[ADT/MathExtras] Introduce PowerOf2Ceil

[davide]

Summary

To be used in https://reviews.llvm.org/D26372 (and probably somewhere else in llvm).

[ruiu]

LGTM

llvm/trunk/include/llvm/Support/MathExtras.h

 
 /// 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;
   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) {
+  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.
 ///
 /// If non-zero \p Skew is specified, the return value will be a minimal
 /// integer that is greater than or equal to \p Value and equal to
 /// \p Align * N + \p Skew for some integer N. If \p Skew is larger than
 /// \p Align, its value is adjusted to '\p Skew mod \p Align'.
 ///

llvm/trunk/unittests/Support/MathExtrasTest.cpp

 
 TEST(MathExtras, isPowerOf2_64) {
   EXPECT_TRUE(isPowerOf2_64(1LL << 46));
   EXPECT_TRUE(isPowerOf2_64(1LL << 12));
   EXPECT_FALSE(isPowerOf2_64((1LL << 53) + 3));
   EXPECT_FALSE(isPowerOf2_64(0xABCDEF0ABCDEF0LL));
 }
 
+TEST(MathExtras, PowerOf2Ceil) {
+  EXPECT_EQ(0, PowerOf2Ceil(0));
+  EXPECT_EQ(8, PowerOf2Ceil(8));
+  EXPECT_EQ(8, PowerOf2Ceil(7));
+}
+
 TEST(MathExtras, ByteSwap_32) {
   EXPECT_EQ(0x44332211u, ByteSwap_32(0x11223344));
   EXPECT_EQ(0xDDCCBBAAu, ByteSwap_32(0xAABBCCDD));
 }
 
 TEST(MathExtras, ByteSwap_64) {
   EXPECT_EQ(0x8877665544332211ULL, ByteSwap_64(0x1122334455667788LL));
   EXPECT_EQ(0x1100FFEEDDCCBBAAULL, ByteSwap_64(0xAABBCCDDEEFF0011LL));