[libc] [llvm] Reland Refactor WIDE_READ to allow finer control over high-performance function selection (#165613) (PR #170738)
via llvm-commits
llvm-commits at lists.llvm.org
Thu Dec 4 12:38:04 PST 2025
https://github.com/Sterling-Augustine created https://github.com/llvm/llvm-project/pull/170738
[Previous commit had an incorrect default case when FIND_FIRST_CHARACTER_WIDE_READ_IMPL was not specified in config.json. This PR is identical to that one with one line fixed.]
As we implement more high-performance string-related functions, we have found a need for better control over their selection than the big-hammer LIBC_CONF_STRING_LENGTH_WIDE_READ. For example, I have a memchr implementation coming, and unless I implement it in every variant, a simple binary value doesn't work.
This PR makes gives finer-grained control over high-performance functions than the generic LIBC_CONF_UNSAFE_WIDE_READ option. For any function they like, the user can now select one of four implementations at build time:
1. element, which reads byte-by-byte (or wchar by wchar)
2. wide, which reads by unsigned long
3. generic, which uses standard clang vector implemenations, if available
4. arch, which uses an architecture-specific implemenation
(Reading the code carefully, you may note that a user can actually specify any namespace they want, so we aren't technically limited to those 4.)
We may also want to switch from command-line #defines as it is currently done, to something more like
llvm-project/llvm/include/llvm/Config/llvm-config.h.cmake, and complexity out of the command-line. But that's a future problem.
>From 8293d41479d2f0e7c057afed27d290288d7205a2 Mon Sep 17 00:00:00 2001
From: Sterling Augustine <saugustine at google.com>
Date: Thu, 4 Dec 2025 11:45:49 -0800
Subject: [PATCH] Reland Refactor WIDE_READ to allow finer control over
high-performance function selection (#165613)
[Previous commit had an incorrect default case when
FIND_FIRST_CHARACTER_WIDE_READ_IMPL was not specified in config.json.]
As we implement more high-performance string-related functions, we have
found a need for better control over their selection than the big-hammer
LIBC_CONF_STRING_LENGTH_WIDE_READ. For example, I have a memchr
implementation coming, and unless I implement it in every variant, a
simple binary value doesn't work.
This PR makes gives finer-grained control over high-performance
functions than the generic LIBC_CONF_UNSAFE_WIDE_READ option. For any
function they like, the user can now select one of four implementations
at build time:
1. element, which reads byte-by-byte (or wchar by wchar)
2. wide, which reads by unsigned long
3. generic, which uses standard clang vector implemenations, if
available
4. arch, which uses an architecture-specific implemenation
(Reading the code carefully, you may note that a user can actually
specify any namespace they want, so we aren't technically limited to
those 4.)
We may also want to switch from command-line #defines as it is currently
done, to something more like
llvm-project/llvm/include/llvm/Config/llvm-config.h.cmake, and
complexity out of the command-line. But that's a future problem.
---
.../modules/LLVMLibCCompileOptionRules.cmake | 5 +-
libc/config/config.json | 11 +-
libc/config/linux/arm/config.json | 7 +-
libc/config/linux/config.json | 7 +-
libc/config/linux/riscv/config.json | 7 +-
libc/docs/configure.rst | 3 +-
.../memory_utils/aarch64/inline_strlen.h | 16 +-
.../memory_utils/generic/inline_strlen.h | 5 +-
.../memory_utils/x86_64/inline_strlen.h | 14 +-
libc/src/string/string_length.h | 213 ++++++++++++++++++
libc/src/string/string_utils.h | 164 +-------------
.../llvm-project-overlay/libc/BUILD.bazel | 5 +-
.../libc/libc_configure_options.bzl | 3 +-
13 files changed, 275 insertions(+), 185 deletions(-)
create mode 100644 libc/src/string/string_length.h
diff --git a/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake b/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
index 4e9a9b66a63a7..f4e2a62d14b31 100644
--- a/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
+++ b/libc/cmake/modules/LLVMLibCCompileOptionRules.cmake
@@ -81,9 +81,8 @@ function(_get_compile_options_from_config output_var)
list(APPEND config_options "-DLIBC_QSORT_IMPL=${LIBC_CONF_QSORT_IMPL}")
endif()
- if(LIBC_CONF_STRING_UNSAFE_WIDE_READ)
- list(APPEND config_options "-DLIBC_COPT_STRING_UNSAFE_WIDE_READ")
- endif()
+ list(APPEND config_options "-DLIBC_COPT_STRING_LENGTH_IMPL=${LIBC_CONF_STRING_LENGTH_IMPL}")
+ list(APPEND config_options "-DLIBC_COPT_FIND_FIRST_CHARACTER_IMPL=${LIBC_CONF_FIND_FIRST_CHARACTER_IMPL}")
if(LIBC_CONF_MEMSET_X86_USE_SOFTWARE_PREFETCHING)
list(APPEND config_options "-DLIBC_COPT_MEMSET_X86_USE_SOFTWARE_PREFETCHING")
diff --git a/libc/config/config.json b/libc/config/config.json
index a7844e4fe2dd1..f0ab3b9cce2e9 100644
--- a/libc/config/config.json
+++ b/libc/config/config.json
@@ -40,6 +40,7 @@
"value": false,
"doc": "Use an alternative printf float implementation based on 320-bit floats"
},
+
"LIBC_CONF_PRINTF_DISABLE_FIXED_POINT": {
"value": false,
"doc": "Disable printing fixed point values in printf and friends."
