Memset hook (#866)

This PR adds a hook that implements `memset` for the llvm backend. We
intend to use this to perform copies while avoiding the overhead of an
extra buffer copy in the C semantics.

---------

Co-authored-by: rv-jenkins <admin@runtimeverification.com>

runtime/strings/bytes.cpp

@@ -182,6 +182,33 @@ SortBytes hook_BYTES_replaceAt(SortBytes b, SortInt start, SortBytes b2) {
   return b;
 }
 
+SortBytes
+hook_BYTES_memset(SortBytes b, SortInt start, SortInt count, SortInt value) {
+  uint64_t ustart = get_ui(start);
+  uint64_t ucount = get_ui(count);
+  uint64_t uend = ustart + ucount;
+  if ((uend < ustart) || (uend < ucount)) {
+    KLLVM_HOOK_INVALID_ARGUMENT(
+        "Exception on memset: unsigned addition start {} plus count {} wraps "
+        "around: uend= {}",
+        ustart, ucount, uend);
+  }
+  uint64_t input_len = len(b);
+  if (uend > input_len) {
+    KLLVM_HOOK_INVALID_ARGUMENT(
+        "Buffer overflow on memset: start {} plus count {} is greater "
+        "than buffer length {}",
+        ustart, ucount, input_len);
+  }
+  int v = mpz_get_si(value);
+  if ((v < -128) || (v > 127)) {
+    KLLVM_HOOK_INVALID_ARGUMENT(
+        "Not a valid value for a byte in memset: v={}", v);
+  }
+  memset(b->data + ustart, v, ucount);
+  return b;
+}
+
 SortInt hook_BYTES_length(SortBytes a) {
   mpz_t result;
   mpz_init_set_ui(result, len(a));

unittests/runtime-strings/bytestest.cpp

@@ -23,6 +23,7 @@ string *hook_BYTES_string2bytes(string *s);
 string *hook_BYTES_substr(string *b, mpz_t start, mpz_t end);
 string *hook_BYTES_replaceAt(string *b, mpz_t start, string *b2);
 string *hook_BYTES_update(string *b, mpz_t off, mpz_t val);
+string *hook_BYTES_memset(string *b, mpz_t start, mpz_t cnt, mpz_t val);
 mpz_ptr hook_BYTES_get(string *b, mpz_t off);
 mpz_ptr hook_BYTES_length(string *b);
 string *hook_BYTES_padRight(string *b, mpz_t len, mpz_t v);
@@ -227,6 +228,34 @@ BOOST_AUTO_TEST_CASE(update) {
   BOOST_CHECK_EQUAL(0, memcmp(res->data, "1204", 4));
 }
 
+BOOST_AUTO_TEST_CASE(memset) {
+  auto _12345 = makeString("12345");
+  mpz_t _0, _1, _3;
+  mpz_init_set_si(_0, '0');
+  mpz_init_set_ui(_1, 1);
+  mpz_init_set_ui(_3, 3);
+
+  auto res = hook_BYTES_memset(_12345, _1, _3, _0);
+  BOOST_CHECK_EQUAL(_12345, res);
+  BOOST_CHECK_EQUAL(5, len(res));
+  BOOST_CHECK_EQUAL(0, memcmp(res->data, "10005", 5));
+
+  mpz_t neg1;
+  mpz_init_set_si(neg1, -1);
+  res = hook_BYTES_memset(_12345, _1, _1, neg1);
+  BOOST_CHECK_EQUAL(_12345, res);
+  BOOST_CHECK_EQUAL(5, len(res));
+  BOOST_CHECK_EQUAL((unsigned char)_12345->data[1], 255);
+
+  BOOST_CHECK_THROW(
+      hook_BYTES_memset(_12345, _3, _3, _0), std::invalid_argument);
+
+  mpz_t uint64max;
+  mpz_init_set_ui(uint64max, UINT64_MAX);
+  BOOST_CHECK_THROW(
+      hook_BYTES_memset(_12345, _1, uint64max, _0), std::invalid_argument);
+}
+
 BOOST_AUTO_TEST_CASE(get) {
   auto _1234 = makeString("1234");
   mpz_t _0;