[libc] Add strchr implementation. Fixes bug in memchr.

Summary: [libc] Adds strchr implementation with unit tests. Fixes signed character bug in memchr.

Reviewers: sivachandra, PaulkaToast

Reviewed By: sivachandra

Subscribers: mgorny, tschuett, ecnelises, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D83353

libc/config/linux/aarch64/entrypoints.txt

@@ -10,6 +10,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.string.strcat
     libc.src.string.strlen
     libc.src.string.memchr
+    libc.src.string.strchr
 )
 
 set(TARGET_LIBM_ENTRYPOINTS

libc/config/linux/x86_64/entrypoints.txt

@@ -28,6 +28,7 @@ set(TARGET_LIBC_ENTRYPOINTS
     libc.src.string.strlen
     libc.src.string.strcmp
     libc.src.string.memchr
+    libc.src.string.strchr
 
     # sys/mman.h entrypoints
     libc.src.sys.mman.mmap

libc/src/string/CMakeLists.txt

@@ -52,6 +52,15 @@ add_entrypoint_object(
     memchr.h
 )
 
+add_entrypoint_object(
+  strchr
+  SRCS
+    strchr.cpp
+  HDRS
+    strchr.h
+  DEPENDS
+    .strlen
+)
 
 # Helper to define a function with multiple implementations
 # - Computes flags to satisfy required/rejected features and arch,

libc/src/string/memchr.cpp

@@ -15,7 +15,8 @@ namespace __llvm_libc {
 // TODO: Look at performance benefits of comparing words.
 void *LLVM_LIBC_ENTRYPOINT(memchr)(const void *src, int c, size_t n) {
   const unsigned char *str = reinterpret_cast<const unsigned char *>(src);
-  for (; n && *str != c; --n, ++str)
+  const unsigned char ch = c;
+  for (; n && *str != ch; --n, ++str)
     ;
   return n ? const_cast<unsigned char *>(str) : nullptr;
 }

libc/src/string/strchr.cpp

@@ -0,0 +1,26 @@
+//===-- Implementation of strchr ------------------------------------------===//
+//
+// 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/strchr.h"
+#include "src/string/strlen.h"
+
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+// TODO: Look at performance benefits of comparing words.
+char *LLVM_LIBC_ENTRYPOINT(strchr)(const char *src, int c) {
+  unsigned char *str =
+      const_cast<unsigned char *>(reinterpret_cast<const unsigned char *>(src));
+  const unsigned char ch = c;
+  for (; *str && *str != ch; ++str)
+    ;
+  return *str == ch ? reinterpret_cast<char *>(str) : nullptr;
+}
+
+} // namespace __llvm_libc

libc/src/string/strchr.h

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

libc/test/src/string/CMakeLists.txt

@@ -52,6 +52,16 @@ add_libc_unittest(
     libc.src.string.memchr
 )
 
+add_libc_unittest(
+  strchr_test
+  SUITE
+    libc_string_unittests
+  SRCS
+    strchr_test.cpp
+  DEPENDS
+    libc.src.string.strchr
+)
+
 # 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/memchr_test.cpp

@@ -111,3 +111,12 @@ TEST(MemChrTest, SingleRepeatedCharacterShouldReturnFirst) {
   // Should return original string since X is first character.
   ASSERT_STREQ(call_memchr(dups, 'X', size), dups);
 }
+
+TEST(MemChrTest, SignedCharacterFound) {
+  char c = -1;
+  const size_t size = 1;
+  char src[size] = {c};
+  const char *actual = call_memchr(src, c, size);
+  // Should find the first character 'c'.
+  ASSERT_EQ(actual[0], c);
+}

libc/test/src/string/strchr_test.cpp

@@ -0,0 +1,87 @@
+//===-- Unittests for strchr ----------------------------------------------===//
+//
+// 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/strchr.h"
+#include "utils/UnitTest/Test.h"
+
+TEST(StrChrTest, FindsFirstCharacter) {
+  const char *src = "abcde";
+  const char *src_copy = src;
+
+  // Should return original string since 'a' is the first character.
+  ASSERT_STREQ(__llvm_libc::strchr(src, 'a'), "abcde");
+  // Source string should not change.
+  ASSERT_STREQ(src, src_copy);
+}
+
+TEST(StrChrTest, FindsMiddleCharacter) {
+  const char *src = "abcde";
+  const char *src_copy = src;
+
+  // Should return characters after (and including) 'c'.
+  ASSERT_STREQ(__llvm_libc::strchr(src, 'c'), "cde");
+  // Source string should not change.
+  ASSERT_STREQ(src, src_copy);
+}
+
+TEST(StrChrTest, FindsLastCharacterThatIsNotNullTerminator) {
+  const char *src = "abcde";
+  const char *src_copy = src;
+
+  // Should return 'e' and null-terminator.
+  ASSERT_STREQ(__llvm_libc::strchr(src, 'e'), "e");
+  // Source string should not change.
+  ASSERT_STREQ(src, src_copy);
+}
+
+TEST(StrChrTest, FindsNullTerminator) {
+  const char *src = "abcde";
+  const char *src_copy = src;
+
+  // Should return null terminator.
+  ASSERT_STREQ(__llvm_libc::strchr(src, '\0'), "");
+  // Source string should not change.
+  ASSERT_STREQ(src, src_copy);
+}
+
+TEST(StrChrTest, CharacterNotWithinStringShouldReturnNullptr) {
+  // Since 'z' is not within the string, should return nullptr.
+  ASSERT_STREQ(__llvm_libc::strchr("123?", 'z'), nullptr);
+}
+
+TEST(StrChrTest, TheSourceShouldNotChange) {
+  const char *src = "abcde";
+  const char *src_copy = src;
+  // When the character is found, the source string should not change.
+  __llvm_libc::strchr(src, 'd');
+  ASSERT_STREQ(src, src_copy);
+  // Same case for when the character is not found.
+  __llvm_libc::strchr(src, 'z');
+  ASSERT_STREQ(src, src_copy);
+  // Same case for when looking for nullptr.
+  __llvm_libc::strchr(src, '\0');
+  ASSERT_STREQ(src, src_copy);
+}
+
+TEST(StrChrTest, ShouldFindFirstOfDuplicates) {
+  // '1' is duplicated in the string, but it should find the first copy.
+  ASSERT_STREQ(__llvm_libc::strchr("abc1def1ghi", '1'), "1def1ghi");
+
+  const char *dups = "XXXXX";
+  // Should return original string since 'X' is the first character.
+  ASSERT_STREQ(__llvm_libc::strchr(dups, 'X'), dups);
+}
+
+TEST(StrChrTest, EmptyStringShouldOnlyMatchNullTerminator) {
+  // Null terminator should match.
+  ASSERT_STREQ(__llvm_libc::strchr("", '\0'), "");
+  // All other characters should not match.
+  ASSERT_STREQ(__llvm_libc::strchr("", 'Z'), nullptr);
+  ASSERT_STREQ(__llvm_libc::strchr("", '3'), nullptr);
+  ASSERT_STREQ(__llvm_libc::strchr("", '*'), nullptr);
+}