Index: clang/lib/AST/ExprConstant.cpp
===================================================================
--- clang/lib/AST/ExprConstant.cpp
+++ clang/lib/AST/ExprConstant.cpp
@@ -9357,7 +9357,7 @@
       APSInt N;
       if (!EvaluateInteger(E->getArg(2), N, Info))
         return false;
-      MaxLength = N.getExtValue();
+      MaxLength = N.getZExtValue();
     }
     // We cannot find the value if there are no candidates to match against.
     if (MaxLength == 0u)
@@ -12381,7 +12381,7 @@
       APSInt N;
       if (!EvaluateInteger(E->getArg(2), N, Info))
         return false;
-      MaxLength = N.getExtValue();
+      MaxLength = N.getZExtValue();
     }
 
     // Empty substrings compare equal by definition.
Index: clang/test/CodeGen/gh64876.cpp
===================================================================
--- /dev/null
+++ clang/test/CodeGen/gh64876.cpp
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -triple x86_64 -S -emit-llvm -disable-llvm-passes -o - %s | FileCheck %s
+
+void f(const char* C, const wchar_t *WC) {
+  int x1 = __builtin_strncmp(C, "b", 0xffffffffffffffff);
+// CHECK: {{.*}}= call i32 @strncmp{{.*}}i64 noundef -1
+  int x2 = __builtin_memcmp(C, "b", 0xffffffffffffffff);
+// CHECK: {{.*}}= call i32 @memcmp{{.*}}i64 noundef -1
+  int x3 = __builtin_bcmp(C, "b", 0xffffffffffffffff);
+// CHECK: {{.*}}= call i32 @bcmp{{.*}}i64 noundef -1
+  int x4 = __builtin_wmemcmp(WC, L"b", 0xffffffffffffffff);
+// CHECK: {{.*}}= call i32 @wmemcmp{{.*}}i64 noundef -1
+  auto x5 = __builtin_memchr(C, (int)'a', 0xffffffffffffffff);
+// CHECK: {{.*}}= call ptr @memchr{{.*}}i64 noundef -1
+  auto x6 = __builtin_wmemchr(WC, (int)'a', 0xffffffffffffffff);
+// CHECK: {{.*}}= call ptr @wmemchr{{.*}}i64 noundef -1
+}
Index: clang/test/SemaCXX/constexpr-string.cpp
===================================================================
--- clang/test/SemaCXX/constexpr-string.cpp
+++ clang/test/SemaCXX/constexpr-string.cpp
@@ -676,3 +676,17 @@
   }
   static_assert(test_address_of_incomplete_struct_type()); // expected-error {{constant}} expected-note {{in call}}
 }
+
+#pragma clang diagnostic push
+#pragma clang diagnostic ignored "-Wconstant-conversion"
+namespace GH64876 {
+void f() {
+  __builtin_strncmp(0, 0, 0xffffffffffffffff);
+  __builtin_memcmp(0, 0, 0xffffffffffffffff);
+  __builtin_bcmp(0, 0, 0xffffffffffffffff);
+  __builtin_wmemcmp(0, 0, 0xffffffffffffffff);
+  __builtin_memchr((const void*)0, 1, 0xffffffffffffffff);
+  __builtin_wmemchr((const wchar_t*)0, 1, 0xffffffffffffffff);
+}
+}
+#pragma clang diagnostic pop