || defined(__GNUC__)
+#if defined(_MSC_VER)
 template <typename T> struct LeadingZerosCounter<T, 4> {
   static unsigned count(T Val, ZeroBehavior ZB) {
+#else
+template <typename T> struct LeadingZerosCounter<T, 4> {
+  static constexpr unsigned count(T Val, ZeroBehavior ZB) {
+#endif
     if (ZB != ZB_Undefined && Val == 0)
       return 32;
 
 #if __has_builtin(__builtin_clz) || defined(__GNUC__)
     return __builtin_clz(Val);
-#elif defined(_MSC_VER)
+#else // defined(_MSC_VER)
     unsigned long Index;
     _BitScanReverse(&Index, Val);
     return Index ^ 31;
 #endif
   }
 };
+#endif
 
-#if !defined(_MSC_VER) || defined(_M_X64)
+#if (defined(_MSC_VER) && defined(_M_X64)) ||                                  \
+    __has_builtin(__builtin_clzll) || defined(__GNUC__)
+#if defined(_MSC_VER) && defined(_M_X64)
 template <typename T> struct LeadingZerosCounter<T, 8> {
   static unsigned count(T Val, ZeroBehavior ZB) {
+#else
+template <typename T> struct LeadingZerosCounter<T, 8> {
+  static constexpr unsigned count(T Val, ZeroBehavior ZB) {
+#endif
     if (ZB != ZB_Undefined && Val == 0)
       return 64;
 
 #if __has_builtin(__builtin_clzll) || defined(__GNUC__)
     return __builtin_clzll(Val);
-#elif defined(_MSC_VER)
+#else // defined(_MSC_VER) && defined(_M_X64)
     unsigned long Index;
     _BitScanReverse64(&Index, Val);
     return Index ^ 63;
 #endif
   }
 };
 #endif
-#endif
 } // namespace detail
 
 /// Count number of 0's from the most significant bit to the least
 ///   stopping at the first 1.
 ///
 /// Only unsigned integral types are allowed.
 ///
 /// \param ZB the behavior on an input of 0. Only ZB_Width and ZB_Undefined are
 ///   valid arguments.
+
+#if defined(_MSC_VER) || __has_builtin(__builtin_clz) || defined(__GNUC__)
+#if defined(_MSC_VER)
 template <typename T>
 unsigned countLeadingZeros(T Val, ZeroBehavior ZB = ZB_Width) {
+#else
+template <typename T>
+constexpr unsigned countLeadingZeros(T Val, ZeroBehavior ZB = ZB_Width) {
+#endif
   static_assert(std::numeric_limits<T>::is_integer &&
                     !std::numeric_limits<T>::is_signed,
                 "Only unsigned integral types are allowed.");
   return llvm::detail::LeadingZerosCounter<T, sizeof(T)>::count(Val, ZB);
 }
+#endif
 
 /// Get the index of the first set bit starting from the least
 ///   significant bit.
 ///
 /// Only unsigned integral types are allowed.
 ///
 /// \param ZB the behavior on an input of 0. Only ZB_Max and ZB_Undefined are
 ///   valid arguments.
+
+#if defined(_MSC_VER) || __has_builtin(__builtin_ctz) || defined(__GNUC__)
+#if defined(_MSC_VER)
 template <typename T> T findFirstSet(T Val, ZeroBehavior ZB = ZB_Max) {
+#else
+template <typename T>
+constexpr T findFirstSet(T Val, ZeroBehavior ZB = ZB_Max) {
+#endif
+
   if (ZB == ZB_Max && Val == 0)
     return std::numeric_limits<T>::max();
 
   return countTrailingZeros(Val, ZB_Undefined);
 }
+#endif
 
 /// Create a bitmask with the N right-most bits set to 1, and all other
 /// bits set to 0.  Only unsigned types are allowed.
 template <typename T> T maskTrailingOnes(unsigned N) {
   static_assert(std::is_unsigned<T>::value, "Invalid type!");
   const unsigned Bits = CHAR_BIT * sizeof(T);
   assert(N <= Bits && "Invalid bit index");
   return N == 0 ? 0 : (T(-1) >> (Bits - N));

 
 /// Get the index of the last set bit starting from the least
 ///   significant bit.
 ///
 /// Only unsigned integral types are allowed.
 ///
 /// \param ZB the behavior on an input of 0. Only ZB_Max and ZB_Undefined are
 ///   valid arguments.
+
+#if defined(_MSC_VER) || __has_builtin(__builtin_clz) || defined(__GNUC__)
+#if defined(_MSC_VER)
 template <typename T> T findLastSet(T Val, ZeroBehavior ZB = ZB_Max) {
+#else
+template <typename T> constexpr T findLastSet(T Val, ZeroBehavior ZB = ZB_Max) {
+#endif
   if (ZB == ZB_Max && Val == 0)
     return std::numeric_limits<T>::max();
 
   // Use ^ instead of - because both gcc and llvm can remove the associated ^
   // in the __builtin_clz intrinsic on x86.
   return countLeadingZeros(Val, ZB_Undefined) ^
          (std::numeric_limits<T>::digits - 1);
 }
+#endif
 
 /// Macro compressed bit reversal table for 256 bits.
 ///
 /// http://graphics.stanford.edu/~seander/bithacks.html#BitReverseTable
 static const unsigned char BitReverseTable256[256] = {
 #define R2(n) n, n + 2 * 64, n + 1 * 64, n + 3 * 64
 #define R4(n) R2(n), R2(n + 2 * 16), R2(n + 1 * 16), R2(n + 3 * 16)
 #define R6(n) R4(n), R4(n + 2 * 4), R4(n + 1 * 4), R4(n + 3 * 4)
     R6(0), R6(2), R6(1), R6(3)
 #undef R2
 #undef R4
 #undef R6
 };
 
 /// Reverse the bits in \p Val.
-template <typename T>
-T reverseBits(T Val) {
+template <typename T> T reverseBits(T Val) {
   unsigned char in[sizeof(Val)];
   unsigned char out[sizeof(Val)];
   std::memcpy(in, &Val, sizeof(Val));
   for (unsigned i = 0; i < sizeof(Val); ++i)
     out[(sizeof(Val) - i) - 1] = BitReverseTable256[in[i]];
   std::memcpy(&Val, out, sizeof(Val));
   return Val;
 }
 
 #if __has_builtin(__builtin_bitreverse8)
-template<>
-inline uint8_t reverseBits<uint8_t>(uint8_t Val) {
+template <> inline uint8_t reverseBits<uint8_t>(uint8_t Val) {
   return __builtin_bitreverse8(Val);
 }
 #endif
 
 #if __has_builtin(__builtin_bitreverse16)
-template<>
-inline uint16_t reverseBits<uint16_t>(uint16_t Val) {
+template <> inline uint16_t reverseBits<uint16_t>(uint16_t Val) {
   return __builtin_bitreverse16(Val);
 }
 #endif
 
 #if __has_builtin(__builtin_bitreverse32)
-template<>
-inline uint32_t reverseBits<uint32_t>(uint32_t Val) {
+template <> inline uint32_t reverseBits<uint32_t>(uint32_t Val) {
   return __builtin_bitreverse32(Val);
 }
 #endif
 
 #if __has_builtin(__builtin_bitreverse64)
-template<>
-inline uint64_t reverseBits<uint64_t>(uint64_t Val) {
+template <> inline uint64_t reverseBits<uint64_t>(uint64_t Val) {
   return __builtin_bitreverse64(Val);
 }
 #endif
 
 // NOTE: The following support functions use the _32/_64 extensions instead of
 // type overloading so that signed and unsigned integers can be used without
 // ambiguity.
 
 /// Return the high 32 bits of a 64 bit value.
 constexpr inline uint32_t Hi_32(uint64_t Value) {
   return static_cast<uint32_t>(Value >> 32);
 }
 
 /// Return the low 32 bits of a 64 bit value.
 constexpr inline uint32_t Lo_32(uint64_t Value) {
   return static_cast<uint32_t>(Value);
 }
 
 /// Make a 64-bit integer from a high / low pair of 32-bit integers.
 constexpr inline uint64_t Make_64(uint32_t High, uint32_t Low) {
-  return ((uint64_t)High << 32) | (uint64_t)Low;
+  return (static_cast<uint64_t>(High) << 32) | static_cast<uint64_t>(Low);
 }
 
 /// Checks if an integer fits into the given bit width.
 template <unsigned N> constexpr inline bool isInt(int64_t x) {
-  return N >= 64 || (-(INT64_C(1)<<(N-1)) <= x && x < (INT64_C(1)<<(N-1)));
+  return N >= 64 ||
+         (-(INT64_C(1) << (N - 1)) <= x && x < (INT64_C(1) << (N - 1)));
 }
 // Template specializations to get better code for common cases.
 template <> constexpr inline bool isInt<8>(int64_t x) {
   return static_cast<int8_t>(x) == x;
 }
 template <> constexpr inline bool isInt<16>(int64_t x) {
   return static_cast<int16_t>(x) == x;
 }