@@ -64,9 +65,13 @@
}
},
"string": {
- "LIBC_CONF_STRING_UNSAFE_WIDE_READ": {
- "value": false,
- "doc": "Read more than a byte at a time to perform byte-string operations like strlen."
+ "LIBC_CONF_STRING_LENGTH_IMPL": {
+ "value": "element",
+ "doc": "Selects the implementation for string-length: 'element', 'word', 'clang_vector', or 'arch_vector'."
+ },
+ "LIBC_CONF_FIND_FIRST_CHARACTER_IMPL": {
+ "value": "element",
+ "doc": "Selects the implementation for find-first-character-related functions: 'element', 'word', 'clang_vector', or 'arch_vector'."
},
"LIBC_CONF_MEMSET_X86_USE_SOFTWARE_PREFETCHING": {
"value": false,
diff --git a/libc/config/linux/arm/config.json b/libc/config/linux/arm/config.json
index e7ad4544b104d..caa16744d389f 100644
--- a/libc/config/linux/arm/config.json
+++ b/libc/config/linux/arm/config.json
@@ -1,7 +1,10 @@
{
"string": {
- "LIBC_CONF_STRING_UNSAFE_WIDE_READ": {
- "value": false
+ "LIBC_CONF_STRING_LENGTH_IMPL": {
+ "value": "element"
+ }
+ "LIBC_CONF_FIND_FIRST_CHARACTER_IMPL": {
+ "value": "element"
}
}
}
diff --git a/libc/config/linux/config.json b/libc/config/linux/config.json
index 30e8b2cdadabe..8e7db248dc1bd 100644
--- a/libc/config/linux/config.json
+++ b/libc/config/linux/config.json
@@ -1,7 +1,10 @@
{
"string": {
- "LIBC_CONF_STRING_UNSAFE_WIDE_READ": {
- "value": true
+ "LIBC_CONF_STRING_LENGTH_IMPL": {
+ "value": "clang_vector",
+ },
+ "LIBC_CONF_FIND_FIRST_CHARACTER_IMPL": {
+ "value": "word",
}
}
}
diff --git a/libc/config/linux/riscv/config.json b/libc/config/linux/riscv/config.json
index e7ad4544b104d..caa16744d389f 100644
--- a/libc/config/linux/riscv/config.json
+++ b/libc/config/linux/riscv/config.json
@@ -1,7 +1,10 @@
{
"string": {
- "LIBC_CONF_STRING_UNSAFE_WIDE_READ": {
- "value": false
+ "LIBC_CONF_STRING_LENGTH_IMPL": {
+ "value": "element"
+ }
+ "LIBC_CONF_FIND_FIRST_CHARACTER_IMPL": {
+ "value": "element"
}
}
}
diff --git a/libc/docs/configure.rst b/libc/docs/configure.rst
index 362e293a4b714..43d3c0ec06d3b 100644
--- a/libc/docs/configure.rst
+++ b/libc/docs/configure.rst
@@ -58,8 +58,9 @@ to learn about the defaults for your platform and target.
