[SYCL] Fix Q8_0 reorder: add missing dequantize path for GEMM

The Q8_0 reorder optimization (ggml-org#21527) was missing a reorder-aware
dequantizer for the GEMM code path used during prompt processing.
After token generation reordered Q8_0 weights (via DMMV/MMVQ), the
next prompt processing pass would read them with the standard
dequantizer, producing garbage output.

Add dequantize_block_q8_0_reorder() and wire it into both
ggml_get_to_fp16_sycl() and ggml_get_to_fp32_sycl(), matching the
pattern already used by Q4_0, Q4_K, and Q6_K.

Fixes ggml-org#21589

AI (Claude) was used to assist with root cause investigation and
writing the kernel code. All code was human-reviewed and tested
on real hardware.

Author: PMZFX

ggml/src/ggml-sycl/convert.cpp

@@ -151,6 +151,25 @@ static void dequantize_row_q4_0_sycl_reorder(const void *vx, dst_t *y, const int
 
 }
 
+template <typename dst_t>
+static void dequantize_row_q8_0_sycl_reorder(const void *vx, dst_t *y, const int64_t k,
+                                     dpct::queue_ptr stream) {
+
+    dpct::has_capability_or_fail(stream->get_device(),
+                                    {sycl::aspect::fp16});
+
+    int constexpr WARP_K = WARP_SIZE * QK8_0;
+    const int n_warp = (k + WARP_K - 1) / WARP_K;
+    GGML_ASSERT(k % QK8_0 == 0);
+    stream->parallel_for(sycl::nd_range<3>(sycl::range<3>(1, 1, n_warp) *
+        sycl::range<3>(1, 1, WARP_SIZE),
+        sycl::range<3>(1, 1, WARP_SIZE)),
+        [=](sycl::nd_item<3> item_ct1) [[sycl::reqd_sub_group_size(WARP_SIZE)]]{
+            dequantize_block_q8_0_reorder(vx, y, k, item_ct1);
+        });
+
+}
+
 template <typename dst_t>
 static void dequantize_row_q4_1_sycl(const void *vx, dst_t *y, const int64_t k,
                                      dpct::queue_ptr stream) {
@@ -614,7 +633,12 @@ to_fp16_sycl_t ggml_get_to_fp16_sycl(ggml_type type, ggml_tensor * dst) {
         case GGML_TYPE_Q5_1:
             return dequantize_block_sycl<QK5_1, QR5_1, dequantize_q5_1>;
         case GGML_TYPE_Q8_0:
-            return dequantize_block_sycl<QK8_0, QR8_0, dequantize_q8_0>;
+            if (dst->src[0]->extra &&
+                ((ggml_tensor_extra_gpu *) dst->src[0]->extra)->optimized_feature.reorder) {
+                return dequantize_row_q8_0_sycl_reorder;
+            } else {
+                return dequantize_block_sycl<QK8_0, QR8_0, dequantize_q8_0>;
+            }
         case GGML_TYPE_Q2_K:
             return dequantize_row_q2_K_sycl;
         case GGML_TYPE_Q3_K:
@@ -683,7 +707,12 @@ to_fp32_sycl_t ggml_get_to_fp32_sycl(ggml_type type, ggml_tensor *dst) {
         case GGML_TYPE_Q5_1:
             return dequantize_block_sycl<QK5_1, QR5_1, dequantize_q5_1>;
         case GGML_TYPE_Q8_0:
-            return dequantize_block_sycl<QK8_0, QR8_0, dequantize_q8_0>;
+            if (dst->src[0]->extra &&
+                ((ggml_tensor_extra_gpu*)dst->src[0]->extra)->optimized_feature.reorder) {
+                return dequantize_row_q8_0_sycl_reorder;
+            } else {
+                return dequantize_block_sycl<QK8_0, QR8_0, dequantize_q8_0>;
+            }
         case GGML_TYPE_Q2_K:
             return dequantize_row_q2_K_sycl;
         case GGML_TYPE_Q3_K:

ggml/src/ggml-sycl/dequantize.hpp

@@ -238,6 +238,34 @@ static void dequantize_block_q4_0_reorder(const void * __restrict__ vx, dst_t *
 
 }
 
+// Dequantize Q8_0 from reorder layout: [all qs (k bytes)][all d values]
+// Each thread handles one block of QK8_0 elements.
+template<typename dst_t>
+static void dequantize_block_q8_0_reorder(const void * __restrict__ vx, dst_t * __restrict__ yy, int64_t k,
+                                  const sycl::nd_item<3> &item_ct1) {
+
+    const int64_t i = item_ct1.get_group(2);
+    const int64_t tid = item_ct1.get_local_id(2);
+    const int lane_ib = i * WARP_SIZE + tid;
+
+    if (lane_ib >= k / QK8_0) {
+        return;
+    }
+
+    dst_t * y_ptr = yy + lane_ib * QK8_0;
+
+    auto qs = (const int8_t*)vx + lane_ib * QK8_0;
+    auto s_ptr = (const sycl::half*)((const uint8_t*)vx + k) + lane_ib;
+
+    const float d = float(*s_ptr);
+
+#pragma unroll
+    for (int l = 0; l < QK8_0; ++l) {
+        y_ptr[l] = d * qs[l];
+    }
+
+}
+
 template<typename dst_t>
 static void dequantize_block_q4_1(const void * __restrict__ vx, dst_t * __restrict__ yy, int64_t nb32,
                                   const sycl::nd_item<3> &item_ct1) {