Replace compareMemory() with memcmp(); NFC

WG14 N3322 (https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3322.pdf)
was adopted for C2y, which makes zero-length operations on null
pointers well-defined. The paper was also adopted as the WG14
equivalent of a WG21 defect report, so it applies in older language
modes as well. We are not aware of any C standard library which takes
advantage of this former UB, so this workaround is no longer needed.

Author: AaronBallman

llvm/include/llvm/ADT/StringRef.h

@@ -66,13 +66,6 @@ namespace llvm {
     /// The length of the string.
     size_t Length = 0;
 
-    // Workaround memcmp issue with null pointers (undefined behavior)
-    // by providing a specialized version
-    static int compareMemory(const char *Lhs, const char *Rhs, size_t Length) {
-      if (Length == 0) { return 0; }
-      return ::memcmp(Lhs,Rhs,Length);
-    }
-
   public:
     /// @name Constructors
     /// @{
@@ -182,8 +175,7 @@ namespace llvm {
     /// the \p RHS.
     [[nodiscard]] int compare(StringRef RHS) const {
       // Check the prefix for a mismatch.
-      if (int Res =
-              compareMemory(data(), RHS.data(), std::min(size(), RHS.size())))
+      if (int Res = ::memcmp(data(), RHS.data(), std::min(size(), RHS.size())))
         return Res < 0 ? -1 : 1;
 
       // Otherwise the prefixes match, so we only need to check the lengths.
@@ -264,7 +256,7 @@ namespace llvm {
     /// Check if this string starts with the given \p Prefix.
     [[nodiscard]] bool starts_with(StringRef Prefix) const {
       return size() >= Prefix.size() &&
-             compareMemory(data(), Prefix.data(), Prefix.size()) == 0;
+             ::memcmp(data(), Prefix.data(), Prefix.size()) == 0;
     }
     [[nodiscard]] bool starts_with(char Prefix) const {
       return !empty() && front() == Prefix;
@@ -276,8 +268,7 @@ namespace llvm {
     /// Check if this string ends with the given \p Suffix.
     [[nodiscard]] bool ends_with(StringRef Suffix) const {
       return size() >= Suffix.size() &&
-             compareMemory(end() - Suffix.size(), Suffix.data(),
-                           Suffix.size()) == 0;
+             ::memcmp(end() - Suffix.size(), Suffix.data(), Suffix.size()) == 0;
     }
     [[nodiscard]] bool ends_with(char Suffix) const {
       return !empty() && back() == Suffix;

llvm/lib/Support/StringRef.cpp

@@ -76,7 +76,7 @@ int StringRef::compare_numeric(StringRef RHS) const {
           break;
       }
       // The two number sequences have the same length (J-I), just memcmp them.
-      if (int Res = compareMemory(data() + I, RHS.data() + I, J - I))
+      if (int Res = ::memcmp(data() + I, RHS.data() + I, J - I))
         return Res < 0 ? -1 : 1;
       // Identical number sequences, continue search after the numbers.
       I = J - 1;