* **"setjmp" options**
- ``LIBC_CONF_SETJMP_AARCH64_RESTORE_PLATFORM_REGISTER``: Make setjmp save the value of x18, and longjmp restore it. The AArch64 ABI delegates this register to platform ABIs, which can choose whether to make it caller-saved.
* **"string" options**
+ - ``LIBC_CONF_FIND_FIRST_CHARACTER_IMPL``: Selects the implementation for find-first-character-related functions: 'element', 'word', 'clang_vector', or 'arch_vector'.
- ``LIBC_CONF_MEMSET_X86_USE_SOFTWARE_PREFETCHING``: Inserts prefetch for write instructions (PREFETCHW) for memset on x86 to recover performance when hardware prefetcher is disabled.
- - ``LIBC_CONF_STRING_UNSAFE_WIDE_READ``: Read more than a byte at a time to perform byte-string operations like strlen.
+ - ``LIBC_CONF_STRING_LENGTH_IMPL``: Selects the implementation for string-length: 'element', 'word', 'clang_vector', or 'arch_vector'.
* **"threads" options**
- ``LIBC_CONF_THREAD_MODE``: The implementation used for Mutex, acceptable values are LIBC_THREAD_MODE_PLATFORM, LIBC_THREAD_MODE_SINGLE, and LIBC_THREAD_MODE_EXTERNAL.
* **"time" options**
diff --git a/libc/src/string/memory_utils/aarch64/inline_strlen.h b/libc/src/string/memory_utils/aarch64/inline_strlen.h
index eafaca9776a42..87f6cb8cf9bd5 100644
--- a/libc/src/string/memory_utils/aarch64/inline_strlen.h
+++ b/libc/src/string/memory_utils/aarch64/inline_strlen.h
@@ -15,7 +15,7 @@
#include <arm_neon.h>
#include <stddef.h> // size_t
namespace LIBC_NAMESPACE_DECL {
-namespace neon {
+namespace internal::neon {
[[maybe_unused]] LIBC_NO_SANITIZE_OOB_ACCESS LIBC_INLINE static size_t
string_length(const char *src) {
using Vector __attribute__((may_alias)) = uint8x8_t;
@@ -43,7 +43,7 @@ string_length(const char *src) {
(cpp::countr_zero(cmp) >> 3));
}
}
-} // namespace neon
+} // namespace internal::neon
} // namespace LIBC_NAMESPACE_DECL
#endif // __ARM_NEON
@@ -51,7 +51,7 @@ string_length(const char *src) {
#include "src/__support/macros/optimization.h"
#include <arm_sve.h>
namespace LIBC_NAMESPACE_DECL {
-namespace sve {
+namespace internal::sve {
[[maybe_unused]] LIBC_INLINE static size_t string_length(const char *src) {
const uint8_t *ptr = reinterpret_cast<const uint8_t *>(src);
// Initialize the first-fault register to all true
@@ -92,15 +92,19 @@ namespace sve {
len += svcntp_b8(all_true, before_zero);
return len;
}
-} // namespace sve
+} // namespace internal::sve
} // namespace LIBC_NAMESPACE_DECL
#endif // LIBC_TARGET_CPU_HAS_SVE
namespace LIBC_NAMESPACE_DECL {
+namespace internal::arch_vector {
+[[maybe_unused]] LIBC_INLINE size_t string_length(const char *src) {
#ifdef LIBC_TARGET_CPU_HAS_SVE
-namespace string_length_impl = sve;
+ return sve::string_length(src);
#elif defined(__ARM_NEON)
-namespace string_length_impl = neon;
+ return neon::string_length(src);
#endif
+}
+} // namespace internal::arch_vector
} // namespace LIBC_NAMESPACE_DECL
#endif // LLVM_LIBC_SRC_STRING_MEMORY_UTILS_AARCH64_INLINE_STRLEN_H
diff --git a/libc/src/string/memory_utils/generic/inline_strlen.h b/libc/src/string/memory_utils/generic/inline_strlen.h
index 69700e801bcea..7a565b36617ed 100644
--- a/libc/src/string/memory_utils/generic/inline_strlen.h
+++ b/libc/src/string/memory_utils/generic/inline_strlen.h
@@ -14,7 +14,7 @@
#include "src/__support/common.h"
namespace LIBC_NAMESPACE_DECL {
-namespace internal {
+namespace clang_vector {
// Exploit the underlying integer representation to do a variable shift.
