lookahead : add example for lookahead decoding (#4207)

* lookahead : init

* lookahead : generate and store n-grams

* lookahead : use loop instead recursion to generate n-grams

* lookahead : initial working implementation

* lookahead : filter repeating n-grams

* lookahead : use deterministic init

* lookahead : add to Makefile

* lookahead : fix a bug in the seq_id of the lookahead tokens

* lookahead : add comments

---------

Co-authored-by: slaren <slarengh@gmail.com>

.gitignore

@@ -47,6 +47,7 @@ models-mnt
 /libllama.so
 /llama-bench
 /llava-cli
+/lookahead
 /main
 /metal
 /perplexity

Makefile

@@ -2,7 +2,7 @@
 BUILD_TARGETS = \
 	main quantize quantize-stats perplexity embedding vdot q8dot train-text-from-scratch convert-llama2c-to-ggml \
 	simple batched batched-bench save-load-state server gguf llama-bench libllava.a llava-cli baby-llama beam-search  \
-	speculative infill tokenize benchmark-matmult parallel finetune export-lora tests/test-c.o
+	speculative infill tokenize benchmark-matmult parallel finetune export-lora lookahead tests/test-c.o
 
 # Binaries only useful for tests
 TEST_TARGETS = \
@@ -657,6 +657,9 @@ speculative: examples/speculative/speculative.cpp ggml.o llama.o $(COMMON_DEPS)
 parallel: examples/parallel/parallel.cpp ggml.o llama.o $(COMMON_DEPS) $(OBJS)
 	$(CXX) $(CXXFLAGS) $(filter-out %.h,$^) -o $@ $(LDFLAGS)
 
+lookahead: examples/lookahead/lookahead.cpp ggml.o llama.o $(COMMON_DEPS) $(OBJS)
+	$(CXX) $(CXXFLAGS) $(filter-out %.h,$^) -o $@ $(LDFLAGS)
+
 ifdef LLAMA_METAL
 metal: examples/metal/metal.cpp ggml.o $(OBJS)
 	$(CXX) $(CXXFLAGS) $^ -o $@ $(LDFLAGS)

examples/CMakeLists.txt

@@ -32,6 +32,7 @@ else()
     add_subdirectory(save-load-state)
     add_subdirectory(simple)
     add_subdirectory(speculative)
+    add_subdirectory(lookahead)
     add_subdirectory(train-text-from-scratch)
     if (LLAMA_METAL)
         add_subdirectory(metal)

examples/lookahead/CMakeLists.txt

@@ -0,0 +1,5 @@
+set(TARGET lookahead)
+add_executable(${TARGET} lookahead.cpp)
+install(TARGETS ${TARGET} RUNTIME)
+target_link_libraries(${TARGET} PRIVATE common llama ${CMAKE_THREAD_LIBS_INIT})
+target_compile_features(${TARGET} PRIVATE cxx_std_11)

