tint/utils: Add Log2 and NextPowerOfTwo

Shamelessly adapted from dawn's implementations.

Change-Id: I77aff9bd1c852c14b0009295024973aa17c50d55
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/116873
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Dan Sinclair <dsinclair@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>

src/tint/utils/math.h

@@ -26,7 +26,7 @@ namespace tint::utils {
 /// @return `value` rounded to the next multiple of `alignment`
 /// @note `alignment` must be positive. An alignment of zero will cause a DBZ.
 template <typename T>
-inline T RoundUp(T alignment, T value) {
+inline constexpr T RoundUp(T alignment, T value) {
     return ((value + alignment - 1) / alignment) * alignment;
 }
 
@@ -34,10 +34,57 @@ inline T RoundUp(T alignment, T value) {
 /// @returns true if `value` is a power-of-two
 /// @note `value` must be positive if `T` is signed
 template <typename T>
-inline bool IsPowerOfTwo(T value) {
+inline constexpr bool IsPowerOfTwo(T value) {
     return (value & (value - 1)) == 0;
 }
 
+/// @param value the input value
+/// @returns the base-2 logarithm of @p value
+inline constexpr uint32_t Log2(uint64_t value) {
+#if defined(__clang__) || defined(__GNUC__)
+    return 63 - static_cast<uint32_t>(__builtin_clzll(value));
+#elif defined(_MSC_VER) && !defined(__clang__) && __cplusplus >= 202002L  // MSVC and C++20+
+    // note: std::is_constant_evaluated() added in C++20
+    //       required here as _BitScanReverse64 is not constexpr
+    if constexpr (!std::is_constant_evaluated()) {
+        // NOLINTNEXTLINE(runtime/int)
+        if constexpr (sizeof(unsigned long) == 8) {  // 64-bit
+            // NOLINTNEXTLINE(runtime/int)
+            unsigned long first_bit_index = 0;
+            _BitScanReverse64(&first_bit_index, value);
+            return first_bit_index;
+        } else {  // 32-bit
+            // NOLINTNEXTLINE(runtime/int)
+            unsigned long first_bit_index = 0;
+            if (_BitScanReverse(&first_bit_index, value >> 32)) {
+                return first_bit_index + 32;
+            }
+            _BitScanReverse(&first_bit_index, value & 0xffffffff);
+            return first_bit_index;
+        }
+    }
+#endif
+
+    // Non intrinsic (slow) path. Supports constexpr evaluation.
+    for (size_t clz = 0; clz < 64; clz++) {
+        size_t bit = 63 - clz;
+        if (value & (static_cast<size_t>(1u) << bit)) {
+            return bit;
+        }
+    }
+    return 64;
+}
+
+/// @param value the input value
+/// @returns the next power of two number greater or equal to @p value
+inline constexpr uint64_t NextPowerOfTwo(uint64_t value) {
+    if (value <= 1) {
+        return 1;
+    } else {
+        return static_cast<uint64_t>(1) << (Log2(value - 1) + 1);
+    }
+}
+
 /// @param value the input value
 /// @returns the largest power of two that `value` is a multiple of
 template <typename T>

src/tint/utils/math_test.cc

@@ -57,6 +57,70 @@ TEST(MathTests, IsPowerOfTwo) {
     EXPECT_EQ(IsPowerOfTwo(9), false);
 }
 
+TEST(MathTests, Log2) {
+    EXPECT_EQ(Log2(1), 0u);
+    EXPECT_EQ(Log2(2), 1u);
+    EXPECT_EQ(Log2(3), 1u);
+    EXPECT_EQ(Log2(4), 2u);
+    EXPECT_EQ(Log2(5), 2u);
+    EXPECT_EQ(Log2(6), 2u);
+    EXPECT_EQ(Log2(7), 2u);
+    EXPECT_EQ(Log2(8), 3u);
+    EXPECT_EQ(Log2(9), 3u);
+    EXPECT_EQ(Log2(0x7fffffffu), 30u);
+    EXPECT_EQ(Log2(0x80000000u), 31u);
+    EXPECT_EQ(Log2(0x80000001u), 31u);
+    EXPECT_EQ(Log2(0x7fffffffffffffffu), 62u);
+    EXPECT_EQ(Log2(0x8000000000000000u), 63u);
+
+    static_assert(Log2(1) == 0u);
+    static_assert(Log2(2) == 1u);
+    static_assert(Log2(3) == 1u);
+    static_assert(Log2(4) == 2u);
+    static_assert(Log2(5) == 2u);
+    static_assert(Log2(6) == 2u);
+    static_assert(Log2(7) == 2u);
+    static_assert(Log2(8) == 3u);
+    static_assert(Log2(9) == 3u);
+    static_assert(Log2(0x7fffffffu) == 30u);
+    static_assert(Log2(0x80000000u) == 31u);
+    static_assert(Log2(0x80000001u) == 31u);
+    static_assert(Log2(0x7fffffffffffffffu) == 62u);
+    static_assert(Log2(0x8000000000000000u) == 63u);
+}
+
+TEST(MathTests, NextPowerOfTwo) {
+    EXPECT_EQ(NextPowerOfTwo(0), 1u);
+    EXPECT_EQ(NextPowerOfTwo(1), 1u);
+    EXPECT_EQ(NextPowerOfTwo(2), 2u);
+    EXPECT_EQ(NextPowerOfTwo(3), 4u);
+    EXPECT_EQ(NextPowerOfTwo(4), 4u);
+    EXPECT_EQ(NextPowerOfTwo(5), 8u);
+    EXPECT_EQ(NextPowerOfTwo(6), 8u);
+    EXPECT_EQ(NextPowerOfTwo(7), 8u);
+    EXPECT_EQ(NextPowerOfTwo(8), 8u);
+    EXPECT_EQ(NextPowerOfTwo(9), 16u);
+    EXPECT_EQ(NextPowerOfTwo(0x7fffffffu), 0x80000000u);
+    EXPECT_EQ(NextPowerOfTwo(0x80000000u), 0x80000000u);
+    EXPECT_EQ(NextPowerOfTwo(0x80000001u), 0x100000000u);
+    EXPECT_EQ(NextPowerOfTwo(0x7fffffffffffffffu), 0x8000000000000000u);
+
+    static_assert(NextPowerOfTwo(0) == 1u);
+    static_assert(NextPowerOfTwo(1) == 1u);
+    static_assert(NextPowerOfTwo(2) == 2u);
+    static_assert(NextPowerOfTwo(3) == 4u);
+    static_assert(NextPowerOfTwo(4) == 4u);
+    static_assert(NextPowerOfTwo(5) == 8u);
+    static_assert(NextPowerOfTwo(6) == 8u);
+    static_assert(NextPowerOfTwo(7) == 8u);
+    static_assert(NextPowerOfTwo(8) == 8u);
+    static_assert(NextPowerOfTwo(9) == 16u);
+    static_assert(NextPowerOfTwo(0x7fffffffu) == 0x80000000u);
+    static_assert(NextPowerOfTwo(0x80000000u) == 0x80000000u);
+    static_assert(NextPowerOfTwo(0x80000001u) == 0x100000000u);
+    static_assert(NextPowerOfTwo(0x7fffffffffffffffu) == 0x8000000000000000u);
+}
+
 TEST(MathTests, MaxAlignOf) {
     EXPECT_EQ(MaxAlignOf(0u), 1u);
     EXPECT_EQ(MaxAlignOf(1u), 1u);