[libc] [llvm] Add vector-based strlen implementation for x86_64 and aarch64 (PR #152389)

[via llvm-commits]

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@@ -0,0 +1,113 @@
+//===-- Strlen implementation for x86_64 ----------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+#ifndef LLVM_LIBC_SRC_STRING_MEMORY_UTILS_X86_64_INLINE_STRLEN_H
+#define LLVM_LIBC_SRC_STRING_MEMORY_UTILS_X86_64_INLINE_STRLEN_H
+
+#include "src/string/memory_utils/op_x86.h" // K_AVX
+
+#include <stddef.h> // size_t
+#include <x86intrin.h>
+namespace LIBC_NAMESPACE_DECL {
+
+#if defined(__SSE2__)
+[[maybe_unused]] LIBC_INLINE size_t string_length_sse2(const char *src) {
+  using Vector __attribute__((may_alias)) = __m128i;
+  Vector z = _mm_setzero_si128();
+  uintptr_t misalign_bytes = reinterpret_cast<uintptr_t>(src) % sizeof(Vector);
+  const Vector *block_ptr =
+      reinterpret_cast<const Vector *>(src - misalign_bytes);
+  if (misalign_bytes) {
+    Vector v = _mm_load_si128(block_ptr);
+    Vector vcmp = _mm_cmpeq_epi8(z, v);
+    // shift away results in irrelevant bytes.
+    int cmp = _mm_movemask_epi8(vcmp) >> misalign_bytes;
+    if (cmp)
+      return __builtin_ctz(cmp);
+    block_ptr++;
+  }
+  while (true) {
+    Vector v = _mm_load_si128(block_ptr);
+    Vector vcmp = _mm_cmpeq_epi8(z, v);
+    int cmp = _mm_movemask_epi8(vcmp);
+    if (cmp)
+      return static_cast<size_t>(reinterpret_cast<uintptr_t>(block_ptr) -
+                                 reinterpret_cast<uintptr_t>(src) +
+                                 __builtin_ctz(cmp));
+    block_ptr++;
+  }
+}
+#endif
+
+#if defined(__AVX2__)
+[[maybe_unused]] LIBC_INLINE size_t string_length_avx2(const char *src) {
+  using Vector __attribute__((may_alias)) = __mm256i;
+  Vector z = _mm256_setzero_si256();
+  uintptr_t misalign_bytes = reinterpret_cast<uintptr_t>(src) % sizeof(Vector);
+  const Vector *block_ptr =
+      reinterpret_cast<const Vector *>(src - misalign_bytes);
+  if (misalign_bytes) {
+    Vector v = _mm256_load_si256(block_ptr);
+    Vector vcmp = _mm256_cmpeq_epi8(z, v);
+    // shift away results in irrelevant bytes.
+    int cmp = _mm256_movemask_epi8(vcmp) >> misalign_bytes;
+    if (cmp)
+      return __builtin_ctz(cmp);
+    block_ptr++;
+  }
+  while (true) {
+    Vector v = _mm256_load_si256(block_ptr);
+    Vector vcmp = _mm256_cmpeq_epi8(z, v);
+    int cmp = _mm256_movemask_epi8(vcmp);
+    if (cmp)
+      return static_cast<size_t>(reinterpret_cast<uintptr_t>(block_ptr) -
+                                 reinterpret_cast<uintptr_t>(src) +
+                                 __builtin_ctz(cmp));
+    block_ptr++;
+  }
+}
+#endif // __AVX__
+
+#if defined(__AVX512F__)
+[[maybe_unused]] LIBC_INLINE size_t string_length_avx512(const char *src) {
+  using Vector __attribute__((may_alias)) = __mm512i;
+  Vector z = _mm512_setzero_si512();
+  uintptr_t misalign_bytes = reinterpret_cast<uintptr_t>(src) % sizeof(Vector);
+  const Vector *block_ptr =
+      reinterpret_cast<const Vector *>(src - misalign_bytes);
+  if (misalign_bytes) {
+    Vector v = _mm512_load_si512(block_ptr);
+    __mmask64 cmp = _mm512_cmp_epu8_mask(z, v, _MM_CMPINT_EQ) >> misalign_bytes;
+    if (cmp)
+      return __builtin_ctzl(cmp);
+    block_ptr++;
+  }
+  while (true) {
+    Vector v = _mm512_load_si512(block_ptr);
+    __mmask64 cmp = _mm512_cmp_epu8_mask(z, v, _MM_CMPINT_EQ);
+    if (cmp)
+      return static_cast<size_t>(reinterpret_cast<uintptr_t>(block_ptr) -
+                                 reinterpret_cast<uintptr_t>(src) +
+                                 __builtin_ctz(cmp));
+    block_ptr++;
+  }
+}
+#endif // __AVX512F__
+
+template <typename T> LIBC_INLINE size_t string_length_x86_64(const char *src) {
+#if defined(__AVX512F__)
+  return string_length_avx512(src);
+#endif
+#if defined(__AVX__)
----------------
overmighty wrote:

```suggestion
#elif defined(__AVX2__)
```

https://github.com/llvm/llvm-project/pull/152389