examples/lookahead/lookahead.cpp

@@ -0,0 +1,487 @@
+#include "common.h"
+#include "llama.h"
+
+#include <cmath>
+#include <cstdio>
+#include <string>
+#include <vector>
+
+struct ngram_data {
+    bool active = false;
+
+    llama_seq_id seq_id = -1;
+
+    std::vector<int> i_batch;
+
+    std::vector<llama_token> tokens;
+};
+
+// n-gram container
+struct ngram_container {
+    ngram_container(int n_vocab, int N, int G) {
+        cnt.resize(n_vocab);
+        head.resize(n_vocab);
+        tokens.resize(n_vocab * G * (N - 1));
+    }
+
+    int n_total = 0;
+
+    std::vector<int> cnt;
+    std::vector<int> head;
+
+    // [n_vocab][G][N - 1]
+    // for each token of the vocab, keep a ring-buffer of capacity G of n-grams of size N - 1
+    std::vector<llama_token> tokens;
+};
+
+int main(int argc, char ** argv) {
+    gpt_params params;
+
+    if (gpt_params_parse(argc, argv, params) == false) {
+        return 1;
+    }
+
+    const int W = 15; // lookahead window
+    const int N = 5;  // n-gram size
+    const int G = 15; // max verification n-grams
+
+    const bool dump_kv_cache = params.dump_kv_cache;
+
+#ifndef LOG_DISABLE_LOGS
+    log_set_target(log_filename_generator("lookahead", "log"));
+    LOG_TEE("Log start\n");
+    log_dump_cmdline(argc, argv);
+#endif // LOG_DISABLE_LOGS
+
+    // init llama.cpp
+    llama_backend_init(params.numa);
+
+    llama_model * model = NULL;
+    llama_context * ctx = NULL;
+
+    // load the target model
+    std::tie(model, ctx) = llama_init_from_gpt_params(params);
+
+    // Tokenize the prompt
+    const bool add_bos = llama_should_add_bos_token(model);
+    LOG("add_bos tgt: %d\n", add_bos);
+
+    std::vector<llama_token> inp;
+    std::vector<llama_token> all;
+
+    inp = ::llama_tokenize(ctx, params.prompt, add_bos, true);
+    all = inp;
+
+    const int max_context_size     = llama_n_ctx(ctx);
+    const int max_tokens_list_size = max_context_size - 4;
+
+    if ((int) inp.size() > max_tokens_list_size) {
+        fprintf(stderr, "%s: error: prompt too long (%d tokens, max %d)\n", __func__, (int) inp.size(), max_tokens_list_size);
+        return 1;
+    }
+
+    fprintf(stderr, "\n\n");
+
+    for (auto id : inp) {
+        fprintf(stderr, "%s", llama_token_to_piece(ctx, id).c_str());
+    }
+
+    fflush(stderr);
+
+    const int n_input = inp.size();
+
+    const auto t_enc_start = ggml_time_us();
+
+    // eval the prompt
+    llama_decode(ctx, llama_batch_get_one( inp.data(), n_input - 1, 0,           0));
+    llama_decode(ctx, llama_batch_get_one(&inp.back(),           1, n_input - 1, 0));
+
+    for (int s = 1; s < W + G + 1; ++s) {
+        llama_kv_cache_seq_cp(ctx, 0, s, -1, -1);
+    }
+
+    const auto t_enc_end = ggml_time_us();
+
+    int n_predict = 0;
+    int n_accept  = 0;
+
+    int n_past = inp.size();
+
+    llama_token id = 0;
+
+    // used to determine end of generation
+    bool has_eos = false;
+
+    // for each decoded batch, we have at most W + G + 1 distinct sequences:
+    // seq_id == 0           : the current input token
+    // seq_id [1, W]         : tokens from the past N - 1 Jacobi iterations
+    // seq_id [W + 1, W + G] : verification n-grams
+    llama_batch batch = llama_batch_init(params.n_ctx, 0, W + G + 1);
+
+    // target model sampling context
+    struct llama_sampling_context * ctx_sampling = llama_sampling_init(params.sparams);
+
+    // verification n-grams
+    std::vector<ngram_data> ngrams_cur(G);
+
+    // tokens for the past N - 1 Jacobi iterations
+    std::vector<llama_token> tokens_j_prev(W);
+    std::vector<std::vector<llama_token>> tokens_j(N - 1);
+    for (int j = 0; j < N - 1; j++) {
+        tokens_j[j].resize(W);
+
+        for (int i = 0; i < W; i++) {
+            // there are different ways to init these tokens
+            if (0) {
+                // initialize randomly from the prompt tokens
+                tokens_j[j][i] = all[1 + rand() % (all.size() - 1)];
+            } else {
+                // initialize with a sequence of increasing numbers
+                tokens_j[j][i] = 100 + i;
+            }
+        }
+    }
+
+    std::vector<llama_seq_id> seq_id_look;
+
+    // the input token belongs both to all sequences
+    std::vector<llama_seq_id> seq_id_all(W + G + 1);
+    for (int i = 0; i < W + G + 1; i++) {
+        seq_id_all[i] = i;
+    }
+
+    // here we keep adding new n-grams as we go
+    ngram_container ngrams_observed(llama_n_vocab(model), N, G);
+
+    // debug
+    struct llama_kv_cache_view kvc_view = llama_kv_cache_view_init(ctx, W + G + 1);
+
+    const auto t_dec_start = ggml_time_us();
+
+    // sample first token
+    {
+        id = llama_sampling_sample(ctx_sampling, ctx, NULL, 0);
+
+        llama_sampling_accept(ctx_sampling, ctx, id, true);
+
+        {
+            const std::string token_str = llama_token_to_piece(ctx, id);
+
+            printf("%s", token_str.c_str());
+            fflush(stdout);
+        }
+    }
+
+    while (true) {
+        // debug
+        if (dump_kv_cache) {
+            llama_kv_cache_view_update(ctx, &kvc_view);
+            dump_kv_cache_view_seqs(kvc_view, 40);
+        }
+
+        // build the mask from https://lmsys.org/blog/2023-11-21-lookahead-decoding/
+        //
+        // Example for W = 5, N = 4, G = 2:
+        // (I = input, L = lookahead, V = verification)
+        //
+        // Batch:  0  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20
+        // T:        -2 -2 -2 -2 -1 -1 -1 -1 -1  0  0  0  0  0  0
+        // Info:   I  L  L  L  L  L  L  L  L  L  L  L  L  L  L  V  V  V  V  V  V
+        // Pos:    0  1  2  3  4  1  2  3  4  5  2  3  4  5  6  1  2  3  1  2  3   (+ n_past)
+        // Logits: 1  0  0  0  0  0  0  0  0  0  1  1  1  1  1  1  1  1  1  1  1
+        // ---------------------------------------------------------------------
+        // Seq:    0
+        //         1              1              1
+        //         2  2              2              2
