[libc] Add strxfrm Implementation

[Caslyn]

Summary

Introduce strxfrm and unit tests. The current implementation is
introduced without locale support.

The simplified function performs a memcpy if the n value is large
enough to store the source len + '\0', otherwise dest is unmodified.

Ticket: https://fxbug.dev/124217

[Caslyn]

Amended commit message

[Caslyn]

Fix typo, unneeded import

[michaelrj]

LGTM after fixing one issue.

libc/src/string/strxfrm.cpp

19

The pointer arguments for strxfrm need to be tagged with __restrict both here and in the header.

[Caslyn]

Add __restrict to pointer args

libc/config/linux/riscv64/entrypoints.txt

set(TARGET_LIBC_ENTRYPOINTS
     libc.src.string.strpbrk
     libc.src.string.strrchr
     libc.src.string.strsep
     libc.src.string.strsignal
     libc.src.string.strspn
     libc.src.string.strstr
     libc.src.string.strtok
     libc.src.string.strtok_r
+    libc.src.string.strxfrm
 
     # inttypes.h entrypoints
     libc.src.inttypes.imaxabs
     libc.src.inttypes.imaxdiv
     libc.src.inttypes.strtoimax
     libc.src.inttypes.strtoumax
 
     # stdlib.h entrypoints

libc/config/linux/x86_64/entrypoints.txt

set(TARGET_LIBC_ENTRYPOINTS
     libc.src.string.strpbrk
     libc.src.string.strrchr
     libc.src.string.strsep
     libc.src.string.strsignal
     libc.src.string.strspn
     libc.src.string.strstr
     libc.src.string.strtok
     libc.src.string.strtok_r
+    libc.src.string.strxfrm
 
     # inttypes.h entrypoints
     libc.src.inttypes.imaxabs
     libc.src.inttypes.imaxdiv
     libc.src.inttypes.strtoimax
     libc.src.inttypes.strtoumax
 
     # stdlib.h entrypoints

libc/src/string/CMakeLists.txt

add_entrypoint_object(
   SRCS
     strtok_r.cpp
   HDRS
     strtok_r.h
   DEPENDS
     .string_utils
 )
 
+add_entrypoint_object(
+  strxfrm
+  SRCS
+    strxfrm.cpp
+  HDRS
+    strxfrm.h
+  DEPENDS
+    .string_utils
+    .memory_utils.memcpy_implementation
+)
+
 # Helper to define a function with multiple implementations
 # - Computes flags to satisfy required/rejected features and arch,
 # - Declares an entry point,
 # - Attach the REQUIRE_CPU_FEATURES property to the target,
 # - Add the fully qualified target to `${name}_implementations` global property for tests.
 function(add_implementation name impl_name)
   cmake_parse_arguments(
     "ADD_IMPL"

libc/src/string/strxfrm.h

+//===-- Implementation header for strxfrm -----------------------*- 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_STRXFRM_H
+#define LLVM_LIBC_SRC_STRING_STRXFRM_H
+
+#include <stddef.h> // For size_t
+namespace __llvm_libc {
+
+size_t strxfrm(char *__restrict dest, const char *__restrict src, size_t n);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STRING_STRXFRM_H

libc/src/string/strxfrm.cpp

+//===-- Implementation of strxfrm -----------------------------------------===//
+//
+// 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/strxfrm.h"
+#include "src/string/memory_utils/memcpy_implementations.h"
+#include "src/string/string_utils.h"
+
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+// TODO: Add support for locales.
+LLVM_LIBC_FUNCTION(size_t, strxfrm,
+                   (char *__restrict dest, const char *__restrict src,
+                    size_t n)) {
+  size_t len = internal::string_length(src);
+  if (n > len)
+    inline_memcpy(dest, src, len + 1);
+  return len;
+}
+
+} // namespace __llvm_libc

libc/test/src/string/CMakeLists.txt

add_libc_unittest(
   SUITE
     libc_string_unittests
   SRCS
     strtok_r_test.cpp
   DEPENDS
     libc.src.string.strtok_r
 )
 
+add_libc_unittest(
+  strxfrm_test
+  SUITE
+    libc_string_unittests
+  SRCS
+    strxfrm_test.cpp
+  DEPENDS
+    libc.src.string.strxfrm
+)
+
 # 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)
   foreach(fq_config_name IN LISTS fq_implementations)
     get_target_property(required_cpu_features ${fq_config_name} REQUIRE_CPU_FEATURES)
     cpu_supports(can_run "${required_cpu_features}")
     if(can_run)
       add_libc_unittest(

libc/test/src/string/strxfrm_test.cpp

+//===-- Unittests for strxfrm ---------------------------------------------===//
+//
+// 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/strxfrm.h"
+#include "test/UnitTest/Test.h"
+
+#include "src/string/string_utils.h"
+
+// TODO: Add more comprehensive tests once locale support is added.
+
+TEST(LlvmLibcStrxfrmTest, SimpleTestSufficientlySizedN) {
+  const char *src = "abc";
+  const size_t n = 5;
+
+  char dest[n];
+  size_t result = __llvm_libc::strxfrm(dest, src, n);
+  ASSERT_EQ(result, __llvm_libc::internal::string_length(src));
+  ASSERT_STREQ(dest, src);
+}
+
+TEST(LlvmLibcStrxfrmTest, SimpleTestExactSizedN) {
+  const char *src = "abc";
+  const size_t n = 4;
+
+  char dest[n];
+  size_t result = __llvm_libc::strxfrm(dest, src, n);
+  ASSERT_EQ(result, __llvm_libc::internal::string_length(src));
+  ASSERT_STREQ(dest, src);
+}
+
+TEST(LlvmLibcStrxfrmTest, SimpleTestInsufficientlySizedN) {
+  const char *src = "abc";
+  const size_t n = 3;
+
+  // Verify strxfrm does not modify dest if src len >= n
+  char dest[n] = {'x', 'x', '\0'};
+  size_t result = __llvm_libc::strxfrm(dest, src, n);
+  ASSERT_GE(result, n);
+  ASSERT_STREQ(dest, "xx");
+}