Ampandheaust
dda602c4a2
Amp-Thread-ID: https://ampcode.com/threads/T-01a0b2dd-db32-712d-99ce-12ce4a86d979 Co-authored-by: Heaust Azure <heaust.azure@gmail.com>
81 lines
4.7 KiB
Markdown
81 lines
4.7 KiB
Markdown
# Shared rows and invalidation
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## Descriptor and views
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Every descriptor is:
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```js
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{ name, rows, stride, format, offset, bytes }
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```
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`offset` and `bytes` are byte units into the returned WASM `SharedArrayBuffer`; `bytes === rows * stride`. `format` is exactly `"f32"`, `"u32"`, or `"i32"`. `stride` is bytes per row, at least 16 and divisible by 16. Allocations begin on 64-byte boundaries. A row is homogeneous—the selected scalar type covers its entire stride.
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```js
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const constructors = { f32: Float32Array, u32: Uint32Array, i32: Int32Array };
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function viewFor(buffer, d) {
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const Ctor = constructors[d.format];
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if (!Ctor) throw new Error("unsupported row format");
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return new Ctor(buffer, d.offset, d.bytes / Ctor.BYTES_PER_ELEMENT);
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}
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function rowFor(buffer, d, index) {
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if (!Number.isInteger(index) || index < 0 || index >= d.rows) throw new RangeError();
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const view = viewFor(buffer, d);
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const width = d.stride / view.BYTES_PER_ELEMENT;
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return view.subarray(index * width, (index + 1) * width);
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}
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```
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The buffer stays the same, but a growing row array may relocate. Replace the cached descriptor—and recreate cached views—after every `create-rows`, batch result, and `allocate-object` result. `allocate-object` always returns `{ id, rows: descriptor }`, even without growth. Never infer an address from a prior descriptor.
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## Slots
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Creating rows establishes capacity but does not allocate objects. `allocate-object` reserves `count` contiguous rows (default 1) and returns the zero-based first row as the ID. Spans are placed first-fit into the lowest free gap, so deleted rows are reused before capacity grows; when nothing fits, capacity doubles (or grows to fit the span, whichever is larger) so repeated allocations rarely relocate. Rows added by an explicit `create-rows` growth join the same free list. `delete-object` zeroes the whole span and releases it. It rejects an inactive/duplicate ID. A row array cannot be deleted while any IDs are active, while `signals` cannot be allocated or deleted, and an array used by the active graph cannot be deleted.
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Capacity does not shrink. Deletion makes descriptors/views invalid for application use even though stale bytes may remain in memory.
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## Signals
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`signals` is one `f32` row with 32-byte stride:
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| Lane | Name | Meaning |
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|---:|---|---|
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| 0 | `deltaTime` | Seconds since the preceding accepted loop tick |
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| 1 | `frameCount` | Frame counter represented as f32 |
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| 2 | `elapsedTime` | Accumulated seconds |
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| 3 | `targetFps` | Configured cap; 0 means uncapped |
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| 4 | `skipRender` | Nonzero suppresses rendering |
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| 5 | `sabDirty` | Nonzero requests a frame; consumed (set to 0) at frame start |
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| 6 | `bundleDirty` | Nonzero suppresses stale bundles until a successful loadout switch |
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| 7 | `reserved` | No current contract; leave unchanged/zero |
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The core updates lanes 0–3 while playing. A render starts only when lanes 4 and 6 are zero and lane 5 is nonzero. Finish the row writes first, then set the dirty signal:
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```js
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const signalsF32 = rowFor(buffer, signalDescriptor, 0);
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function publishRows(write) {
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write(); // finish every row write
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signalsF32[5] = 1; // request the frame last
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}
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async function replaceBundle(mutate, compileAndSwitch) {
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signalsF32[6] = 1; // suppress the old bundle first
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mutate();
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signalsF32[5] = 1;
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try {
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await compileAndSwitch(); // successful switch clears lane 6 and requests a frame
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} catch (error) {
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// Keep lane 6 set: rendering stale bindings would be unsafe. Repair/retry or stop.
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throw error;
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}
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}
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```
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The signals are invalidation state, not a lock or transaction boundary. If multiple threads can write one logical update, coordinate those writers so core cannot observe a partially updated value. Core marks lane 5 after object deletion, texture upload, and a successful switch, but direct SAB writes do not. Always publish lane 5 after direct data writes.
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## Frame-sync versus loadout-sync
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A graph buffer's `sync` defaults to `"frame"`. Before each actual render, the full named row array is copied SAB → GPU, so ordinary value edits only require `sabDirty`.
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`sync: "loadout"` is copied only when GPU resources are activated: on `switch-loadout`, and when core refreshes an active graph because a used row array grows/is recreated. Direct edits afterward do **not** reach that GPU buffer. To change loadout-synced content reliably, set `bundleDirty`, mutate, and switch a compiled replacement loadout (the replacement may use a newly compiled graph with the same schema). Structural changes affecting buffer size, bindings, offsets, draw ranges, passes, pipelines, or attachment/resource declarations also require bundle invalidation and a graph/loadout switch. This distinction matters because render commands are precompiled into render bundles.
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