diff --git a/deps/zlib/BUILD.gn b/deps/zlib/BUILD.gn
index 8ed0807a994b1e..46627bca7eb158 100644
--- a/deps/zlib/BUILD.gn
+++ b/deps/zlib/BUILD.gn
@@ -3,6 +3,7 @@
 # found in the LICENSE file.
 
 import("//build/config/compiler/compiler.gni")
+import("//build/config/dcheck_always_on.gni")
 
 declare_args() {
   # Expose zlib's symbols, used by Node.js to provide zlib APIs for its native
@@ -33,7 +34,7 @@ config("zlib_internal_config") {
     # Build code using -O3, see: crbug.com/1084371.
     configs = [ "//build/config/compiler:optimize_speed" ]
   }
-  if (is_debug || use_fuzzing_engine) {
+  if (is_debug || dcheck_always_on || use_fuzzing_engine) {
     # Enable zlib's asserts in debug and fuzzer builds.
     defines += [ "ZLIB_DEBUG" ]
   }
diff --git a/deps/zlib/contrib/tests/utils_unittest.cc b/deps/zlib/contrib/tests/utils_unittest.cc
index d06dbc9812ad92..4a8027717920f6 100644
--- a/deps/zlib/contrib/tests/utils_unittest.cc
+++ b/deps/zlib/contrib/tests/utils_unittest.cc
@@ -1025,6 +1025,61 @@ TEST(ZlibTest, DeflateZFixedCorruption) {
       0);
 }
 
+TEST(ZlibTest, DeflateCopy) {
+  // Check that deflateCopy() works.
+
+  z_stream stream1;
+  stream1.zalloc = Z_NULL;
+  stream1.zfree = Z_NULL;
+  int ret =
+      deflateInit(&stream1, Z_DEFAULT_COMPRESSION);
+  ASSERT_EQ(ret, Z_OK);
+  std::vector<uint8_t> compressed(
+      deflateBound(&stream1, strlen(zFixedCorruptionData)));
+  stream1.next_out = compressed.data();
+  stream1.avail_out = compressed.size();
+
+  // Compress the first 1000 bytes.
+  stream1.next_in = (uint8_t*)zFixedCorruptionData;
+  stream1.avail_in = 1000;
+  ret = deflate(&stream1, Z_NO_FLUSH);
+  ASSERT_EQ(ret, Z_OK);
+
+  // Copy the stream state.
+  z_stream stream2;
+  ret = deflateCopy(&stream2, &stream1);
+  ASSERT_EQ(ret, Z_OK);
+  deflateEnd(&stream1);
+
+  // Compress the remaining bytes.
+  stream2.next_in = (uint8_t*)zFixedCorruptionData + (1000 - stream2.avail_in);
+  stream2.avail_in = strlen(zFixedCorruptionData) - (1000 - stream2.avail_in);
+  ret = deflate(&stream2, Z_FINISH);
+  ASSERT_EQ(ret, Z_STREAM_END);
+  size_t compressed_sz = compressed.size() - stream2.avail_out;
+  deflateEnd(&stream2);
+
+  // Check that decompression is successful.
+  std::vector<uint8_t> decompressed(strlen(zFixedCorruptionData));
+  z_stream stream;
+  stream.zalloc = Z_NULL;
+  stream.zfree = Z_NULL;
+  ret = inflateInit(&stream);
+  ASSERT_EQ(ret, Z_OK);
+  stream.next_in = compressed.data();
+  stream.avail_in = compressed_sz;
+  stream.next_out = decompressed.data();
+  stream.avail_out = decompressed.size();
+  ret = inflate(&stream, Z_FINISH);
+  ASSERT_EQ(ret, Z_STREAM_END);
+  inflateEnd(&stream);
+
+  EXPECT_EQ(decompressed.size(), strlen(zFixedCorruptionData));
+  EXPECT_EQ(
+      memcmp(zFixedCorruptionData, decompressed.data(), decompressed.size()),
+      0);
+}
+
 // TODO(gustavoa): make these tests run standalone.
 #ifndef CMAKE_STANDALONE_UNITTESTS
 
