Fix Windows build

Author: fboemer

hexl/util/clang.hpp

@@ -53,9 +53,9 @@ inline uint64_t MultiplyUInt64Hi(uint64_t x, uint64_t y) {
   return static_cast<uint64_t>(product >> BitShift);
 }
 
-// Returns maximum number of possible significant bits given modulus
-inline uint64_t MSB(uint64_t modulus) {
-  return static_cast<uint64_t>(std::log2l(modulus));
+// Returns most-significant bit of the input
+inline uint64_t MSB(uint64_t input) {
+  return static_cast<uint64_t>(std::log2l(input));
 }
 
 #define HEXL_LOOP_UNROLL_4 _Pragma("clang loop unroll_count(4)")

hexl/util/gcc.hpp

@@ -54,9 +54,9 @@ inline uint64_t MultiplyUInt64Hi(uint64_t x, uint64_t y) {
   return static_cast<uint64_t>(product >> BitShift);
 }
 
-// Returns maximum number of possible significant bits given modulus
-inline uint64_t MSB(uint64_t modulus) {
-  return static_cast<uint64_t>(std::log2l(modulus));
+// Returns most-significant bit of the input
+inline uint64_t MSB(uint64_t input) {
+  return static_cast<uint64_t>(std::log2l(input));
 }
 
 #define HEXL_LOOP_UNROLL_4 _Pragma("GCC unroll 4")

hexl/util/msvc.hpp

@@ -267,9 +267,11 @@ inline uint64_t DivideUInt128UInt64Lo(const uint64_t numerator_hi,
   return quotient[0];
 }
 
-// Returns maximum number of possible significant bits given modulus
-inline uint64_t MSB(uint64_t modulus) {
-  return static_cast<uint64_t>(floorl(std::log2l(modulus)) - 1);
+// Returns most-significant bit of the input
+inline uint64_t MSB(uint64_t input) {
+  unsigned long index{0};  // NOLINT(runtime/int)
+  _BitScanReverse64(&index, input);
+  return index;
 }
 
 #define HEXL_LOOP_UNROLL_4 \

test/test-number-theory.cpp

@@ -428,6 +428,9 @@ TEST(NumberTheory, MSB) {
   EXPECT_EQ(60ULL, MSB(2305843009213689601));  // 2**61 - 4351
   EXPECT_EQ(59ULL, MSB(1152921504606844417));  // 2**60 - 2559
   EXPECT_EQ(59ULL, MSB(1152921504606844289));  // 2**60 - 2687
+  EXPECT_EQ(40ULL, MSB((1ULL << 40) + 1));
+  EXPECT_EQ(40ULL, MSB(1ULL << 40));
+  EXPECT_EQ(39ULL, MSB((1ULL << 40) - 1));
   EXPECT_EQ(8ULL, MSB(256));
   EXPECT_EQ(0ULL, MSB(1));
 }