Fixed part of the implementation and added more tests.

Author: Sriya Pratipati

libc/src/wchar/wmemcmp.cpp

@@ -17,9 +17,9 @@ namespace LIBC_NAMESPACE_DECL {
 
 LLVM_LIBC_FUNCTION(int, wmemcmp,
                    (const wchar_t *s1, const wchar_t *s2, size_t n)) {
-  for (size_t i = 0; i < n; ++s1, ++s2, ++i) {
-    if (*s1 != *s2)
-      return (int)(*s1 - *s2);
+  for (size_t i = 0; i < n; ++i) {
+    if (s1[i] != s2[i])
+      return (int)(s1[i] - s2[i]);
   }
   // If it reaches the end, all n values must be the same.
   return 0;

libc/test/src/wchar/wmemcmp_test.cpp

@@ -38,3 +38,31 @@ TEST(LlvmLibcWMemcmpTest, LhsAfterRhsLexically) {
   const wchar_t *rhs = L"ab";
   EXPECT_GT(LIBC_NAMESPACE::wmemcmp(lhs, rhs, 2), 0);
 }
+
+TEST(LlvmLibcWMemcmpTest, CompareToEmpty) {
+  // lhs is nonempty, should result in a value greater than 0.
+  const wchar_t *lhs = L"az";
+  const wchar_t *rhs = L"";
+  EXPECT_GT(LIBC_NAMESPACE::wmemcmp(lhs, rhs, 2), 0);
+}
+
+TEST(LlvmLibcWMemcmpTest, LhsAfterRhsLexicallyLong) {
+  // b after a, should result in a value greater than 0.
+  const wchar_t *lhs = L"aaaaaaaaaaaaab";
+  const wchar_t *rhs = L"aaaaaaaaaaaaaa";
+  EXPECT_GT(LIBC_NAMESPACE::wmemcmp(lhs, rhs, 15), 0);
+}
+
+TEST(LlvmLibcWMemcmpTest, RhsAfterLhsLexicallyLong) {
+  // b after a, should result in a value less than 0.
+  const wchar_t *lhs = L"aaaaaaaaaaaaaa";
+  const wchar_t *rhs = L"aaaaaaaaaaaaab";
+  EXPECT_LT(LIBC_NAMESPACE::wmemcmp(lhs, rhs, 15), 0);
+}
+
+TEST(LlvmLibcWMemcmpTest, LhsRhsAreTheSameLong) {
+  // Comparing strings of equal value should result in 0.
+  const wchar_t *lhs = L"aaaaaaaaaaaaaa";
+  const wchar_t *rhs = L"aaaaaaaaaaaaaa";
+  EXPECT_EQ(LIBC_NAMESPACE::wmemcmp(lhs, rhs, 15), 0);
+}