[libc-commits] [libc] [libc] Add generic cpp::byteswap to CPP/bit.h (PR #196274)
via libc-commits
libc-commits at lists.llvm.org
Thu May 7 03:37:49 PDT 2026
llvmorg-github-actions[bot] wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libc
Author: Jeff Bailey (kaladron)
<details>
<summary>Changes</summary>
Added a constexpr byteswap template using recursive half-swap decomposition. Accepts all integral types, matching C++23 std::byteswap semantics. Signed types delegate to the unsigned path via static_cast, which the compiler elides entirely.
A single recursive template handles all sizes from 8 to 128 bits with no per-width specialisations or builtin fallbacks needed. Produces optimal bswap/rolw instructions on Clang at -O2. A static_assert rejects types larger than 128 bits.
Refactored endian_internal.h to call cpp::byteswap directly, replacing the explicit template specialisations and builtin dispatch.
Assisted-by: Automated tooling, human reviewed.
---
Full diff: https://github.com/llvm/llvm-project/pull/196274.diff
4 Files Affected:
- (modified) libc/src/__support/CMakeLists.txt (+1)
- (modified) libc/src/__support/CPP/bit.h (+31-1)
- (modified) libc/src/__support/endian_internal.h (+7-38)
- (modified) libc/test/src/__support/CPP/bit_test.cpp (+54)
``````````diff
diff --git a/libc/src/__support/CMakeLists.txt b/libc/src/__support/CMakeLists.txt
index 6eab12ec8b637..098fb6ef86936 100644
--- a/libc/src/__support/CMakeLists.txt
+++ b/libc/src/__support/CMakeLists.txt
@@ -103,6 +103,7 @@ add_header_library(
macros/config.h
DEPENDS
libc.hdr.stdint_proxy
+ libc.src.__support.CPP.bit
)
add_header_library(
diff --git a/libc/src/__support/CPP/bit.h b/libc/src/__support/CPP/bit.h
index 0ba8b9219a317..3dfa81017472d 100644
--- a/libc/src/__support/CPP/bit.h
+++ b/libc/src/__support/CPP/bit.h
@@ -6,7 +6,7 @@
//
//===----------------------------------------------------------------------===//
// This is inspired by LLVM ADT/bit.h header.
-// Some functions are missing, we can add them as needed (popcount, byteswap).
+// Some functions are missing, we can add them as needed.
#ifndef LLVM_LIBC_SRC___SUPPORT_CPP_BIT_H
#define LLVM_LIBC_SRC___SUPPORT_CPP_BIT_H
@@ -330,6 +330,36 @@ ADD_SPECIALIZATION(unsigned long long, __builtin_popcountll)
#endif // __builtin_popcountg
#undef ADD_SPECIALIZATION
+/// Reverses the bytes in the given integer value.
+///
+/// All integral types are allowed, matching C++23 std::byteswap semantics.
+/// Signed types delegate to the unsigned path via static_cast.
+///
+/// The recursive decomposition generates optimal 'bswap' or 'rolw'
+/// instructions on Clang at -O2 without requiring compiler intrinsics.
