[libc] Adds strrchr implementation.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D84875

libc/config/linux/aarch64/entrypoints.txt

@@ -20,6 +20,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.string.strcpy
     libc.src.string.strlen
     libc.src.string.strnlen
+    libc.src.string.strrchr
     libc.src.string.strstr
 )
 

libc/config/linux/x86_64/entrypoints.txt

@@ -38,6 +38,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.string.strcpy
     libc.src.string.strlen
     libc.src.string.strnlen
+    libc.src.string.strrchr
     libc.src.string.strstr
 
     # sys/mman.h entrypoints

libc/src/string/CMakeLists.txt

@@ -86,6 +86,14 @@ add_entrypoint_object(
     memrchr.h
 )
 
+add_entrypoint_object(
+  strrchr
+  SRCS
+    strrchr.cpp
+  HDRS
+    strrchr.h
+)
+
 # Helper to define a function with multiple implementations
 # - Computes flags to satisfy required/rejected features and arch,
 # - Declares an entry point,

libc/src/string/strrchr.cpp

@@ -0,0 +1,28 @@
+//===-- Implementation of strrchr------------------------------------------===//
+//
+// 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/strrchr.h"
+
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+char *LLVM_LIBC_ENTRYPOINT(strrchr)(const char *src, int c) {
+  unsigned char *str =
+      const_cast<unsigned char *>(reinterpret_cast<const unsigned char *>(src));
+  const unsigned char ch = c;
+
+  unsigned char *last_occurrence = nullptr;
+  do {
+    if (*str == ch)
+      last_occurrence = str;
+  } while (*str++);
+  return reinterpret_cast<char *>(last_occurrence);
+}
+
+} // namespace __llvm_libc

libc/src/string/strrchr.h

@@ -0,0 +1,18 @@
+//===-- Implementation header for strrchr -----------------------*- 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_STRRCHR_H
+#define LLVM_LIBC_SRC_STRING_STRRCHR_H
+
+namespace __llvm_libc {
+
+char *strrchr(const char *src, int c);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STRING_STRRCHR_H

libc/test/src/string/CMakeLists.txt

@@ -92,6 +92,16 @@ add_libc_unittest(
     libc.src.string.memrchr
 )
 
+add_libc_unittest(
+  strrchr_test
+  SUITE
+    libc_string_unittests
+  SRCS
+    strrchr_test.cpp
+  DEPENDS
+    libc.src.string.strrchr
+)
+
 # Tests all implementations that can run on the host.
 function(add_libc_multi_impl_test name)
   get_property(fq_implementations GLOBAL PROPERTY ${name}_implementations)

libc/test/src/string/strrchr_test.cpp

@@ -0,0 +1,81 @@
+//===-- Unittests for strrchr ---------------------------------------------===//
+//
+// 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/strrchr.h"
+#include "utils/UnitTest/Test.h"
+
+TEST(StrRChrTest, FindsFirstCharacter) {
+  const char *src = "abcde";
+  const char *src_copy = src;
+
+  // Should return original string since 'a' is the first character.
+  ASSERT_STREQ(__llvm_libc::strrchr(src, 'a'), "abcde");
+  // Source string should not change.
+  ASSERT_STREQ(src, src_copy);
+}
+
+TEST(StrRChrTest, FindsMiddleCharacter) {
+  const char *src = "abcde";
+  const char *src_copy = src;
+
+  // Should return characters after (and including) 'c'.
+  ASSERT_STREQ(__llvm_libc::strrchr(src, 'c'), "cde");
+  // Source string should not change.
+  ASSERT_STREQ(src, src_copy);
+}
+
+TEST(StrRChrTest, FindsLastCharacterThatIsNotNullTerminator) {
+  const char *src = "abcde";
+  const char *src_copy = src;
+
+  // Should return 'e' and null-terminator.
+  ASSERT_STREQ(__llvm_libc::strrchr(src, 'e'), "e");
+  // Source string should not change.
+  ASSERT_STREQ(src, src_copy);
+}
+
+TEST(StrRChrTest, FindsNullTerminator) {
+  const char *src = "abcde";
+  const char *src_copy = src;
+
+  // Should return null terminator.
+  ASSERT_STREQ(__llvm_libc::strrchr(src, '\0'), "");
+  // Source string should not change.
+  ASSERT_STREQ(src, src_copy);
+}
+
+TEST(StrRChrTest, FindsLastNullTerminator) {
+  const char src[5] = {'a', '\0', 'b', '\0', 'c'};
+  // 'b' is behind a null terminator, so should not be found.
+  ASSERT_STREQ(__llvm_libc::strrchr(src, 'b'), nullptr);
+  // Same goes for 'c'.
+  ASSERT_STREQ(__llvm_libc::strrchr(src, 'c'), nullptr);
+}
+
+TEST(StrRChrTest, CharacterNotWithinStringShouldReturnNullptr) {
+  // Since 'z' is not within the string, should return nullptr.
+  ASSERT_STREQ(__llvm_libc::strrchr("123?", 'z'), nullptr);
+}
+
+TEST(StrRChrTest, ShouldFindLastOfDuplicates) {
+  // '1' is duplicated in the string, but it should find the last copy.
+  ASSERT_STREQ(__llvm_libc::strrchr("abc1def1ghi", '1'), "1ghi");
+
+  const char *dups = "XXXXX";
+  // Should return the last occurrence of 'X'.
+  ASSERT_STREQ(__llvm_libc::strrchr(dups, 'X'), "X");
+}
+
+TEST(StrRChrTest, EmptyStringShouldOnlyMatchNullTerminator) {
+  // Null terminator should match.
+  ASSERT_STREQ(__llvm_libc::strrchr("", '\0'), "");
+  // All other characters should not match.
+  ASSERT_STREQ(__llvm_libc::strrchr("", 'A'), nullptr);
+  ASSERT_STREQ(__llvm_libc::strrchr("", '2'), nullptr);
+  ASSERT_STREQ(__llvm_libc::strrchr("", '*'), nullptr);
+}