…l tiles

Pull Request resolved: #20492

**Skip the QK contraction for fully-masked causal tiles** — at S=128 prefill ~48% of the (query, key) tiles are entirely above the diagonal and contribute nothing; this elides their dot products (prefill-only; bit-identical output).

**Problem**: For causal prefill, ~half the (query S-tile, key context-tile) pairs are entirely above the diagonal, yet the kernel still computes their full `d4` dot product before masking the result to `NEG_INF`.

**Solution**: Skip the contraction for fully-masked tiles; the existing per-element mask still writes the sentinel:
- **Before**: every `(s0, c0)` tile runs the full `d4` dot-product loop, then `store_qk` masks above-diagonal elements to `NEG_INF`.
- **After**: a fully-masked tile (`c0 > s0 + TM-1 + input_pos`) breaks the `d4` loop immediately (`acc` stays 0); `store_qk` masks every element to `NEG_INF` exactly as before.

**Implementation**:
- Add `skip_tile = c0 > s0 + (TM - 1) + params.input_pos`, folded into the `d4` loop break condition.
- Store loop unchanged — runs unconditionally, so no scratch entry is left stale.
- Mirrors Vulkan `sdpa_compute_attn_weights_tiled.glsl` (`tile_in_mask_region`).

**Constraints**:
- No KV-cache, host, dispatch, or uniform change (all tiles still launch; the skip is in-shader).
- Prefill-only: decode `S=1` never triggers it (`c0 <= input_pos < input_pos + TM - 1`).
- `NEG_INF` stays the WGSL-safe `-1.0e30` (WGSL forbids a literal `-inf`); does not copy Vulkan's `-1.0/0.0`.

Co-authored with Claude Code.
ghstack-source-id: 396792509
@exported-using-ghexport

Differential Revision: [D109517773](https://our.internmc.facebook.com/intern/diff/D109517773/)

…l tiles

Pull Request resolved: #20492

**Skip the QK contraction for fully-masked causal tiles** — at S=128 prefill ~48% of the (query, key) tiles are entirely above the diagonal and contribute nothing; this elides their dot products (prefill-only; bit-identical output).

**Problem**: For causal prefill, ~half the (query S-tile, key context-tile) pairs are entirely above the diagonal, yet the kernel still computes their full `d4` dot product before masking the result to `NEG_INF`.

**Solution**: Skip the contraction for fully-masked tiles; the existing per-element mask still writes the sentinel:
- **Before**: every `(s0, c0)` tile runs the full `d4` dot-product loop, then `store_qk` masks above-diagonal elements to `NEG_INF`.
- **After**: a fully-masked tile (`c0 > s0 + TM-1 + input_pos`) breaks the `d4` loop immediately (`acc` stays 0); `store_qk` masks every element to `NEG_INF` exactly as before.

**Implementation**:
- Add `skip_tile = c0 > s0 + (TM - 1) + params.input_pos`, folded into the `d4` loop break condition.
- Store loop unchanged — runs unconditionally, so no scratch entry is left stale.
- Mirrors Vulkan `sdpa_compute_attn_weights_tiled.glsl` (`tile_in_mask_region`).

**Constraints**:
- No KV-cache, host, dispatch, or uniform change (all tiles still launch; the skip is in-shader).
- Prefill-only: decode `S=1` never triggers it (`c0 <= input_pos < input_pos + TM - 1`).
- `NEG_INF` stays the WGSL-safe `-1.0e30` (WGSL forbids a literal `-inf`); does not copy Vulkan's `-1.0/0.0`.

Co-authored with Claude Code.
ghstack-source-id: 396792509
@exported-using-ghexport

Differential Revision: [D109517773](https://our.internmc.facebook.com/intern/diff/D109517773/)