+template <typename T>
+[[nodiscard]] LIBC_INLINE constexpr cpp::enable_if_t<cpp::is_integral_v<T>, T>
+byteswap(T value) {
+ static_assert(sizeof(T) <= 16, "byteswap: unsupported type size");
+ if constexpr (!cpp::is_unsigned_v<T>) {
+ using U = cpp::make_unsigned_t<T>;
+ return static_cast<T>(byteswap(static_cast<U>(value)));
+ } else if constexpr (sizeof(T) == 1) {
+ return value;
+ } else {
+ constexpr unsigned half_bits = sizeof(T) * 8 / 2;
+ using Half = cpp::conditional_t<
+ sizeof(T) == 2, uint8_t,
+ cpp::conditional_t<
+ sizeof(T) == 4, uint16_t,
+ cpp::conditional_t<sizeof(T) == 8, uint32_t, uint64_t>>>;
+ Half lo = static_cast<Half>(value);
+ Half hi = static_cast<Half>(value >> half_bits);
+ return static_cast<T>((static_cast<T>(byteswap(lo)) << half_bits) |
+ static_cast<T>(byteswap(hi)));
+ }
+}
+
} // namespace cpp
} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/__support/endian_internal.h b/libc/src/__support/endian_internal.h
index 07cde7b905c4d..8d81329fd243d 100644
--- a/libc/src/__support/endian_internal.h
+++ b/libc/src/__support/endian_internal.h
@@ -10,6 +10,7 @@
#define LLVM_LIBC_SRC___SUPPORT_ENDIAN_INTERNAL_H
#include "hdr/stdint_proxy.h"
+#include "src/__support/CPP/bit.h"
#include "src/__support/common.h"
#include "src/__support/macros/config.h"
@@ -29,38 +30,6 @@ namespace LIBC_NAMESPACE_DECL {
namespace internal {
-template <typename T> LIBC_INLINE T byte_swap(T value);
-
-template <> LIBC_INLINE uint16_t byte_swap<uint16_t>(uint16_t value) {
-#if __has_builtin(__builtin_bswap16)
- return __builtin_bswap16(value);
-#else
- return (value << 8) | (value >> 8);
-#endif // __builtin_bswap16
-}
-
-template <> LIBC_INLINE uint32_t byte_swap<uint32_t>(uint32_t value) {
-#if __has_builtin(__builtin_bswap32)
- return __builtin_bswap32(value);
-#else
- return byte_swap<uint16_t>(static_cast<uint16_t>(value >> 16)) ||
- (static_cast<uint32_t>(
- byte_swap<uint16_t>(static_cast<uint16_t>(value)))
- << 16);
-#endif // __builtin_bswap64
-}
-
-template <> LIBC_INLINE uint64_t byte_swap<uint64_t>(uint64_t value) {
-#if __has_builtin(__builtin_bswap64)
- return __builtin_bswap64(value);
-#else
- return byte_swap<uint32_t>(static_cast<uint32_t>(value >> 32)) ||
- (static_cast<uint64_t>(
- byte_swap<uint32_t>(static_cast<uint32_t>(value)))
- << 32);
-#endif // __builtin_bswap64
-}
-
// Converts uint8_t, uint16_t, uint32_t, uint64_t to its big or little endian
// counterpart.
// We use explicit template specialization:
@@ -91,7 +60,7 @@ template <>
template <>
LIBC_INLINE uint16_t
Endian<__ORDER_LITTLE_ENDIAN__>::to_big_endian<uint16_t>(uint16_t v) {
- return byte_swap<uint16_t>(v);
+ return cpp::byteswap(v);
}
template <>
template <>
@@ -103,7 +72,7 @@ template <>
template <>
LIBC_INLINE uint32_t
Endian<__ORDER_LITTLE_ENDIAN__>::to_big_endian<uint32_t>(uint32_t v) {
- return byte_swap<uint32_t>(v);
+ return cpp::byteswap(v);
}
template <>
template <>
@@ -115,7 +84,7 @@ template <>
template <>
LIBC_INLINE uint64_t
Endian<__ORDER_LITTLE_ENDIAN__>::to_big_endian<uint64_t>(uint64_t v) {
- return byte_swap<uint64_t>(v);
+ return cpp::byteswap(v);
}
template <>
