[llvm] [ADT] Add bit_ceil_constexpr (PR #164115)

[Kazu Hirata via llvm-commits]

https://github.com/kazutakahirata created https://github.com/llvm/llvm-project/pull/164115

This patch adds llvm::bit_ceil_constexpr, a constexpr version of
llvm::bit_ceil.

The new function is intended to serve as a marker.  When we switch to
C++20, we will most likely go through functions in llvm/ADT/bit.h and
replace them with their counterparts from <bit>.  With
llvm::bit_ceil_constexpr, we can easily replace its use with
std::bit_ceil.

This patch replaces RoundUpToPowerOfTwo in SmallPtrSet.h the new
function.


>From f71e972e6665bf89b5211b259222cf7ee5ee262d Mon Sep 17 00:00:00 2001
From: Kazu Hirata <kazu at google.com>
Date: Wed, 8 Oct 2025 11:09:44 -0700
Subject: [PATCH] [ADT] Add bit_ceil_constexpr

This patch adds llvm::bit_ceil_constexpr, a constexpr version of
llvm::bit_ceil.

The new function is intended to serve as a marker.  When we switch to
C++20, we will most likely go through functions in llvm/ADT/bit.h and
replace them with their counterparts from <bit>.  With
llvm::bit_ceil_constexpr, we can easily replace its use with
std::bit_ceil.

This patch replaces RoundUpToPowerOfTwo in SmallPtrSet.h the new
function.
---
 llvm/include/llvm/ADT/SmallPtrSet.h | 12 +-----------
 llvm/include/llvm/ADT/bit.h         | 15 +++++++++++++++
 llvm/unittests/ADT/BitTest.cpp      | 16 ++++++++++++++++
 3 files changed, 32 insertions(+), 11 deletions(-)

diff --git a/llvm/include/llvm/ADT/SmallPtrSet.h b/llvm/include/llvm/ADT/SmallPtrSet.h
index f588a77a53b2a..8e7c8b30293b2 100644
--- a/llvm/include/llvm/ADT/SmallPtrSet.h
+++ b/llvm/include/llvm/ADT/SmallPtrSet.h
@@ -532,18 +532,8 @@ class SmallPtrSet : public SmallPtrSetImpl<PtrType> {
 
   using BaseT = SmallPtrSetImpl<PtrType>;
 
-  // A constexpr version of llvm::bit_ceil.
-  // TODO: Replace this with std::bit_ceil once C++20 is available.
-  static constexpr size_t RoundUpToPowerOfTwo(size_t X) {
-    size_t C = 1;
-    size_t CMax = C << (std::numeric_limits<size_t>::digits - 1);
-    while (C < X && C < CMax)
-      C <<= 1;
-    return C;
-  }
-
   // Make sure that SmallSize is a power of two, round up if not.
-  static constexpr size_t SmallSizePowTwo = RoundUpToPowerOfTwo(SmallSize);
+  static constexpr size_t SmallSizePowTwo = llvm::bit_ceil_constexpr(SmallSize);
   /// SmallStorage - Fixed size storage used in 'small mode'.
   const void *SmallStorage[SmallSizePowTwo];
 
diff --git a/llvm/include/llvm/ADT/bit.h b/llvm/include/llvm/ADT/bit.h
index 8c68d0a90c753..8b60b6998ca0b 100644
--- a/llvm/include/llvm/ADT/bit.h
+++ b/llvm/include/llvm/ADT/bit.h
@@ -336,6 +336,21 @@ template <typename T> [[nodiscard]] T bit_ceil(T Value) {
   return T(1) << llvm::bit_width<T>(Value - 1u);
 }
 
+/// Returns the smallest integral power of two no smaller than Value if Value is
+/// nonzero.  Returns 1 otherwise.
+///
+/// Ex. bit_ceil(5) == 8.
+///
+/// The return value is undefined if the input is larger than the largest power
+/// of two representable in T.
+template <typename T> [[nodiscard]] constexpr T bit_ceil_constexpr(T Value) {
+  static_assert(std::is_unsigned_v<T>,
+                "Only unsigned integral types are allowed.");
+  if (Value < 2)
+    return 1;
+  return T(1) << llvm::bit_width_constexpr<T>(Value - 1u);
+}
+
 template <typename T, typename = std::enable_if_t<std::is_unsigned_v<T>>>
 [[nodiscard]] constexpr T rotl(T V, int R) {
   constexpr unsigned N = std::numeric_limits<T>::digits;
diff --git a/llvm/unittests/ADT/BitTest.cpp b/llvm/unittests/ADT/BitTest.cpp
index eaed4e1fe327d..5b3df918e62b0 100644
--- a/llvm/unittests/ADT/BitTest.cpp
+++ b/llvm/unittests/ADT/BitTest.cpp
@@ -270,6 +270,22 @@ TEST(BitTest, BitWidthConstexpr) {
       llvm::bit_width_constexpr(std::numeric_limits<uint64_t>::max()) == 64);
 }
 
+TEST(BitTest, BitCeilConstexpr) {
+  static_assert(llvm::bit_ceil_constexpr(0u) == 1);
+  static_assert(llvm::bit_ceil_constexpr(1u) == 1);
+  static_assert(llvm::bit_ceil_constexpr(2u) == 2);
+  static_assert(llvm::bit_ceil_constexpr(3u) == 4);
+  static_assert(llvm::bit_ceil_constexpr(4u) == 4);
+  static_assert(llvm::bit_ceil_constexpr(5u) == 8);
+  static_assert(llvm::bit_ceil_constexpr(6u) == 8);
+  static_assert(llvm::bit_ceil_constexpr(7u) == 8);
+  static_assert(llvm::bit_ceil_constexpr(8u) == 8);
+
+  static_assert(llvm::bit_ceil_constexpr(255u) == 256);
+  static_assert(llvm::bit_ceil_constexpr(256u) == 256);
+  static_assert(llvm::bit_ceil_constexpr(257u) == 512);
+}
+
 TEST(BitTest, CountlZero) {
   uint8_t Z8 = 0;
   uint16_t Z16 = 0;