[ExecuTorch][Vulkan] Tests for et_vk.q4gsw_requant

Pull Request resolved: #20946

**Correctness tests for the Vulkan `et_vk.q4gsw_requant` kernel** (stacked above the op diff).

**Coverage:** the golden codes are computed with ATen (`round`/`clamp`, zero-scale -> code 8), mirroring `quant_nibble`, then packed into the expected W_4X8 int buffer with a small bit-packing reference (data-reshaping only, no hand-rolled math). The kernel output is compared int-for-int against that buffer, which locks the exact byte layout the forward reads. The latent is built as `code * scale` so `round()` is unambiguous (no `.5` tie-break divergence).

Cases:
- `test_tile_aligned` — single group, tile-aligned N/K.
- `test_grouped` — multiple quantization groups along K.
- `test_odd_n4` — `N % 8 != 0` (odd N4 -> padded stride + bias-zero OOB tile).
- `test_zero_scale` — a zero scale must yield code 8, not a divide-by-zero.

Also wires `q4gsw_requant_test` into `targets.bzl` + `CMakeLists.txt`.
ghstack-source-id: 405059122
@exported-using-ghexport

Differential Revision: [D111797526](https://our.internmc.facebook.com/intern/diff/D111797526/)

Author: JCNTH

backends/vulkan/test/op_tests/CMakeLists.txt

@@ -129,6 +129,10 @@ if(TARGET vulkan_backend AND LIB_TORCH)
     quantized_linear_backward_test
     ${CMAKE_CURRENT_SOURCE_DIR}/quantized_linear_backward_test.cpp test_utils
   )
+  vulkan_op_test(
+    q4gsw_requant_test ${CMAKE_CURRENT_SOURCE_DIR}/q4gsw_requant_test.cpp
+    test_utils
+  )
 
   # Only build generated op tests if a path to tags.yaml and
   # native_functions.yaml is provided. These files are required for codegen.