 /// Count the number of ones from the most significant bit to the first
 /// zero bit.
 ///
 /// Ex. countLeadingOnes(0xFF0FFF00) == 8.
 /// Only unsigned integral types are allowed.
 ///
 /// \param ZB the behavior on an input of all ones. Only ZB_Width and
 /// ZB_Undefined are valid arguments.
+#if defined(_MSC_VER) || __has_builtin(__builtin_clz) || defined(__GNUC__)
+#if defined(_MSC_VER)
 template <typename T>
 unsigned countLeadingOnes(T Value, ZeroBehavior ZB = ZB_Width) {
+#else
+template <typename T>
+constexpr unsigned countLeadingOnes(T Value, ZeroBehavior ZB = ZB_Width) {
+#endif
   static_assert(std::numeric_limits<T>::is_integer &&
                     !std::numeric_limits<T>::is_signed,
                 "Only unsigned integral types are allowed.");
   return countLeadingZeros<T>(~Value, ZB);
 }
+#endif
 
 /// Count the number of ones from the least significant bit to the first
 /// zero bit.
 ///
 /// Ex. countTrailingOnes(0x00FF00FF) == 8.
 /// Only unsigned integral types are allowed.
 ///
 /// \param ZB the behavior on an input of all ones. Only ZB_Width and
 /// ZB_Undefined are valid arguments.
+
+#if defined(_MSC_VER) || __has_builtin(__builtin_ctz) || defined(__GNUC__)
+#if defined(_MSC_VER)
 template <typename T>
 unsigned countTrailingOnes(T Value, ZeroBehavior ZB = ZB_Width) {
+#else
+template <typename T>
+constexpr unsigned countTrailingOnes(T Value, ZeroBehavior ZB = ZB_Width) {
+#endif
+
   static_assert(std::numeric_limits<T>::is_integer &&
                     !std::numeric_limits<T>::is_signed,
                 "Only unsigned integral types are allowed.");
   return countTrailingZeros<T>(~Value, ZB);
 }
+#endif
 
 namespace detail {
 template <typename T, std::size_t SizeOfT> struct PopulationCounter {
-  static unsigned count(T Value) {
+  static constexpr unsigned count(T Value) {
     // Generic version, forward to 32 bits.
     static_assert(SizeOfT <= 4, "Not implemented!");
-#if defined(__GNUC__)
+#if __has_builtin(__builtin_popcount) || defined(__GNUC__)
     return __builtin_popcount(Value);
 #else
     uint32_t v = Value;
     v = v - ((v >> 1) & 0x55555555);
     v = (v & 0x33333333) + ((v >> 2) & 0x33333333);
     return ((v + (v >> 4) & 0xF0F0F0F) * 0x1010101) >> 24;
 #endif
   }
 };
 
 template <typename T> struct PopulationCounter<T, 8> {
-  static unsigned count(T Value) {
-#if defined(__GNUC__)
+  static constexpr unsigned count(T Value) {
+#if __has_builtin(__builtin_popcountll) || defined(__GNUC__)
     return __builtin_popcountll(Value);
 #else
     uint64_t v = Value;
     v = v - ((v >> 1) & 0x5555555555555555ULL);
     v = (v & 0x3333333333333333ULL) + ((v >> 2) & 0x3333333333333333ULL);
     v = (v + (v >> 4)) & 0x0F0F0F0F0F0F0F0FULL;
-    return unsigned((uint64_t)(v * 0x0101010101010101ULL) >> 56);
+    return unsigned(static_cast<uint64_t>(v * 0x0101010101010101ULL) >> 56);
 #endif
   }
 };
 } // namespace detail
 
 /// Count the number of set bits in a value.
 /// Ex. countPopulation(0xF000F000) = 8
 /// Returns 0 if the word is zero.
-template <typename T>
-inline unsigned countPopulation(T Value) {
+template <typename T> constexpr inline unsigned countPopulation(T Value) {
   static_assert(std::numeric_limits<T>::is_integer &&
                     !std::numeric_limits<T>::is_signed,
                 "Only unsigned integral types are allowed.");
   return detail::PopulationCounter<T, sizeof(T)>::count(Value);
 }
 
 /// Compile time Log2.
 /// Valid only for positive powers of two.

 #else
   return log2(Value);
 #endif
 }
 
 /// Return the floor log base 2 of the specified value, -1 if the value is zero.
 /// (32 bit edition.)
 /// Ex. Log2_32(32) == 5, Log2_32(1) == 0, Log2_32(0) == -1, Log2_32(6) == 2
+
+#if defined(_MSC_VER) || __has_builtin(__builtin_clz) || defined(__GNUC__)
+#if defined(_MSC_VER)
 inline unsigned Log2_32(uint32_t Value) {
+#else
+constexpr inline unsigned Log2_32(uint32_t Value) {
+#endif
   return 31 - countLeadingZeros(Value);
 }
+#endif
 
 /// Return the floor log base 2 of the specified value, -1 if the value is zero.
 /// (64 bit edition.)
+
+#if (defined(_MSC_VER) && defined(_M_X64)) ||                                  \
+    __has_builtin(__builtin_clzll) || defined(__GNUC__)
+#if defined(_MSC_VER) && defined(_M_X64)
 inline unsigned Log2_64(uint64_t Value) {
+#else
+constexpr inline unsigned Log2_64(uint64_t Value) {
+#endif
   return 63 - countLeadingZeros(Value);
 }
+#endif
 
 /// Return the ceil log base 2 of the specified value, 32 if the value is zero.
 /// (32 bit edition).
 /// Ex. Log2_32_Ceil(32) == 5, Log2_32_Ceil(1) == 0, Log2_32_Ceil(6) == 3
+#if defined(_MSC_VER) || __has_builtin(__builtin_clz) || defined(__GNUC__)
+#if defined(_MSC_VER)
 inline unsigned Log2_32_Ceil(uint32_t Value) {
+#else
+constexpr inline unsigned Log2_32_Ceil(uint32_t Value) {
+#endif
   return 32 - countLeadingZeros(Value - 1);
 }
+#endif
 
 /// Return the ceil log base 2 of the specified value, 64 if the value is zero.
 /// (64 bit edition.)
+#if (defined(_MSC_VER) && defined(_M_X64)) ||                                  \
+    __has_builtin(__builtin_clzll) || defined(__GNUC__)
+#if defined(_MSC_VER) && defined(_M_X64)
 inline unsigned Log2_64_Ceil(uint64_t Value) {
+#else
+constexpr inline unsigned Log2_64_Ceil(uint64_t Value) {
+#endif
   return 64 - countLeadingZeros(Value - 1);
 }
+#endif
 
 /// Return the greatest common divisor of the values using Euclid's algorithm.
-template <typename T>
-inline T greatestCommonDivisor(T A, T B) {
+template <typename T> constexpr inline T greatestCommonDivisor(T A, T B) {
   while (B) {
     T Tmp = B;
     B = A % B;
     A = Tmp;
   }
   return A;
 }
 
-inline uint64_t GreatestCommonDivisor64(uint64_t A, uint64_t B) {
+constexpr inline uint64_t GreatestCommonDivisor64(uint64_t A, uint64_t B) {
   return greatestCommonDivisor<uint64_t>(A, B);
 }
 
 /// This function takes a 64-bit integer and returns the bit equivalent double.
 inline double BitsToDouble(uint64_t Bits) {
   double D;
   static_assert(sizeof(uint64_t) == sizeof(double), "Unexpected type sizes");
   memcpy(&D, &Bits, sizeof(Bits));
constexpr inline uint64_t MinAlign(uint64_t A, uint64_t B) {
   // Replace "-Value" by "1+~Value" in the following commented code to avoid
   // MSVC warning C4146
   //    return (A | B) & -(A | B);
   return (A | B) & (1 + ~(A | B));
 }
 
 /// Returns the next power of two (in 64-bits) that is strictly greater than A.
 /// Returns zero on overflow.
-inline uint64_t NextPowerOf2(uint64_t A) {
+constexpr inline uint64_t NextPowerOf2(uint64_t A) {
   A |= (A >> 1);
   A |= (A >> 2);
   A |= (A >> 4);
   A |= (A >> 8);
   A |= (A >> 16);
   A |= (A >> 32);
   return A + 1;
 }
 
 /// Returns the power of two which is less than or equal to the given value.
 /// Essentially, it is a floor operation across the domain of powers of two.
-inline uint64_t PowerOf2Floor(uint64_t A) {
-  if (!A) return 0;
+constexpr inline uint64_t PowerOf2Floor(uint64_t A) {
+  if (!A)
+    return 0;
   return 1ull << (63 - countLeadingZeros(A, ZB_Undefined));
 }
 