diff --git a/deps/zlib/deflate.c b/deps/zlib/deflate.c
index adb2488224516a..4920e7007af887 100644
--- a/deps/zlib/deflate.c
+++ b/deps/zlib/deflate.c
@@ -1345,7 +1345,11 @@ int ZEXPORT deflateCopy(z_streamp dest, z_streamp source) {
     ds->window = (Bytef *) ZALLOC(dest, ds->w_size, 2*sizeof(Byte));
     ds->prev   = (Posf *)  ZALLOC(dest, ds->w_size, sizeof(Pos));
     ds->head   = (Posf *)  ZALLOC(dest, ds->hash_size, sizeof(Pos));
+#ifdef LIT_MEM
+    ds->pending_buf = (uchf *) ZALLOC(dest, ds->lit_bufsize, 5);
+#else
     ds->pending_buf = (uchf *) ZALLOC(dest, ds->lit_bufsize, 4);
+#endif
 
     if (ds->window == Z_NULL || ds->prev == Z_NULL || ds->head == Z_NULL ||
         ds->pending_buf == Z_NULL) {
@@ -1356,7 +1360,11 @@ int ZEXPORT deflateCopy(z_streamp dest, z_streamp source) {
     zmemcpy(ds->window, ss->window, ds->w_size * 2 * sizeof(Byte));
     zmemcpy((voidpf)ds->prev, (voidpf)ss->prev, ds->w_size * sizeof(Pos));
     zmemcpy((voidpf)ds->head, (voidpf)ss->head, ds->hash_size * sizeof(Pos));
+#ifdef LIT_MEM
+    zmemcpy(ds->pending_buf, ss->pending_buf, (uInt)ds->lit_bufsize * 5);
+#else
     zmemcpy(ds->pending_buf, ss->pending_buf, (uInt)ds->pending_buf_size);
+#endif
 
     ds->pending_out = ds->pending_buf + (ss->pending_out - ss->pending_buf);
 #ifdef LIT_MEM
diff --git a/deps/zlib/deflate.h b/deps/zlib/deflate.h
index 654d0944cda29a..eb7f0724015cc7 100644
--- a/deps/zlib/deflate.h
+++ b/deps/zlib/deflate.h
@@ -25,7 +25,7 @@
 
 /* define LIT_MEM to slightly increase the speed of deflate (order 1% to 2%) at
    the cost of a larger memory footprint */
-/* #define LIT_MEM */
+#define LIT_MEM
 
 /* ===========================================================================
  * Internal compression state.
diff --git a/deps/zlib/google/compression_utils.cc b/deps/zlib/google/compression_utils.cc
index 279ea0732980c3..c2b17e4ced6f22 100644
--- a/deps/zlib/google/compression_utils.cc
+++ b/deps/zlib/google/compression_utils.cc
@@ -4,7 +4,6 @@
 
 #include "third_party/zlib/google/compression_utils.h"
 
-#include "base/bit_cast.h"
 #include "base/check_op.h"
 #include "base/process/memory.h"
 #include "base/sys_byteorder.h"
@@ -24,8 +23,8 @@ bool GzipCompress(base::span<const char> input,
   // uLongf can be larger than size_t.
   uLongf compressed_size_long = static_cast<uLongf>(output_buffer_size);
   if (zlib_internal::GzipCompressHelper(
-          base::bit_cast<Bytef*>(output_buffer), &compressed_size_long,
-          base::bit_cast<const Bytef*>(input.data()),
+          reinterpret_cast<Bytef*>(output_buffer), &compressed_size_long,
+          reinterpret_cast<const Bytef*>(input.data()),
           static_cast<uLongf>(input.size()), malloc_fn, free_fn) != Z_OK) {
     return false;
   }
@@ -55,7 +54,7 @@ bool GzipCompress(base::span<const uint8_t> input, std::string* output) {
 
