f comment more details why

Author: TheBlueMatt

lightning/src/routing/scoring.rs

@@ -1658,6 +1658,14 @@ mod bucketed_history {
 	#[derive(Clone, Copy)]
 	#[repr(C)] // Force the fields in memory to be in the order we specify.
 	pub(super) struct HistoricalLiquidityTracker {
+		// This struct sites inside a `(u64, ChannelLiquidity)` in memory, and we first read the
+		// liquidity offsets in `ChannelLiquidity` when calculating the non-historical score. This
+		// means that the first handful of bytes of this struct will already be sitting in cache by
+		// the time we go to look at them.
+		//
+		// Because the first thing we do is check if `total_valid_points` is sufficient to consider
+		// the data here at all, and can return early if it is not, we want this to go first to
+		// avoid hitting a second cache line load entirely in that case.
 		total_valid_points_tracked: u64,
 		min_liquidity_offset_history: HistoricalBucketRangeTracker,
 		max_liquidity_offset_history: HistoricalBucketRangeTracker,