 /// Returns the power of two which is greater than or equal to the given value.
 /// Essentially, it is a ceil operation across the domain of powers of two.
-inline uint64_t PowerOf2Ceil(uint64_t A) {
+constexpr inline uint64_t PowerOf2Ceil(uint64_t A) {
   if (!A)
     return 0;
   return NextPowerOf2(A - 1);
 }
 
 /// Returns the next integer (mod 2**64) that is greater than or equal to
 /// \p Value and is a multiple of \p Align. \p Align must be non-zero.
 ///

 }
 
 /// Returns the integer ceil(Numerator / Denominator).
 inline uint64_t divideCeil(uint64_t Numerator, uint64_t Denominator) {
   return alignTo(Numerator, Denominator) / Denominator;
 }
 
 /// Returns the integer nearest(Numerator / Denominator).
-inline uint64_t divideNearest(uint64_t Numerator, uint64_t Denominator) {
+constexpr inline uint64_t divideNearest(uint64_t Numerator,
+                                        uint64_t Denominator) {
   return (Numerator + (Denominator / 2)) / Denominator;
 }
 
 /// Returns the largest uint64_t less than or equal to \p Value and is
 /// \p Skew mod \p Align. \p Align must be non-zero
 inline uint64_t alignDown(uint64_t Value, uint64_t Align, uint64_t Skew = 0) {
   assert(Align != 0u && "Align can't be 0.");
   Skew %= Align;
inline int64_t SignExtend64(uint64_t X, unsigned B) {
   assert(B > 0 && "Bit width can't be 0.");
   assert(B <= 64 && "Bit width out of range.");
   return int64_t(X << (64 - B)) >> (64 - B);
 }
 
 /// Subtract two unsigned integers, X and Y, of type T and return the absolute
 /// value of the result.
 template <typename T>
-std::enable_if_t<std::is_unsigned<T>::value, T> AbsoluteDifference(T X, T Y) {
+constexpr std::enable_if_t<std::is_unsigned<T>::value, T>
+AbsoluteDifference(T X, T Y) {
   return X > Y ? (X - Y) : (Y - X);
 }
 
 /// Add two unsigned integers, X and Y, of type T.  Clamp the result to the
 /// maximum representable value of T on overflow.  ResultOverflowed indicates if
 /// the result is larger than the maximum representable value of type T.
 template <typename T>
 std::enable_if_t<std::is_unsigned<T>::value, T>
if (Overflowed)
     return Product;
 
   return SaturatingAdd(A, Product, &Overflowed);
 }
 
 /// Use this rather than HUGE_VALF; the latter causes warnings on MSVC.
 extern const float huge_valf;
 
-
 /// Add two signed integers, computing the two's complement truncated result,
 /// returning true if overflow occured.
 template <typename T>
 std::enable_if_t<std::is_signed<T>::value, T> AddOverflow(T X, T Y, T &Result) {
 #if __has_builtin(__builtin_add_overflow)
   return __builtin_add_overflow(X, Y, &Result);
 #else
   // Perform the unsigned addition.
std::enable_if_t<std::is_signed<T>::value, T> MulOverflow(T X, T Y, T &Result) {
   // Check how the max allowed absolute value (2^n for negative, 2^(n-1) for
   // positive) divided by an argument compares to the other.
   if (IsNegative)
     return UX > (static_cast<U>(std::numeric_limits<T>::max()) + U(1)) / UY;
   else
     return UX > (static_cast<U>(std::numeric_limits<T>::max())) / UY;
 }
 
-} // End llvm namespace
+} // namespace llvm
 
 #endif

llvm/lib/Support/MathExtras.cpp

 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Support/MathExtras.h"
 
 #ifdef _MSC_VER
 #include <limits>
 #else
-#include <math.h>
+#include <cmath>
 #endif
 
 namespace llvm {
 
-#if defined(_MSC_VER)
-  // Visual Studio defines the HUGE_VAL class of macros using purposeful
-  // constant arithmetic overflow, which it then warns on when encountered.
-  const float huge_valf = std::numeric_limits<float>::infinity();
+#ifdef _MSC_VER
 #else
-  const float huge_valf = HUGE_VALF;
+constexpr float huge_valf = HUGE_VALF;
 #endif
 
 } // namespace llvm