   if (zlib_internal::GzipCompressHelper(
           compressed_data, &compressed_data_size,
-          base::bit_cast<const Bytef*>(input.data()), input_size, nullptr,
+          reinterpret_cast<const Bytef*>(input.data()), input_size, nullptr,
           nullptr) != Z_OK) {
     free(compressed_data);
     return false;
@@ -82,8 +81,8 @@ bool GzipUncompress(const std::string& input, std::string* output) {
 
   uncompressed_output.resize(uncompressed_size);
   if (zlib_internal::GzipUncompressHelper(
-          base::bit_cast<Bytef*>(uncompressed_output.data()),
-          &uncompressed_size, base::bit_cast<const Bytef*>(input.data()),
+          reinterpret_cast<Bytef*>(uncompressed_output.data()),
+          &uncompressed_size, reinterpret_cast<const Bytef*>(input.data()),
           static_cast<uLongf>(input.length())) == Z_OK) {
     output->swap(uncompressed_output);
     return true;
@@ -102,8 +101,8 @@ bool GzipUncompress(base::span<const uint8_t> input,
   if (uncompressed_size > output.size())
     return false;
   return zlib_internal::GzipUncompressHelper(
-             base::bit_cast<Bytef*>(output.data()), &uncompressed_size,
-             base::bit_cast<const Bytef*>(input.data()),
+             reinterpret_cast<Bytef*>(const_cast<uint8_t*>(output.data())),
+             &uncompressed_size, reinterpret_cast<const Bytef*>(input.data()),
              static_cast<uLongf>(input.size())) == Z_OK;
 }
 
@@ -117,8 +116,8 @@ bool GzipUncompress(base::span<const uint8_t> input, std::string* output) {
   uLongf uncompressed_size = GetUncompressedSize(input);
   output->resize(uncompressed_size);
   return zlib_internal::GzipUncompressHelper(
-             base::bit_cast<Bytef*>(output->data()), &uncompressed_size,
-             base::bit_cast<const Bytef*>(input.data()),
+             reinterpret_cast<Bytef*>(output->data()), &uncompressed_size,
+             reinterpret_cast<const Bytef*>(input.data()),
              static_cast<uLongf>(input.size())) == Z_OK;
 }
 
@@ -128,7 +127,8 @@ uint32_t GetUncompressedSize(base::span<const char> compressed_data) {
 
 uint32_t GetUncompressedSize(base::span<const uint8_t> compressed_data) {
   return zlib_internal::GetGzipUncompressedSize(
-      base::bit_cast<Bytef*>(compressed_data.data()), compressed_data.size());
+      reinterpret_cast<const Bytef*>(compressed_data.data()),
+      compressed_data.size());
 }
 
 }  // namespace compression
diff --git a/deps/zlib/trees.c b/deps/zlib/trees.c
index 5ca23e9381a3a3..38135273c9c3bc 100644
--- a/deps/zlib/trees.c
+++ b/deps/zlib/trees.c
@@ -938,7 +938,8 @@ local void compress_block(deflate_state *s, const ct_data *ltree,
 
         /* Check for no overlay of pending_buf on needed symbols */
 #ifdef LIT_MEM
-        Assert(s->pending < (s->lit_bufsize << 1) + sx, "pendingBuf overflow");
+        Assert(s->pending < (s->lit_bufsize << 1) + (sx << 1),
+               "pendingBuf overflow");
 #else
         Assert(s->pending < s->lit_bufsize + sx, "pendingBuf overflow");
 #endif
diff --git a/src/zlib_version.h b/src/zlib_version.h
index b15d94fdfbe88d..10aa3283c81aab 100644
--- a/src/zlib_version.h
+++ b/src/zlib_version.h
@@ -2,5 +2,5 @@
 // Refer to tools/dep_updaters/update-zlib.sh
 #ifndef SRC_ZLIB_VERSION_H_
 #define SRC_ZLIB_VERSION_H_
-#define ZLIB_VERSION "1.3.0.1-motley-dd5fc13"
+#define ZLIB_VERSION "1.3.0.1-motley-40e35a7"
 #endif  // SRC_ZLIB_VERSION_H_