template <>
@@ -147,7 +116,7 @@ template <>
template <>
LIBC_INLINE uint16_t
Endian<__ORDER_BIG_ENDIAN__>::to_little_endian<uint16_t>(uint16_t v) {
- return byte_swap<uint16_t>(v);
+ return cpp::byteswap(v);
}
template <>
template <>
@@ -159,7 +128,7 @@ template <>
template <>
LIBC_INLINE uint32_t
Endian<__ORDER_BIG_ENDIAN__>::to_little_endian<uint32_t>(uint32_t v) {
- return byte_swap<uint32_t>(v);
+ return cpp::byteswap(v);
}
template <>
template <>
@@ -171,7 +140,7 @@ template <>
template <>
LIBC_INLINE uint64_t
Endian<__ORDER_BIG_ENDIAN__>::to_little_endian<uint64_t>(uint64_t v) {
- return byte_swap<uint64_t>(v);
+ return cpp::byteswap(v);
}
} // namespace internal
diff --git a/libc/test/src/__support/CPP/bit_test.cpp b/libc/test/src/__support/CPP/bit_test.cpp
index 891e693e0c953..bf22abe5b7d4a 100644
--- a/libc/test/src/__support/CPP/bit_test.cpp
+++ b/libc/test/src/__support/CPP/bit_test.cpp
@@ -233,5 +233,59 @@ TYPED_TEST(LlvmLibcBitTest, CountOnes, UnsignedTypes) {
cpp::numeric_limits<T>::digits - i);
}
+TEST(LlvmLibcBitTest, Byteswap) {
+ // 8-bit: identity
+ EXPECT_EQ(byteswap(uint8_t(0x00)), uint8_t(0x00));
+ EXPECT_EQ(byteswap(uint8_t(0xAB)), uint8_t(0xAB));
+ EXPECT_EQ(byteswap(uint8_t(0xFF)), uint8_t(0xFF));
+
+ // 16-bit
+ EXPECT_EQ(byteswap(uint16_t(0x0000)), uint16_t(0x0000));
+ EXPECT_EQ(byteswap(uint16_t(0x1234)), uint16_t(0x3412));
+ EXPECT_EQ(byteswap(uint16_t(0xAABB)), uint16_t(0xBBAA));
+ EXPECT_EQ(byteswap(uint16_t(0xFFFF)), uint16_t(0xFFFF));
+
+ // 32-bit
+ EXPECT_EQ(byteswap(uint32_t(0x00000000)), uint32_t(0x00000000));
+ EXPECT_EQ(byteswap(uint32_t(0x12345678)), uint32_t(0x78563412));
+ EXPECT_EQ(byteswap(uint32_t(0xDEADBEEF)), uint32_t(0xEFBEADDE));
+ EXPECT_EQ(byteswap(uint32_t(0xFFFFFFFF)), uint32_t(0xFFFFFFFF));
+
+ // 64-bit
+ EXPECT_EQ(byteswap(uint64_t(0x0000000000000000)),
+ uint64_t(0x0000000000000000));
+ EXPECT_EQ(byteswap(uint64_t(0x0123456789ABCDEF)),
+ uint64_t(0xEFCDAB8967452301));
+ EXPECT_EQ(byteswap(uint64_t(0xFFFFFFFFFFFFFFFF)),
+ uint64_t(0xFFFFFFFFFFFFFFFF));
+}
+
+TEST(LlvmLibcBitTest, ByteswapSigned) {
+ // Signed 16-bit
+ EXPECT_EQ(byteswap(int16_t(0x1234)), int16_t(0x3412));
+
+ // Signed 32-bit
+ EXPECT_EQ(byteswap(int32_t(0x12345678)), int32_t(0x78563412));
+
+ // Signed 64-bit
+ EXPECT_EQ(byteswap(int64_t(0x0123456789ABCDEF)), int64_t(0xEFCDAB8967452301));
+}
+
+using ByteswapTypes = testing::TypeList<
+#if defined(LIBC_TYPES_HAS_INT128)
+ __uint128_t,
+#endif
+ unsigned char, unsigned short, unsigned int, unsigned long,
+ unsigned long long, signed char, short, int, long, long long>;
+
+TYPED_TEST(LlvmLibcBitTest, ByteswapInvolution, ByteswapTypes) {
+ // Byteswap is its own inverse: byteswap(byteswap(x)) == x.
+ EXPECT_EQ(byteswap(byteswap(T(0))), T(0));
+ EXPECT_EQ(byteswap(byteswap(T(1))), T(1));
+ EXPECT_EQ(byteswap(byteswap(T(~0))), T(~0));
+ EXPECT_EQ(byteswap(byteswap(T(0x0123456789ABCDEF & T(~0)))),
+ T(0x0123456789ABCDEF & T(~0)));
+}
+
} // namespace cpp
} // namespace LIBC_NAMESPACE_DECL
``````````
</details>
https://github.com/llvm/llvm-project/pull/196274
More information about the libc-commits
mailing list