[libc][c23] add memset_explicit (#83577)

Author: SchrodingerZhu

libc/config/linux/aarch64/entrypoints.txt

@@ -51,6 +51,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.string.mempcpy
     libc.src.string.memrchr
     libc.src.string.memset
+    libc.src.string.memset_explicit
     libc.src.string.rindex
     libc.src.string.stpcpy
     libc.src.string.stpncpy

libc/config/linux/x86_64/entrypoints.txt

@@ -51,6 +51,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.string.mempcpy
     libc.src.string.memrchr
     libc.src.string.memset
+    libc.src.string.memset_explicit
     libc.src.string.rindex
     libc.src.string.stpcpy
     libc.src.string.stpncpy

libc/spec/stdc.td

@@ -234,6 +234,11 @@ def StdC : StandardSpec<"stdc"> {
               RetValSpec<VoidPtr>,
               [ArgSpec<VoidPtr>, ArgSpec<IntType>, ArgSpec<SizeTType>]
           >,
+          FunctionSpec<
+              "memset_explicit",
+              RetValSpec<VoidPtr>,
+              [ArgSpec<VoidPtr>, ArgSpec<IntType>, ArgSpec<SizeTType>]
+          >,
           FunctionSpec<
               "strcpy",
               RetValSpec<CharPtr>,

libc/src/string/CMakeLists.txt

@@ -441,6 +441,17 @@ add_entrypoint_object(
     .memory_utils.inline_memcpy
 )
 
+add_entrypoint_object(
+  memset_explicit
+  SRCS
+    memset_explicit.cpp
+  HDRS
+    memset_explicit.h
+  DEPENDS
+    .string_utils
+    .memory_utils.inline_memset
+)
+
 # Helper to define a function with multiple implementations
 # - Computes flags to satisfy required/rejected features and arch,
 # - Declares an entry point,

libc/src/string/memset_explicit.cpp

@@ -0,0 +1,25 @@
+//===-- Implementation of memset_explicit ---------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "src/string/memset_explicit.h"
+#include "src/__support/common.h"
+#include "src/string/memory_utils/inline_memset.h"
+
+namespace LIBC_NAMESPACE {
+
+[[gnu::noinline]] LLVM_LIBC_FUNCTION(void *, memset_explicit,
+                                     (void *dst, int value, size_t count)) {
+  // Use the inline memset function to set the memory.
+  inline_memset(dst, static_cast<uint8_t>(value), count);
+  // avoid dead store elimination
+  // The asm itself should also be sufficient to behave as a compiler barrier.
+  asm("" : : "r"(dst) : "memory");
+  return dst;
+}
+
+} // namespace LIBC_NAMESPACE

libc/src/string/memset_explicit.h

@@ -0,0 +1,20 @@
+//===-- Implementation header for memset_explicit ---------------*- 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
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef LLVM_LIBC_SRC_STRING_MEMSET_EXPLICIT_H
+#define LLVM_LIBC_SRC_STRING_MEMSET_EXPLICIT_H
+
+#include <stddef.h> // size_t
+
+namespace LIBC_NAMESPACE {
+
+[[gnu::noinline]] void *memset_explicit(void *ptr, int value, size_t count);
+
+} // namespace LIBC_NAMESPACE
+
+#endif // LLVM_LIBC_SRC_STRING_MEMSET_EXPLICIT_H

libc/test/src/string/CMakeLists.txt

@@ -418,6 +418,16 @@ add_libc_test(
     libc.src.string.strxfrm
 )
 
+add_libc_test(
+  memset_explicit_test
+  SUITE
+    libc-string-tests
+  SRCS
+    memset_explicit_test.cpp
+  DEPENDS
+    libc.src.string.memset_explicit
+)
+
 # Tests all implementations that can run on the target CPU.
 function(add_libc_multi_impl_test name)
   get_property(fq_implementations GLOBAL PROPERTY ${name}_implementations)

libc/test/src/string/memset_explicit_test.cpp

@@ -0,0 +1,31 @@
+//===-- Unittests for memset_explicit -------------------------------------===//
+//
+// 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
+//
+//===----------------------------------------------------------------------===//
+
+#include "memory_utils/memory_check_utils.h"
+#include "src/string/memset_explicit.h"
+#include "test/UnitTest/Test.h"
+
+namespace LIBC_NAMESPACE {
+
+// Apply the same tests as memset
+
+static inline void Adaptor(cpp::span<char> p1, uint8_t value, size_t size) {
+  LIBC_NAMESPACE::memset_explicit(p1.begin(), value, size);
+}
+
+TEST(LlvmLibcmemsetExplicitTest, SizeSweep) {
+  static constexpr size_t kMaxSize = 400;
+  Buffer DstBuffer(kMaxSize);
+  for (size_t size = 0; size < kMaxSize; ++size) {
+    const char value = size % 10;
+    auto dst = DstBuffer.span().subspan(0, size);
+    ASSERT_TRUE((CheckMemset<Adaptor>(dst, value, size)));
+  }
+}
+
+} // namespace LIBC_NAMESPACE