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vulkan: support GPUs without hardware DP4A in mul_mat_vec_ptq1_0 and eliminate dequant divergence - #291
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### Summary
Optimizes `dequant_ptq1_0.comp` by eliminating the dynamic loop, removing per-element branch divergence, and reducing redundant global memory fetches.
### Details & Key Changes
1. **$O(1)$ Trit Unpacking:**
The original implementation used a dynamic loop `for (uint i = 0u; i < n; ++i) v = (v * 3u) & 0xFFu;`. In modular arithmetic $\pmod{256}$, this is equivalent to $v_n = (b \cdot 3^n) \pmod{256}$. Replaced the loop with a compile-time lookup table `pow3 = {1, 3, 9, 27, 81}`, eliminating dynamic loop execution and severe warp/wavefront divergence.
2. **Hoisted Branch Invariants:**
Each thread processes 8 consecutive elements (`8 * il + l`). Thread element ranges never cross block boundaries (`il < 10` is strictly $< 80$, `10 <= il < 15` is strictly $[80..119]$, and `il == 15` is strictly $\ge 120$). Hoisted the `if/else` checks out of the unrolled 8-element loop, evaluating conditions once per thread instead of 8 times.
3. **Memory Access Optimization:**
For `il == 15`, `qh[0]` and `qh[1]` are now loaded into scalar registers once rather than repeatedly fetched from global memory for every trit.
### Verification
Mathematically verified to produce bit-exact identical dequantized values compared to the previous CPU/GPU codec reference. Drastically improves Vulkan decode token rate, particularly on AMD (RADV) and non-CUDA hardware.
… optimize ALU
### Summary
Enables `mul_mat_vec_ptq1_0` to run universally across all Vulkan-capable GPUs (including AMD Polaris GCN 4.0 / Vega / older hardware lacking `VK_KHR_shader_integer_dot_product`) while optimizing dot product ALU throughput.
### Problem
Previously, the shader enforced `#extension GL_EXT_integer_dot_product : require`. On GPUs without hardware DP4A (such as AMD RX 480/580/590, Vega 56/64, Radeon VII), shader pipeline compilation failed at runtime. This caused the engine to silently fall back to the slow CPU-style `dequant_ptq1_0` path, reducing decode speed to <1 token/second. Additionally, the default path enforced saturating dot products (`dotPacked4x8AccSatEXT`), adding overhead on AMD architectures.
### Solution
1. Changed extension declaration to `#extension GL_EXT_integer_dot_product : enable`.
2. Added an emulated `DOT4` fallback using core GLSL 450 `bitfieldExtract`. On AMD GCN/Polaris, this compiles cleanly to native `v_bfe_i32` instructions without pipeline failure.
3. Switched the hardware path to non-saturating `dotPacked4x8EXT` by default: the maximum accumulation across 32 elements is $32 \times 2 \times 127 = 8128 \ll 2^{31}-1$, making integer overflow impossible and allowing direct emission of single-cycle hardware DP4A (`v_dot4_i32_i8`).
4. Set default `MUL3` to single-cycle shift-add `((v << 1u) + (v))` mapping to `v_lshl_add_u32`.
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Tested on an Intel Arc B390 (Panther Lake Xe3 iGPU), Windows 11, driver 32.0.101.8724, GCC 16.2 ucrt64, Correctness: Decode,
No regression on the dot path and the dequant path is within noise. One caveat for whoever reviews the core change: the emulated DOT4 in Unrelated to this PR but visible in the numbers: with integer dot disabled the 27B runs at 1.5 t/s on both trees, so the fallback itself is very slow here. |
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I can validate the fallback path on hardware lacking Tested on:
Before this PR, pipeline creation crashed during Vulkan shader compilation due to the hard requirement on the extension. With this patch, the shaders compile cleanly. The emulated Benchmark timings from As expected on Polaris, it is heavily compute-bound without hardware DP4A (~0.50 t/s for 27B), but the fallback is completely functional and prevents crashes on legacy GCN hardware. |
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Summary
Fixes severe performance degradation (<1 tok/s) and shader compilation crashes on Vulkan GPUs lacking hardware DP4A instructions (such as AMD Polaris RX 400/500 series, Vega, and older architectures), while optimizing the dequantizer fallback.
What is fixed:
mul_mat_vec_ptq1_0.comp:#extension GL_EXT_integer_dot_product : requireto: enable. Previously, GPUs without DP4A support failed to compile the pipeline at runtime, silently falling back to the slow CPU-style dequant path.DOT4fallback using GLSL 450bitfieldExtract(which compiles cleanly to nativev_bfe_i32instructions on AMD GCN/Polaris).dotPacked4x8EXT(saturation is mathematically impossible for 32 trits and added unnecessary instruction overhead).MUL3to single-cycle shift-add((v << 1u) + (v)).dequant_ptq1_0.comp:v = (v * 3u) & 0xFFuwith a constant lookup tableqhbytes into registers.Tested on: