[libc-commits] [libc] [libc][support][UInt] implement 128b popcount (PR #86531)

[Nick Desaulniers via libc-commits]

https://github.com/nickdesaulniers created https://github.com/llvm/llvm-project/pull/86531

We could have used this in the implementation of UInt::has_single_bit, but
has_single_bit has a perhaps useful early return.


>From 3d400c64eb04e0ce52054befb1af8d89f1bb477e Mon Sep 17 00:00:00 2001
From: Nick Desaulniers <ndesaulniers at google.com>
Date: Mon, 25 Mar 2024 09:51:29 -0700
Subject: [PATCH] [libc][support][UInt] implement 128b popcount

We could have used this in the implementation of UInt::has_single_bit, but
has_single_bit has a perhaps useful early return.
---
 libc/src/__support/UInt.h                | 11 +++++++++++
 libc/test/src/__support/CPP/bit_test.cpp |  9 +--------
 2 files changed, 12 insertions(+), 8 deletions(-)

diff --git a/libc/src/__support/UInt.h b/libc/src/__support/UInt.h
index df01e081e3c19e..b04e8d02ad4c16 100644
--- a/libc/src/__support/UInt.h
+++ b/libc/src/__support/UInt.h
@@ -1082,6 +1082,17 @@ bit_cast(const UInt<Bits> &from) {
   return cpp::bit_cast<To>(from.val);
 }
 
+// Specialization of cpp::popcount ('bit.h') for BigInt.
+template <typename T>
+[[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<is_big_int_v<T>, int>
+popcount(T value) {
+  int bits = 0;
+  for (auto word : value.val)
+    if (word)
+      bits += popcount(word);
+  return bits;
+}
+
 // Specialization of cpp::has_single_bit ('bit.h') for BigInt.
 template <typename T>
 [[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<is_big_int_v<T>, bool>
diff --git a/libc/test/src/__support/CPP/bit_test.cpp b/libc/test/src/__support/CPP/bit_test.cpp
index cee5b90c8f4bdb..875b47e6a1980e 100644
--- a/libc/test/src/__support/CPP/bit_test.cpp
+++ b/libc/test/src/__support/CPP/bit_test.cpp
@@ -15,13 +15,6 @@
 
 namespace LIBC_NAMESPACE::cpp {
 
-using UnsignedTypesNoBigInt = testing::TypeList<
-#if defined(LIBC_TYPES_HAS_INT128)
-    __uint128_t,
-#endif // LIBC_TYPES_HAS_INT128
-    unsigned char, unsigned short, unsigned int, unsigned long,
-    unsigned long long>;
-
 using UnsignedTypes = testing::TypeList<
 #if defined(LIBC_TYPES_HAS_INT128)
     __uint128_t,
@@ -228,7 +221,7 @@ TEST(LlvmLibcBitTest, Rotr) {
             rotr<uint64_t>(0x12345678deadbeefULL, -19));
 }
 
-TYPED_TEST(LlvmLibcBitTest, CountOnes, UnsignedTypesNoBigInt) {
+TYPED_TEST(LlvmLibcBitTest, CountOnes, UnsignedTypes) {
   EXPECT_EQ(popcount(T(0)), 0);
   for (int i = 0; i != cpp::numeric_limits<T>::digits; ++i)
     EXPECT_EQ(popcount<T>(cpp::numeric_limits<T>::max() >> i),