westonpace commented on code in PR #35565:
URL: https://github.com/apache/arrow/pull/35565#discussion_r1199258408
##########
cpp/src/arrow/util/align_util.cc:
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@@ -30,12 +32,120 @@ bool CheckAlignment(const Buffer& buffer, int64_t
alignment) {
return buffer.address() % alignment == 0;
}
-bool CheckAlignment(const ArrayData& array, int64_t alignment) {
- for (const auto& buffer : array.buffers) {
- if (buffer) {
- if (!CheckAlignment(*buffer, alignment)) return false;
+namespace {
+
+// Some buffers are frequently type-punned. For example, in an int32 array the
+// values buffer is frequently cast to int32_t*
+//
+// This sort of punning is only valid if the pointer is aligned to a proper
width
+// (e.g. 4 bytes in the case of int32).
+//
+// We generally assume that all buffers are at least 8-bit aligned and so we
only
+// need to worry about buffers that are commonly cast to wider data types.
Note that
+// this alignment is something that is guaranteed by malloc (e.g. new
int32_t[] will
+// return a buffer that is 4 byte aligned) or common libraries (e.g. numpy)
but it is
+// not currently guaranteed by flight (GH-32276).
+//
+// By happy coincedence, for every data type, the only buffer that might need
wider
+// alignment is the second buffer (at index 1). This function returns the
expected
+// alignment (in bits) of the second buffer for the given array to safely
allow this cast.
+//
+// If the array's type doesn't have a second buffer or the second buffer is
not expected
+// to be type punned, then we return 8.
+int GetMallocValuesAlignment(const ArrayData& array) {
Review Comment:
Done, except I return `1` if `buffer_index > 1` also (e.g. binary arrays)
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