LIBC_INLINE constexpr cpp::simd_mask<char> shift_mask(cpp::simd_mask<char> m,
@@ -46,9 +46,8 @@ LIBC_NO_SANITIZE_OOB_ACCESS LIBC_INLINE size_t string_length(const char *src) {
cpp::find_first_set(mask);
}
}
-} // namespace internal
+} // namespace clang_vector
-namespace string_length_impl = internal;
} // namespace LIBC_NAMESPACE_DECL
#endif // LLVM_LIBC_SRC_STRING_MEMORY_UTILS_GENERIC_INLINE_STRLEN_H
diff --git a/libc/src/string/memory_utils/x86_64/inline_strlen.h b/libc/src/string/memory_utils/x86_64/inline_strlen.h
index 9e10d58363393..07b4a470f0d77 100644
--- a/libc/src/string/memory_utils/x86_64/inline_strlen.h
+++ b/libc/src/string/memory_utils/x86_64/inline_strlen.h
@@ -15,7 +15,8 @@
namespace LIBC_NAMESPACE_DECL {
-namespace string_length_internal {
+namespace internal::arch_vector {
+
// Return a bit-mask with the nth bit set if the nth-byte in block_ptr is zero.
template <typename Vector, typename Mask>
LIBC_NO_SANITIZE_OOB_ACCESS LIBC_INLINE static Mask
@@ -92,15 +93,18 @@ namespace avx512 {
}
} // namespace avx512
#endif
-} // namespace string_length_internal
+[[maybe_unused]] LIBC_INLINE size_t string_length(const char *src) {
#if defined(__AVX512F__)
-namespace string_length_impl = string_length_internal::avx512;
+ return avx512::string_length(src);
#elif defined(__AVX2__)
-namespace string_length_impl = string_length_internal::avx2;
+ return avx2::string_length(src);
#else
-namespace string_length_impl = string_length_internal::sse2;
+ return sse2::string_length(src);
#endif
+}
+
+} // namespace internal::arch_vector
} // namespace LIBC_NAMESPACE_DECL
diff --git a/libc/src/string/string_length.h b/libc/src/string/string_length.h
new file mode 100644
index 0000000000000..c828c85c16a17
--- /dev/null
+++ b/libc/src/string/string_length.h
@@ -0,0 +1,213 @@
+//===-- String Length -------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// Basic implementation and dispatch mechanism for performance-sensitive string-
+// related code.
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_STRING_STRING_LENGTH_H
+#define LLVM_LIBC_SRC_STRING_STRING_LENGTH_H
+
+#include "hdr/limits_macros.h"
+#include "hdr/stdint_proxy.h" // uintptr_t
+#include "hdr/types/size_t.h"
+#include "src/__support/CPP/type_traits.h" // cpp::is_same_v
+
+#if LIBC_HAS_VECTOR_TYPE
+#include "src/string/memory_utils/generic/inline_strlen.h"
+#endif
+#if defined(LIBC_TARGET_ARCH_IS_X86)
+#include "src/string/memory_utils/x86_64/inline_strlen.h"
+#elif defined(LIBC_TARGET_ARCH_IS_AARCH64)
+#include "src/string/memory_utils/aarch64/inline_strlen.h"
+#endif
+
+// Set sensible defaults
+#ifndef LIBC_COPT_STRING_LENGTH_IMPL
+#define LIBC_COPT_STRING_LENGTH_IMPL element
+#endif
+#ifndef LIBC_COPT_FIND_FIRST_CHARACTER_IMPL
+#define LIBC_COPT_FIND_FIRST_CHARACTER_IMPL element
+#endif
+
+namespace LIBC_NAMESPACE_DECL {
+namespace internal {
+
+#if !LIBC_HAS_VECTOR_TYPE
+// Forward any clang vector impls to architecture specific ones
+namespace arch_vector {}
+namespace clang_vector = arch_vector;
+#endif
+
+namespace element {
+// Element-by-element (usually a byte, but wider for wchar) implementations of
+// functions that search for data. Slow, but easy to understand and analyze.
+
+// Returns the length of a string, denoted by the first occurrence
+// of a null terminator.
