ggml-zendnn : adaptive fallback to CPU backend for small batch sizes (#22681)

* ggml-zendnn : add runtime env var GGML_ZENDNN_ADAPTIVE_FALLBACK to control adaptive fallback (default: enabled)

* ggml-zendnn : restore original fallback logic when adaptive fallback is disabled

Author: z-sachin

ggml/src/ggml-zendnn/CMakeLists.txt

@@ -28,7 +28,7 @@ if (NOT ZENDNN_ROOT OR ZENDNN_ROOT STREQUAL "" OR ZENDNN_ROOT STREQUAL "OFF")
     ExternalProject_Add(
         zendnn
         GIT_REPOSITORY https://github.com/amd/ZenDNN.git
-        GIT_TAG f79f7321a1add65ced6397a6bfab7edba6e3e14e    # ZenDNN-2026-WW13
+        GIT_TAG ac9e580d9434b7b98985f2627a7ebfb5eba4bb0d    # ZenDNN-2026-WW17
         PREFIX      ${ZENDNN_PREFIX}
         SOURCE_DIR  ${ZENDNN_SOURCE_DIR}
         BINARY_DIR  ${ZENDNN_BUILD_DIR}

ggml/src/ggml-zendnn/ggml-zendnn.cpp

@@ -47,6 +47,7 @@ static bool ggml_zendnn_matmul(ggml_backend_zendnn_context * ctx, int64_t m, int
     params.dtypes.dst = ggml_to_zendnn_type<TC>();
     params.num_threads = ctx->n_threads;
 
+    zendnnl::lowoha::matmul::matmul_batch_params_t batch_params;
     zendnnl::error_handling::status_t status = zendnnl::lowoha::matmul::matmul_direct(
         'r', false, true,   // row-major, don't transpose B, transpose A (because it's column-major)
         n,                  // M: rows of B and C
@@ -59,7 +60,7 @@ static bool ggml_zendnn_matmul(ggml_backend_zendnn_context * ctx, int64_t m, int
         0.0f,               // beta
         C, ldc,             // output C[n,m]
         true,               // is_weights_const
-        {},                 // batch_params
+        batch_params,       // batch_params
         params              // params
     );
 
@@ -520,6 +521,12 @@ static ggml_backend_buffer_t ggml_backend_zendnn_device_buffer_from_host_ptr(ggm
     GGML_UNUSED(max_tensor_size);
 }
 
+static bool ggml_zendnn_adaptive_fallback_enabled() {
+    static const bool enabled = std::getenv("GGML_ZENDNN_ADAPTIVE_FALLBACK") == nullptr ||
+                                std::atoi(std::getenv("GGML_ZENDNN_ADAPTIVE_FALLBACK")) != 0;
+    return enabled;
+}
+
 static bool ggml_backend_zendnn_device_supports_op(ggml_backend_dev_t dev, const struct ggml_tensor * op) {
     switch (op->op) {
         case GGML_OP_NONE:
@@ -538,12 +545,24 @@ static bool ggml_backend_zendnn_device_supports_op(ggml_backend_dev_t dev, const
             const int64_t ne10 = inputs->ne[0];
             const int64_t ne0 = op->ne[0];
             const int64_t ne1 = op->ne[1];
-
             const int64_t min_batch = 1;
-            if (!ggml_is_contiguous(weights) || !ggml_is_contiguous(inputs) ||
-                ne0 < min_batch || ne1 < min_batch || ne10 < min_batch) {
+
+            if(!ggml_is_contiguous(weights) || !ggml_is_contiguous(inputs)) {
+                return false;
+            }
+
+            if (ggml_zendnn_adaptive_fallback_enabled()) {
+                const int64_t K = inputs->ne[0];
+                const int64_t N = (inputs->ne[1]*inputs->ne[2]*inputs->ne[3]);
+                const int64_t M = weights->ne[1];
+                if(K <= 256 || N <= 128 || M <= 96) {
                     return false;
+                }
             }
+            else if (ne0 < min_batch || ne1 < min_batch || ne10 < min_batch) {
+                return false;
+            }
+
             // MUL_MAT_ID performs best with a moderate number of experts due to its
             // gather + batched matmul + scatter approach. Future versions will leverage
             // ZenDNN's grouped_gemm for better scalability with larger expert counts: