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Document and adjust node struct size for better padding usage #207

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Document and adjust node struct size for better padding usage #207

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9 changes: 8 additions & 1 deletion crates/redpiler/src/backend/direct/node.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -149,12 +149,19 @@ impl NonMaxU8 {
}
}

// The size of `Node` is 96 bytes (aligned to 16 bytes boundaries).
// This means 4 nodes fit into 3 cache lines (assuming 128 byte long cache lines).
// For the best possbile memory layout adjust the inline array capacity of `updates: SmallVec<_>` as follows:
// 1. The capacity should be always be odd as one entry is always used internally as discriminant
// 2. Optionally increase or decrease the capacity by 2 to eleminate `Node` padding
// 3. Finally increase or decrease the capacity by multiples of 4 until satisfied
// There are currently 6 bytes unused due to alignment/padding.
#[derive(Debug, Clone)]
pub struct Node {
pub ty: NodeType,
pub default_inputs: NodeInput,
pub side_inputs: NodeInput,
pub updates: SmallVec<[ForwardLink; 10]>,
pub updates: SmallVec<[ForwardLink; 9]>,
pub is_io: bool,

/// Powered or lit
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