backends/vulkan/test/op_tests/q4gsw_requant_test.cpp

@@ -0,0 +1,195 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include <gtest/gtest.h>
+
+#include <ATen/ATen.h>
+
+#include <executorch/backends/vulkan/runtime/api/api.h>
+#include <executorch/backends/vulkan/runtime/graph/ComputeGraph.h>
+#include <executorch/backends/vulkan/runtime/graph/ops/OperatorRegistry.h>
+
+#include "test_utils.h"
+
+//
+// Reference Implementation
+//
+
+// Pack [N, K] codes (0..15) into the W_4X8 block-packed int buffer the forward
+// reads. Mirrors pack_q4_linear_weight__w_4x8.glsl: one ivec4 per (k4, n8),
+// byte b holds an (even-N low nibble, odd-N high nibble) pair at K = k4*4 + b.
+std::vector<int32_t> pack_codes_w4x8(const at::Tensor& codes) {
+  const int64_t N = codes.size(0);
+  const int64_t K = codes.size(1);
+  const int64_t K4 = K / 4;
+  const int64_t N4 = N / 4;
+  const int64_t N4_padded = (N4 + 1) & ~int64_t{1};
+  const int64_t N8 = N4_padded / 2;
+  std::vector<int32_t> buf(K4 * N4_padded * 2, 0);
+  auto ca = codes.accessor<int, 2>();
+
+  auto pack_tile = [&](int64_t k4, int64_t n4, uint32_t& px, uint32_t& py) {
+    px = 0u;
+    py = 0u;
+    for (int ni = 0; ni < 4; ++ni) {
+      const int64_t n = n4 * 4 + ni;
+      for (int b = 0; b < 4; ++b) {
+        const uint32_t code = static_cast<uint32_t>(ca[n][k4 * 4 + b] & 0xF);
+        const int shift = 8 * b + (ni & 1) * 4;
+        if (ni < 2) {
+          px |= code << shift;
+        } else {
+          py |= code << shift;
+        }
+      }
+    }
+  };
+
+  for (int64_t k4 = 0; k4 < K4; ++k4) {
+    for (int64_t n8 = 0; n8 < N8; ++n8) {
+      const int64_t n4_a = 2 * n8;
+      const int64_t n4_b = n4_a + 1;
+      uint32_t px_a, py_a, px_b = 0x88888888u, py_b = 0x88888888u;
+      pack_tile(k4, n4_a, px_a, py_a);
+      if (n4_b < N4) {
+        pack_tile(k4, n4_b, px_b, py_b);
+      }
+      const int64_t base = (k4 * N8 + n8) * 4;
+      buf[base + 0] = static_cast<int32_t>(px_a);
+      buf[base + 1] = static_cast<int32_t>(py_a);
+      buf[base + 2] = static_cast<int32_t>(px_b);
+      buf[base + 3] = static_cast<int32_t>(py_b);
+    }
+  }
+  return buf;
+}
+
+//
+// Test function
+//
+
+void test_vulkan_q4gsw_requant_impl(
+    const int64_t N,
+    const int64_t K,
+    const int64_t group_size,
+    const bool with_zero_scale) {
+  const int64_t num_groups = K / group_size;
+
+  at::Tensor scales =
+      at::rand({num_groups, N}, at::device(at::kCPU).dtype(at::kFloat)) + 0.5;
+  if (with_zero_scale) {
+    scales.index_put_({0, 0}, 0.0);
+  }
+
+  const at::Tensor group_idx =
+      at::arange(K, at::device(at::kCPU).dtype(at::kLong))
+          .div(group_size, "floor");
+  const at::Tensor scale_full =
+      scales.t().contiguous().index_select(1, group_idx); // [N, K]
+
+  // Deterministic quotient targets, each >=0.2 from any .5 tie, so GPU fp32
+  // division (~2.5 ULP, not correctly rounded) and the CPU golden round
+  // identically. Covers round both directions and clamp past [-8, 7].
+  const std::vector<float> pattern = {
+      0.3f, -0.4f, 2.7f, -3.3f, 6.4f, -6.4f, 13.2f, -21.7f};
+  const at::Tensor pat =
+      at::tensor(pattern, at::device(at::kCPU).dtype(at::kFloat));
+  const at::Tensor q_idx =
+      at::arange(N * K, at::device(at::kCPU).dtype(at::kLong))
+          .remainder(static_cast<int64_t>(pattern.size()));
+  const at::Tensor target_q = pat.index_select(0, q_idx).reshape({N, K});
+  at::Tensor latent = target_q * scale_full;
+
+  // Golden codes, mirroring quant_nibble: q=0 where scale==0, else roundEven
+  // (matches at::round half-to-even); clamp to [-8, 7]; code = (q + 8) & 0xF.
+  const at::Tensor nonzero = scale_full != 0;
+  const at::Tensor safe =
+      at::where(nonzero, scale_full, at::ones_like(scale_full));
+  at::Tensor q = at::round(latent / safe);
+  q = at::where(nonzero, q, at::zeros_like(q));
+  q = at::clamp(q, -8, 7);
+  const at::Tensor golden_codes =
+      (q.to(at::kInt) + 8).bitwise_and(0xF); // [N, K] in 0..15
+
+  const std::vector<int32_t> expected = pack_codes_w4x8(golden_codes);
+
+  using namespace vkcompute;
+
+  GraphConfig config;
+  ComputeGraph graph(config);
+
+  IOValueRef r_latent = graph.add_input_tensor(
+      latent.sizes().vec(),
+      from_at_scalartype(latent.scalar_type()),
+      utils::kBuffer);
+  ValueRef r_scales = graph.add_tensorref(
+      scales.sizes().vec(),
+      from_at_scalartype(scales.scalar_type()),
+      scales.const_data_ptr());
+  const ValueRef r_group_size = graph.add_scalar<int64_t>(group_size);
+
+  const int64_t N4 = N / 4;
+  const int64_t N4_padded = (N4 + 1) & ~int64_t{1};
+  const ValueRef r_packed =
+      graph.add_tensor({(K / 4) * N4_padded * 2}, vkapi::kInt, utils::kBuffer);
+
+  VK_GET_OP_FN("et_vk.q4gsw_requant.default")
+  (graph, {r_latent.value, r_scales, r_group_size, r_packed});
+
+  ValueRef staging_out = graph.set_output_tensor(r_packed);
+
+  graph.prepare();
+  graph.prepack();
+  graph.propagate_resize();
+
+  graph.maybe_cast_and_copy_into_staging(
+      r_latent.staging,
+      latent.const_data_ptr(),
+      latent.numel(),
+      from_at_scalartype(latent.scalar_type()));
+
+  graph.execute();
+
+  at::Tensor vk_packed = at::empty(
+      {static_cast<int64_t>(expected.size())},
+      at::device(at::kCPU).dtype(at::kInt));
+  graph.maybe_cast_and_copy_from_staging(
+      staging_out,
+      vk_packed.mutable_data_ptr(),
+      vk_packed.numel(),
+      from_at_scalartype(vk_packed.scalar_type()));
+
+  auto va = vk_packed.accessor<int, 1>();
+  for (size_t i = 0; i < expected.size(); ++i) {
+    ASSERT_EQ(va[i], expected[i]) << "mismatch at packed int " << i;
+  }
+}
+
+// Tile-aligned single-group.
+TEST(VulkanQ4gswRequantTest, test_tile_aligned) {
+  test_vulkan_q4gsw_requant_impl(
+      /*N=*/16, /*K=*/32, /*group_size=*/32, /*with_zero_scale=*/false);
+}
+
+// Multiple quantization groups along K.
+TEST(VulkanQ4gswRequantTest, test_grouped) {
+  test_vulkan_q4gsw_requant_impl(
+      /*N=*/32, /*K=*/64, /*group_size=*/32, /*with_zero_scale=*/false);
+}
+
+// N not a multiple of 8 (odd N4 -> padded stride + bias-zero OOB tile).
+TEST(VulkanQ4gswRequantTest, test_odd_n4) {
+  test_vulkan_q4gsw_requant_impl(
+      /*N=*/12, /*K=*/16, /*group_size=*/16, /*with_zero_scale=*/false);
+}
+
+// A zero scale must produce the bias-zero code (8), not a divide-by-zero.
+TEST(VulkanQ4gswRequantTest, test_zero_scale) {
+  test_vulkan_q4gsw_requant_impl(
+      /*N=*/16, /*K=*/32, /*group_size=*/32, /*with_zero_scale=*/true);
+}

backends/vulkan/test/op_tests/targets.bzl

@@ -204,6 +204,12 @@ def define_common_targets(is_fbcode = False):
             ":test_utils",
         ]
     )
+    define_test_targets(
+        "q4gsw_requant_test",
+        extra_deps = [
+            ":test_utils",
+        ]
+    )
     define_test_targets(
         "rms_norm_test",
         extra_deps = [