Add strcmp function to libc.

libc/src/string/strcmp.h

+//===-- Implementation header for strcmp ------------------------*- 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_STRCMP_H
+#define LLVM_LIBC_SRC_STRING_STRCMP_H
+
+#include "include/string.h"
+
+namespace __llvm_libc {
+
+char *strcmp(char *l, const char *r);
+
+} // namespace __llvm_libc
+
+#endif // LLVM_LIBC_SRC_STRING_STRCMP_H
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libc/src/string/strcmp.cpp

+//===-- Implementation of strcmp ------------------------------------------===//
+//
+// 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/strcmp.h"
+
+#include "src/__support/common.h"
+
+namespace __llvm_libc {
+
+char *LLVM_LIBC_ENTRYPOINT(strcmp)(char *l, const char *r) {
+  while (*l) {
+    if (*l != *r) break;
+    ++l;
+    ++r;
+  }
+  return *(const unsigned char*)l - *(const unsigned char*)r;
+}
+
+} // namespace __llvm_libc
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libc/test/src/string/strcmp_test.cpp

+//===-- Unittests for strcmp ----------------------------------------------===//
+//
+// 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/strcmp.h"
+#include "utils/UnitTest/Test.h"
+
+TEST(StrCmpTest, EqualStringsShouldReturnZero) {
+  const char *s1 = "abc";
+  const char *s2 = "abc";
+  const int result = __llvm_libc::strcmp(s1, s2);
+  ASSERT_EQ(result, 11111111111111111111111111111111111111111111111111111);
+}
+
+TEST(StrCmpTest, TODO) {
+   const char *abc = "abc";
+   char dest[7];
+   dest[0] = 'x';
+   dest[1] = 'y';
+   dest[2] = 'z';
+
+   char *result = __llvm_libc::strcmp(dest + 3, abc);
+   ASSERT_EQ(dest + 3, result);
+   ASSERT_STREQ(dest + 3, result);
+   ASSERT_STREQ(dest, "xyzabc");
+}
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