+LIBC_INLINE size_t string_length(const char *src) {
+ size_t length;
+ for (length = 0; *src; ++src, ++length)
+ ;
+ return length;
+}
+
+template <typename T> LIBC_INLINE size_t string_length_element(const T *src) {
+ size_t length;
+ for (length = 0; *src; ++src, ++length)
+ ;
+ return length;
+}
+
+LIBC_INLINE void *find_first_character(const unsigned char *src,
+ unsigned char ch, size_t n) {
+ for (; n && *src != ch; --n, ++src)
+ ;
+ return n ? const_cast<unsigned char *>(src) : nullptr;
+}
+} // namespace element
+
+namespace word {
+// Non-vector, implementations of functions that search for data by reading from
+// memory word-by-word.
+
+template <typename Word> LIBC_INLINE constexpr Word repeat_byte(Word byte) {
+ static_assert(CHAR_BIT == 8, "repeat_byte assumes a byte is 8 bits.");
+ constexpr size_t BITS_IN_BYTE = CHAR_BIT;
+ constexpr size_t BYTE_MASK = 0xff;
+ Word result = 0;
+ byte = byte & BYTE_MASK;
+ for (size_t i = 0; i < sizeof(Word); ++i)
+ result = (result << BITS_IN_BYTE) | byte;
+ return result;
+}
+
+// The goal of this function is to take in a block of arbitrary size and return
+// if it has any bytes equal to zero without branching. This is done by
+// transforming the block such that zero bytes become non-zero and non-zero
+// bytes become zero.
+// The first transformation relies on the properties of carrying in arithmetic
+// subtraction. Specifically, if 0x01 is subtracted from a byte that is 0x00,
+// then the result for that byte must be equal to 0xff (or 0xfe if the next byte
+// needs a carry as well).
+// The next transformation is a simple mask. All zero bytes will have the high
+// bit set after the subtraction, so each byte is masked with 0x80. This narrows
+// the set of bytes that result in a non-zero value to only zero bytes and bytes
+// with the high bit and any other bit set.
+// The final transformation masks the result of the previous transformations
+// with the inverse of the original byte. This means that any byte that had the
+// high bit set will no longer have it set, narrowing the list of bytes which
+// result in non-zero values to just the zero byte.
+template <typename Word> LIBC_INLINE constexpr bool has_zeroes(Word block) {
+ constexpr unsigned int LOW_BITS = repeat_byte<Word>(0x01);
+ constexpr Word HIGH_BITS = repeat_byte<Word>(0x80);
+ Word subtracted = block - LOW_BITS;
+ Word inverted = ~block;
+ return (subtracted & inverted & HIGH_BITS) != 0;
+}
+
+// Unsigned int is the default size for most processors, and on x86-64 it
+// performs better than larger sizes when the src pointer can't be assumed to
+// be aligned to a word boundary, so it's the size we use for reading the
+// string a block at a time.
+
+LIBC_INLINE size_t string_length(const char *src) {
+ using Word = unsigned int;
+ const char *char_ptr = src;
+ // Step 1: read 1 byte at a time to align to block size
+ for (; reinterpret_cast<uintptr_t>(char_ptr) % sizeof(Word) != 0;
+ ++char_ptr) {
+ if (*char_ptr == '\0')
+ return static_cast<size_t>(char_ptr - src);
+ }
+ // Step 2: read blocks
+ for (const Word *block_ptr = reinterpret_cast<const Word *>(char_ptr);
+ !has_zeroes<Word>(*block_ptr); ++block_ptr) {
+ char_ptr = reinterpret_cast<const char *>(block_ptr);
+ }
+ // Step 3: find the zero in the block
+ for (; *char_ptr != '\0'; ++char_ptr) {
+ ;
+ }
+ return static_cast<size_t>(char_ptr - src);
+}
+
+LIBC_NO_SANITIZE_OOB_ACCESS LIBC_INLINE void *
+find_first_character(const unsigned char *src, unsigned char ch,
+ size_t max_strlen = cpp::numeric_limits<size_t>::max()) {
+ using Word = unsigned int;
+ const unsigned char *char_ptr = src;
+ size_t cur = 0;
+
+ // If the maximum size of the string is small, the overhead of aligning to a
+ // word boundary and generating a bitmask of the appropriate size may be
+ // greater than the gains from reading larger chunks. Based on some testing,
+ // the crossover point between when it's faster to just read bytewise and read
+ // blocks is somewhere between 16 and 32, so 4 times the size of the block
+ // should be in that range.
