src: simplify alignment-handling code

Use a common function to handle alignment computations in
multiple places.

PR-URL: #33884
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

Author: addaleax

src/node_http2.cc

@@ -373,9 +373,7 @@ Origins::Origins(
                               origin_string_len);
 
   // Make sure the start address is aligned appropriately for an nghttp2_nv*.
-  char* start = reinterpret_cast<char*>(
-      RoundUp(reinterpret_cast<uintptr_t>(buf_.data()),
-              alignof(nghttp2_origin_entry)));
+  char* start = AlignUp(buf_.data(), alignof(nghttp2_origin_entry));
   char* origin_contents = start + (count_ * sizeof(nghttp2_origin_entry));
   nghttp2_origin_entry* const nva =
       reinterpret_cast<nghttp2_origin_entry*>(start);

src/node_http_common-inl.h

@@ -31,8 +31,7 @@ NgHeaders<T>::NgHeaders(Environment* env, v8::Local<v8::Array> headers) {
                                  count_ * sizeof(nv_t) +
                                  header_string_len);
 
-  char* start = reinterpret_cast<char*>(
-      RoundUp(reinterpret_cast<uintptr_t>(*buf_), alignof(nv_t)));
+  char* start = AlignUp(buf_.out(), alignof(nv_t));
   char* header_contents = start + (count_ * sizeof(nv_t));
   nv_t* const nva = reinterpret_cast<nv_t*>(start);
 

src/string_bytes.cc

@@ -273,16 +273,14 @@ size_t StringBytes::WriteUCS2(Isolate* isolate,
     return 0;
   }
 
+  uint16_t* const aligned_dst = AlignUp(dst, sizeof(*dst));
   size_t nchars;
-  size_t alignment = reinterpret_cast<uintptr_t>(dst) % sizeof(*dst);
-  if (alignment == 0) {
+  if (aligned_dst == dst) {
     nchars = str->Write(isolate, dst, 0, max_chars, flags);
     *chars_written = nchars;
     return nchars * sizeof(*dst);
   }
 
-  uint16_t* aligned_dst =
-      reinterpret_cast<uint16_t*>(buf + sizeof(*dst) - alignment);
   CHECK_EQ(reinterpret_cast<uintptr_t>(aligned_dst) % sizeof(*dst), 0);
 
   // Write all but the last char

src/util-inl.h

@@ -208,8 +208,7 @@ void SwapBytes16(char* data, size_t nbytes) {
   CHECK_EQ(nbytes % 2, 0);
 
 #if defined(_MSC_VER)
-  int align = reinterpret_cast<uintptr_t>(data) % sizeof(uint16_t);
-  if (align == 0) {
+  if (AlignUp(data, sizeof(uint16_t)) == data) {
     // MSVC has no strict aliasing, and is able to highly optimize this case.
     uint16_t* data16 = reinterpret_cast<uint16_t*>(data);
     size_t len16 = nbytes / sizeof(*data16);
@@ -232,9 +231,8 @@ void SwapBytes32(char* data, size_t nbytes) {
   CHECK_EQ(nbytes % 4, 0);
 
 #if defined(_MSC_VER)
-  int align = reinterpret_cast<uintptr_t>(data) % sizeof(uint32_t);
   // MSVC has no strict aliasing, and is able to highly optimize this case.
-  if (align == 0) {
+  if (AlignUp(data, sizeof(uint32_t)) == data) {
     uint32_t* data32 = reinterpret_cast<uint32_t*>(data);
     size_t len32 = nbytes / sizeof(*data32);
     for (size_t i = 0; i < len32; i++) {
@@ -256,8 +254,7 @@ void SwapBytes64(char* data, size_t nbytes) {
   CHECK_EQ(nbytes % 8, 0);
 
 #if defined(_MSC_VER)
-  int align = reinterpret_cast<uintptr_t>(data) % sizeof(uint64_t);
-  if (align == 0) {
+  if (AlignUp(data, sizeof(uint64_t)) == data) {
     // MSVC has no strict aliasing, and is able to highly optimize this case.
     uint64_t* data64 = reinterpret_cast<uint64_t*>(data);
     size_t len64 = nbytes / sizeof(*data64);

src/util.h

@@ -719,6 +719,13 @@ constexpr T RoundUp(T a, T b) {
   return a % b != 0 ? a + b - (a % b) : a;
 }
 
+// Align ptr to an `alignment`-bytes boundary.
+template <typename T, typename U>
+constexpr T* AlignUp(T* ptr, U alignment) {
+  return reinterpret_cast<T*>(
+      RoundUp(reinterpret_cast<uintptr_t>(ptr), alignment));
+}
+
 class SlicedArguments : public MaybeStackBuffer<v8::Local<v8::Value>> {
  public:
   inline explicit SlicedArguments(