spec: Add benchmark-only synthetic speculative acceptance options (#27711)

* Add benchmark-only synthetic speculative acceptance to llama-server and llama-cli

* Address review comments

* Address review comments

* Add some comments in the code

tools/server/server-context.cpp

@@ -23,6 +23,7 @@
 #include <exception>
 #include <memory>
 #include <filesystem>
+#include <random>
 #include <utility>
 #include <fstream>
 
@@ -51,6 +52,50 @@ static common_speculative_output_limits server_output_limits(const common_params
     return result;
 }
 
+// synthetic draft verification for benchmarking - accept draft tokens at random instead of by match with the target
+// on replay the draft was already accepted before a context checkpoint restore, so repeat the same decisions
+static std::vector<llama_token> server_sample_and_accept_synth(
+        common_sampler * smpl,
+        llama_context * ctx,
+        const std::vector<int32_t> & idxs,
+        const llama_tokens & draft,
+        const std::vector<double> & synth_probs,
+        std::mt19937 & rng,
+        bool is_replay) {
+    GGML_ASSERT(idxs.size() == draft.size() + 1);
+    GGML_ASSERT(synth_probs.size() >= draft.size());
+
+    std::vector<llama_token> result;
+    result.reserve(idxs.size());
+
+    const llama_vocab * vocab = llama_model_get_vocab(llama_get_model(ctx));
+    std::uniform_real_distribution<double> dist(0.0, 1.0);
+    for (size_t i = 0; i < draft.size(); ++i) {
+        const llama_token id = common_sampler_sample(smpl, ctx, idxs[i]);
+        const bool accept = is_replay || dist(rng) < synth_probs[i];
+        // do not accept a drafted EOG token - it would end the generation early
+        // on replay the last token is from the target and can be EOG, so skip this check
+        if (accept && (is_replay || !llama_vocab_is_eog(vocab, draft[i]))) {
+            // synthetic draft tokens do not advance grammar or reasoning state
+            // the last replay token is from the target and must advance both
+            const bool is_replay_target = is_replay && i + 1 == draft.size();
+            common_sampler_accept(smpl, draft[i], is_replay_target);
+            result.push_back(draft[i]);
+            continue;
+        }
+
+        common_sampler_accept(smpl, id, true);
+        result.push_back(id);
+        return result;
+    }
+
+    const llama_token id = common_sampler_sample(smpl, ctx, idxs[draft.size()]);
+    common_sampler_accept(smpl, id, true);
+    result.push_back(id);
+
+    return result;
+}
+
 // state diagram: https://github.com/ggml-org/llama.cpp/pull/9283
 enum slot_state {
     SLOT_STATE_IDLE,
@@ -211,6 +256,7 @@ struct server_slot {
     std::vector<int32_t> spec_i_batch;
     common_prompt_checkpoint spec_ckpt;
     bool spec_is_replay = false;
+    std::mt19937 spec_synth_rng;
 
     // TODO: move members that belong to the task (such as `generated_text`, `has_new_line`) to task_results_state
     //       see https://github.com/ggml-org/llama.cpp/pull/18283#issuecomment-3710175837
@@ -1194,6 +1240,9 @@ private:
                 spec.reset(common_speculative_init(params_base.speculative, params_base.n_parallel));
             } catch (const std::exception & e) {
                 SRV_ERR("failed to initialize speculative decoding context: %s\n", e.what());
+                if (params_base.speculative.has_synth()) {
+                    return false;
+                }
             }
         }
 
@@ -1209,6 +1258,11 @@ private:
             model_dft = nullptr;
         }
 
+        if (!spec && params_base.speculative.has_synth()) {
+            SRV_ERR("%s", "synthetic acceptance requires an initialized speculative decoding context\n");
+            return false;
+        }
+
         for (int i = 0; i < params_base.n_parallel; i++) {
             server_slot & slot = slots[i];
 
@@ -1717,6 +1771,13 @@ private:
 
             SLT_TRC(slot, "sampler chain: %s\n", common_sampler_print(slot.smpl.get()).c_str());
             SLT_TRC(slot, "sampler params: \n%s\n", task.params.sampling.print().c_str());
+
+            if (spec && !common_speculative_get_synth_probs(spec.get()).empty()) {
+                const uint32_t seed = task.params.sampling.seed == LLAMA_DEFAULT_SEED
+                    ? std::random_device{}()
+                    : task.params.sampling.seed;
+                slot.spec_synth_rng.seed(seed);
+            }
         } else {
             slot.smpl.reset();
         }
@@ -3802,7 +3863,12 @@ private:
                 common_sampler_ptr smpl_save(common_sampler_clone(slot.smpl.get()));
 
                 GGML_ASSERT(slot.spec_i_batch.size() == n_draft + 1);
-                auto accepted = common_sampler_sample_and_accept_n(slot.smpl.get(), slot.ctx_tgt, slot.spec_i_batch, slot.spec_draft);
+                const auto & synth_probs = common_speculative_get_synth_probs(spec.get());
+                auto accepted = synth_probs.empty()
+                    ? common_sampler_sample_and_accept_n(slot.smpl.get(), slot.ctx_tgt, slot.spec_i_batch, slot.spec_draft)
+                    : server_sample_and_accept_synth(
+                            slot.smpl.get(), slot.ctx_tgt, slot.spec_i_batch, slot.spec_draft,
+                            synth_probs, slot.spec_synth_rng, slot.spec_is_replay);
                 slot.spec_i_batch.clear();
 
                 GGML_ASSERT(accepted.size() >= 1);
@@ -3868,7 +3934,7 @@ private:
 
             auto & n_accepted_per_pos = slot.n_accepted_per_pos;
             if (n_accepted_per_pos.empty()) {
-                n_accepted_per_pos.resize(common_speculative_n_max(&params_base.speculative), 0);
+                n_accepted_per_pos.resize(common_speculative_n_max(spec.get()), 0);
             }
             for (size_t i = 0; i < n_accepted && i < n_accepted_per_pos.size(); ++i) {
                 n_accepted_per_pos[i]++;