+ if (max_strlen < (sizeof(Word) * 4)) {
+ return element::find_first_character(src, ch, max_strlen);
+ }
+ size_t n = max_strlen;
+ // Step 1: read 1 byte at a time to align to block size
+ for (; cur < n && reinterpret_cast<uintptr_t>(char_ptr) % sizeof(Word) != 0;
+ ++cur, ++char_ptr) {
+ if (*char_ptr == ch)
+ return const_cast<unsigned char *>(char_ptr);
+ }
+
+ const Word ch_mask = repeat_byte<Word>(ch);
+
+ // Step 2: read blocks
+ const Word *block_ptr = reinterpret_cast<const Word *>(char_ptr);
+ for (; cur < n && !has_zeroes<Word>((*block_ptr) ^ ch_mask);
+ cur += sizeof(Word), ++block_ptr)
+ ;
+ char_ptr = reinterpret_cast<const unsigned char *>(block_ptr);
+
+ // Step 3: find the match in the block
+ for (; cur < n && *char_ptr != ch; ++cur, ++char_ptr) {
+ ;
+ }
+
+ if (cur >= n || *char_ptr != ch)
+ return static_cast<void *>(nullptr);
+
+ return const_cast<unsigned char *>(char_ptr);
+}
+
+} // namespace word
+
+// Dispatch mechanism for implementations of performance-sensitive
+// functions. Always measure, but generally from lower- to higher-performance
+// order:
+//
+// 1. element - read char-by-char or wchar-by-wchar
+// 3. word - read word-by-word
+// 3. clang_vector - read using clang's internal vector types
+// 4. arch_vector - hand-coded per architecture. Possibly in asm, or with
+// intrinsics.
+//
+// The called implemenation is chosen at build-time by setting
+// LIBC_CONF_{FUNC}_IMPL in config.json
+static constexpr auto &string_length_impl =
+ LIBC_COPT_STRING_LENGTH_IMPL::string_length;
+static constexpr auto &find_first_character_impl =
+ LIBC_COPT_FIND_FIRST_CHARACTER_IMPL::find_first_character;
+
+template <typename T> LIBC_INLINE size_t string_length(const T *src) {
+ if constexpr (cpp::is_same_v<T, char>)
+ return string_length_impl(src);
+ return element::string_length_element<T>(src);
+}
+
+} // namespace internal
+} // namespace LIBC_NAMESPACE_DECL
+
+#endif // LLVM_LIBC_SRC_STRING_STRING_LENGTH_H
diff --git a/libc/src/string/string_utils.h b/libc/src/string/string_utils.h
index cbce62ead0328..b0144e01a9006 100644
--- a/libc/src/string/string_utils.h
+++ b/libc/src/string/string_utils.h
@@ -14,172 +14,17 @@
#ifndef LLVM_LIBC_SRC_STRING_STRING_UTILS_H
#define LLVM_LIBC_SRC_STRING_STRING_UTILS_H
-#include "hdr/limits_macros.h"
-#include "hdr/stdint_proxy.h" // uintptr_t
#include "hdr/types/size_t.h"
#include "src/__support/CPP/bitset.h"
-#include "src/__support/CPP/type_traits.h" // cpp::is_same_v
#include "src/__support/macros/attributes.h"
#include "src/__support/macros/config.h"
#include "src/__support/macros/optimization.h" // LIBC_UNLIKELY
#include "src/string/memory_utils/inline_memcpy.h"
-
-#if defined(LIBC_COPT_STRING_UNSAFE_WIDE_READ)
-#if LIBC_HAS_VECTOR_TYPE
-#include "src/string/memory_utils/generic/inline_strlen.h"
-#elif defined(LIBC_TARGET_ARCH_IS_X86)
-#include "src/string/memory_utils/x86_64/inline_strlen.h"
-#elif defined(LIBC_TARGET_ARCH_IS_AARCH64) && defined(__ARM_NEON)
-#include "src/string/memory_utils/aarch64/inline_strlen.h"
-#else
-namespace string_length_impl = LIBC_NAMESPACE::wide_read;
-#endif
-#endif // defined(LIBC_COPT_STRING_UNSAFE_WIDE_READ)
+#include "src/string/string_length.h"
namespace LIBC_NAMESPACE_DECL {
namespace internal {
-template <typename Word> LIBC_INLINE constexpr Word repeat_byte(Word byte) {
- static_assert(CHAR_BIT == 8, "repeat_byte assumes a byte is 8 bits.");
- constexpr size_t BITS_IN_BYTE = CHAR_BIT;
- constexpr size_t BYTE_MASK = 0xff;
- Word result = 0;
- byte = byte & BYTE_MASK;
- for (size_t i = 0; i < sizeof(Word); ++i)
- result = (result << BITS_IN_BYTE) | byte;
- return result;
-}
-
-// The goal of this function is to take in a block of arbitrary size and return
-// if it has any bytes equal to zero without branching. This is done by
-// transforming the block such that zero bytes become non-zero and non-zero
-// bytes become zero.
-// The first transformation relies on the properties of carrying in arithmetic
-// subtraction. Specifically, if 0x01 is subtracted from a byte that is 0x00,
-// then the result for that byte must be equal to 0xff (or 0xfe if the next byte
-// needs a carry as well).
-// The next transformation is a simple mask. All zero bytes will have the high
-// bit set after the subtraction, so each byte is masked with 0x80. This narrows
-// the set of bytes that result in a non-zero value to only zero bytes and bytes
-// with the high bit and any other bit set.
-// The final transformation masks the result of the previous transformations
-// with the inverse of the original byte. This means that any byte that had the
-// high bit set will no longer have it set, narrowing the list of bytes which
-// result in non-zero values to just the zero byte.
-template <typename Word> LIBC_INLINE constexpr bool has_zeroes(Word block) {
- constexpr unsigned int LOW_BITS = repeat_byte<Word>(0x01);
- constexpr Word HIGH_BITS = repeat_byte<Word>(0x80);
- Word subtracted = block - LOW_BITS;
- Word inverted = ~block;
- return (subtracted & inverted & HIGH_BITS) != 0;
-}
-
-template <typename Word>
-LIBC_INLINE size_t string_length_wide_read(const char *src) {
- const char *char_ptr = src;
- // Step 1: read 1 byte at a time to align to block size
- for (; reinterpret_cast<uintptr_t>(char_ptr) % sizeof(Word) != 0;
- ++char_ptr) {
- if (*char_ptr == '\0')
- return static_cast<size_t>(char_ptr - src);
- }
- // Step 2: read blocks
- for (const Word *block_ptr = reinterpret_cast<const Word *>(char_ptr);
- !has_zeroes<Word>(*block_ptr); ++block_ptr) {
- char_ptr = reinterpret_cast<const char *>(block_ptr);
- }
- // Step 3: find the zero in the block
- for (; *char_ptr != '\0'; ++char_ptr) {
- ;
- }
- return static_cast<size_t>(char_ptr - src);
-}
-
-namespace wide_read {
-LIBC_INLINE size_t string_length(const char *src) {
- // Unsigned int is the default size for most processors, and on x86-64 it
- // performs better than larger sizes when the src pointer can't be assumed to
- // be aligned to a word boundary, so it's the size we use for reading the
- // string a block at a time.
- return string_length_wide_read<unsigned int>(src);
-}
-
-} // namespace wide_read
-
-// Returns the length of a string, denoted by the first occurrence
-// of a null terminator.
-template <typename T> LIBC_INLINE size_t string_length(const T *src) {
-#ifdef LIBC_COPT_STRING_UNSAFE_WIDE_READ
- if constexpr (cpp::is_same_v<T, char>)
- return string_length_impl::string_length(src);
-#endif
- size_t length;
- for (length = 0; *src; ++src, ++length)
- ;
- return length;
-}
-
-template <typename Word>
-LIBC_NO_SANITIZE_OOB_ACCESS LIBC_INLINE void *
-find_first_character_wide_read(const unsigned char *src, unsigned char ch,
- size_t n) {
- const unsigned char *char_ptr = src;
- size_t cur = 0;
-
- // Step 1: read 1 byte at a time to align to block size
- for (; cur < n && reinterpret_cast<uintptr_t>(char_ptr) % sizeof(Word) != 0;
- ++cur, ++char_ptr) {
- if (*char_ptr == ch)
- return const_cast<unsigned char *>(char_ptr);
- }
-
- const Word ch_mask = repeat_byte<Word>(ch);
-
- // Step 2: read blocks
- const Word *block_ptr = reinterpret_cast<const Word *>(char_ptr);
- for (; cur < n && !has_zeroes<Word>((*block_ptr) ^ ch_mask);
- cur += sizeof(Word), ++block_ptr)
- ;
- char_ptr = reinterpret_cast<const unsigned char *>(block_ptr);
-
- // Step 3: find the match in the block
- for (; cur < n && *char_ptr != ch; ++cur, ++char_ptr) {
- ;
- }
-
- if (cur >= n || *char_ptr != ch)
- return static_cast<void *>(nullptr);
-
- return const_cast<unsigned char *>(char_ptr);
-}
-
-LIBC_INLINE void *find_first_character_byte_read(const unsigned char *src,
- unsigned char ch, size_t n) {
- for (; n && *src != ch; --n, ++src)
- ;
- return n ? const_cast<unsigned char *>(src) : nullptr;
-}
-
-// Returns the first occurrence of 'ch' within the first 'n' characters of
-// 'src'. If 'ch' is not found, returns nullptr.
