[useful] Drop bespoke BitCount implementation (#37083)

use `absl::popcount` instead

Built on #37082 which should be merged first

Closes #37083

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/37083 from ctiller:less-bespoke-2 317a4545f0
PiperOrigin-RevId: 652870573

src/core/lib/gprpp/bitset.h

@@ -138,7 +138,7 @@ class BitSet {
   uint32_t count() const {
     uint32_t count = 0;
     for (size_t i = 0; i < kUnits; i++) {
-      count += BitCount(units_[i]);
+      count += absl::popcount(units_[i]);
     }
     return count;
   }

src/core/util/useful.h

@@ -57,53 +57,6 @@ bool GetBit(T i, size_t n) {
   return (i & (T(1) << n)) != 0;
 }
 
-namespace useful_detail {
-GPR_ATTRIBUTE_ALWAYS_INLINE_FUNCTION inline constexpr uint32_t HexdigitBitcount(
-    uint32_t x) {
-  return (x - ((x >> 1) & 0x77777777) - ((x >> 2) & 0x33333333) -
-          ((x >> 3) & 0x11111111));
-}
-}  // namespace useful_detail
-
-GPR_ATTRIBUTE_ALWAYS_INLINE_FUNCTION inline constexpr uint32_t BitCount(
-    uint32_t i) {
-  return (((useful_detail::HexdigitBitcount(i) +
-            (useful_detail::HexdigitBitcount(i) >> 4)) &
-           0x0f0f0f0f) %
-          255);
-}
-
-GPR_ATTRIBUTE_ALWAYS_INLINE_FUNCTION inline constexpr uint32_t BitCount(
-    uint64_t i) {
-  return BitCount(static_cast<uint32_t>(i)) +
-         BitCount(static_cast<uint32_t>(i >> 32));
-}
-
-GPR_ATTRIBUTE_ALWAYS_INLINE_FUNCTION inline constexpr uint32_t BitCount(
-    uint16_t i) {
-  return BitCount(static_cast<uint32_t>(i));
-}
-GPR_ATTRIBUTE_ALWAYS_INLINE_FUNCTION inline constexpr uint32_t BitCount(
-    uint8_t i) {
-  return BitCount(static_cast<uint32_t>(i));
-}
-GPR_ATTRIBUTE_ALWAYS_INLINE_FUNCTION inline constexpr uint32_t BitCount(
-    int64_t i) {
-  return BitCount(static_cast<uint64_t>(i));
-}
-GPR_ATTRIBUTE_ALWAYS_INLINE_FUNCTION inline constexpr uint32_t BitCount(
-    int32_t i) {
-  return BitCount(static_cast<uint32_t>(i));
-}
-GPR_ATTRIBUTE_ALWAYS_INLINE_FUNCTION inline constexpr uint32_t BitCount(
-    int16_t i) {
-  return BitCount(static_cast<uint16_t>(i));
-}
-GPR_ATTRIBUTE_ALWAYS_INLINE_FUNCTION inline constexpr uint32_t BitCount(
-    int8_t i) {
-  return BitCount(static_cast<uint8_t>(i));
-}
-
 #if GRPC_HAS_BUILTIN(__builtin_ctz)
 GPR_ATTRIBUTE_ALWAYS_INLINE_FUNCTION inline uint32_t CountTrailingZeros(
     uint32_t i) {
@@ -119,12 +72,12 @@ GPR_ATTRIBUTE_ALWAYS_INLINE_FUNCTION inline uint32_t CountTrailingZeros(
 GPR_ATTRIBUTE_ALWAYS_INLINE_FUNCTION inline uint32_t CountTrailingZeros(
     uint32_t i) {
   DCHECK_NE(i, 0);  // __builtin_ctz returns undefined behavior for 0
-  return BitCount((i & -i) - 1);
+  return absl::popcount((i & -i) - 1);
 }
 GPR_ATTRIBUTE_ALWAYS_INLINE_FUNCTION inline uint32_t CountTrailingZeros(
     uint64_t i) {
   DCHECK_NE(i, 0);  // __builtin_ctz returns undefined behavior for 0
-  return BitCount((i & -i) - 1);
+  return absl::popcount((i & -i) - 1);
 }
 #endif
 

test/core/util/useful_test.cc

@@ -48,18 +48,11 @@ TEST(UsefulTest, ArraySize) {
 TEST(UsefulTest, BitOps) {
   uint32_t bitset = 0;
 
-  EXPECT_EQ(BitCount((1u << 31) - 1), 31);
-  EXPECT_EQ(BitCount(1u << 3), 1);
-  EXPECT_EQ(BitCount(0), 0);
   EXPECT_EQ(SetBit(&bitset, 3), 8);
-  EXPECT_EQ(BitCount(bitset), 1);
   EXPECT_EQ(GetBit(bitset, 3), 1);
   EXPECT_EQ(SetBit(&bitset, 1), 10);
-  EXPECT_EQ(BitCount(bitset), 2);
   EXPECT_EQ(ClearBit(&bitset, 3), 2);
-  EXPECT_EQ(BitCount(bitset), 1);
   EXPECT_EQ(GetBit(bitset, 3), 0);
-  EXPECT_EQ(BitCount(std::numeric_limits<uint64_t>::max()), 64);
 }
 
 TEST(UsefulTest, SaturatingAdd) {