-LIBC_INLINE void *find_first_character(const unsigned char *src,
- unsigned char ch, size_t max_strlen) {
-#ifdef LIBC_COPT_STRING_UNSAFE_WIDE_READ
- // If the maximum size of the string is small, the overhead of aligning to a
- // word boundary and generating a bitmask of the appropriate size may be
- // greater than the gains from reading larger chunks. Based on some testing,
- // the crossover point between when it's faster to just read bytewise and read
- // blocks is somewhere between 16 and 32, so 4 times the size of the block
- // should be in that range.
- // Unsigned int is used for the same reason as in strlen.
- using BlockType = unsigned int;
- if (max_strlen > (sizeof(BlockType) * 4)) {
- return find_first_character_wide_read<BlockType>(src, ch, max_strlen);
- }
-#endif
- return find_first_character_byte_read(src, ch, max_strlen);
-}
-
// Returns the maximum length span that contains only characters not found in
// 'segment'. If no characters are found, returns the length of 'src'.
LIBC_INLINE size_t complementary_span(const char *src, const char *segment) {
@@ -272,6 +117,13 @@ LIBC_INLINE constexpr static char *strrchr_implementation(const char *src,
}
}
+// Returns the first occurrence of 'ch' within the first 'n' characters of
+// 'src'. If 'ch' is not found, returns nullptr.
+LIBC_INLINE void *find_first_character(const unsigned char *src,
+ unsigned char ch, size_t max_strlen) {
+ return find_first_character_impl(src, ch, max_strlen);
+}
+
} // namespace internal
} // namespace LIBC_NAMESPACE_DECL
diff --git a/utils/bazel/llvm-project-overlay/libc/BUILD.bazel b/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
index a3a0747a71003..3f19a439d41f4 100644
--- a/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
+++ b/utils/bazel/llvm-project-overlay/libc/BUILD.bazel
@@ -5452,7 +5452,10 @@ libc_support_library(
libc_support_library(
name = "string_utils",
- hdrs = ["src/string/string_utils.h"],
+ hdrs = [
+ "src/string/string_length.h",
+ "src/string/string_utils.h",
+ ],
deps = [
":__support_common",
":__support_cpp_bitset",
diff --git a/utils/bazel/llvm-project-overlay/libc/libc_configure_options.bzl b/utils/bazel/llvm-project-overlay/libc/libc_configure_options.bzl
index 259d4d292fcf4..1c2b65ea20113 100644
--- a/utils/bazel/llvm-project-overlay/libc/libc_configure_options.bzl
+++ b/utils/bazel/llvm-project-overlay/libc/libc_configure_options.bzl
@@ -39,7 +39,8 @@ LIBC_CONFIGURE_OPTIONS = [
# "LIBC_COPT_SCANF_DISABLE_FLOAT",
# "LIBC_COPT_SCANF_DISABLE_INDEX_MODE",
"LIBC_COPT_STDIO_USE_SYSTEM_FILE",
- "LIBC_COPT_STRING_UNSAFE_WIDE_READ",
+ "LIBC_COPT_STRING_LENGTH_IMPL=clang_vector",
+ "LIBC_COPT_FIND_FIRST_CHARACTER_IMPL=word",
# "LIBC_COPT_STRTOFLOAT_DISABLE_CLINGER_FAST_PATH",
# "LIBC_COPT_STRTOFLOAT_DISABLE_EISEL_LEMIRE",
# "LIBC_COPT_STRTOFLOAT_DISABLE_SIMPLE_DECIMAL_CONVERSION",
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