+        //         3  3  3              3              3
+        //         4  4  4  4              4              4
+        //         5  5  5  5  5              5              5
+        //         6                                            6  6  6
+        //         7                                                     7  7  7
+        // ---------------------------------------------------------------------
+        //                                       |  |  |  |  |  |  |  |  |  |  |
+        //                                       V  V  V  V  V  |  |  |  |  |  |
+        //                                         j_tokens     |  |  |  |  |  |
+        //                                                      V  V  V  V  V  V
+        //                                                             id
+        {
+            llama_batch_clear(batch);
+
+            // current token - first token of the first level
+            llama_batch_add(batch, id, n_past, seq_id_all, true);
+
+            // verification n-grams - queue this before the lookahead tokens for less KV cache fragmentation
+            {
+                const int g_cur = ngrams_observed.cnt[id];
+
+                ngrams_cur.resize(g_cur);
+                for (int g = 0; g < g_cur; g++) {
+                    ngrams_cur[g].active = true;
+                    ngrams_cur[g].tokens.resize(N);
+                    ngrams_cur[g].i_batch.resize(N);
+                    ngrams_cur[g].seq_id = W + 1 + g;
+                    ngrams_cur[g].i_batch[0] = 0;
+                    ngrams_cur[g].tokens [0] = id;
+                }
+
+                for (int j = 0; j < N - 1; j++) {
+                    for (int g = 0; g < g_cur; g++) {
+                        const int idx = id*(N - 1)*G + g*(N - 1);
+
+                        const llama_token t = ngrams_observed.tokens[idx + j];
+
+                        ngrams_cur[g].tokens [j + 1] = t;
+                        ngrams_cur[g].i_batch[j + 1] = batch.n_tokens;
+
+                        llama_batch_add(batch, t, n_past + j + 1, { W + 1 + g }, true);
+                    }
+                }
+            }
+
+            // fill the remaining W - 1 tokens for the first level
+            for (int i = 1; i < W; i++) {
+                seq_id_look.resize(W - i);
+                for (int j = 0; j < W - i; j++) {
+                    seq_id_look[j] = i + j + 1;
+                }
+
+                llama_batch_add(batch, tokens_j[0][i], n_past + i, seq_id_look, false);
+            }
+
+            // fill the rest of the levels
+            for (int j = 1; j < N - 1; j++) {
+                for (int i = 0; i < W; i++) {
+                    llama_batch_add(batch, tokens_j[j][i], n_past + j + i, { i + 1 }, j == N - 2);
+                }
+            }
+        }
+
+        if (llama_decode(ctx, batch) != 0) {
+            fprintf(stderr, "\n\n%s: error: llama_decode failed - increase KV cache size\n", __func__);
+            return 1;
+        }
+
+        int seq_id_best = 0;
+
+        for (int v = 0; v < N; ++v) {
+            int i_batch = 0;
+
+            // if no active ngrams are left, it means the sampled token does not pass the verification
+            if (v > 0) {
+                for (int g = 0; g < (int) ngrams_cur.size(); g++) {
+                    if (ngrams_cur[g].active) {
+                        i_batch = ngrams_cur[g].i_batch[v];
+                        seq_id_best = ngrams_cur[g].seq_id;
+
+                        ++n_accept;
+                        break;
+                    }
+                }
+
+                // no more matches -> create a new batch
+                if (i_batch == 0) {
+                    break;
+                }
+            }
+
+            // sample the next token
+            id = llama_sampling_sample(ctx_sampling, ctx, NULL, i_batch);
+
+            llama_sampling_accept(ctx_sampling, ctx, id, true);
+
+            // print
+            {
+                const std::string token_str = llama_token_to_piece(ctx, id);
+
+                if (v == 0) {
+                    printf("%s", token_str.c_str());
+                } else {
+                    // print light cyan
+                    printf("\033[0;96m%s\033[0m", token_str.c_str());
+                }
+                fflush(stdout);
+
+                if (id == llama_token_eos(model)) {
+                    has_eos = true;
+                }
+
+                all.push_back(id);
+            }
+
+            ++n_predict;
+            ++n_past;
+
+            if (n_predict > params.n_predict || has_eos) {
+                break;
+            }
+
+            // verify across active n-grams
+            for (int g = 0; g < (int) ngrams_cur.size(); g++) {
+                if (ngrams_cur[g].active) {
+                    if (v == N - 1) {
+                        ngrams_cur[g].active = false;
+                    } else {
+                        if (id != ngrams_cur[g].tokens[v + 1]) {
+                            ngrams_cur[g].active = false;
+                        }
+                    }
+                }
+            }
+
+            // print known n-grams starting with token id (debug)
+            if (0 && v == 0) {
+                if (ngrams_observed.cnt[id] > 0) {
+                    printf("\n - %d n-grams starting with '%s'\n", ngrams_observed.cnt[id], llama_token_to_piece(ctx, id).c_str());
+                }
+
+                for (int i = 0; i < ngrams_observed.cnt[id]; i++) {
+                    printf("   - ngram %2d: ", i);
+
+                    const int idx = id*(N - 1)*G + i*(N - 1);
+
+                    for (int j = 0; j < N - 1; j++) {
+                        const std::string token_str = llama_token_to_piece(ctx, ngrams_observed.tokens[idx + j]);
+
+                        printf("%s", token_str.c_str());
+                    }
+
+                    printf("\n");
+                }
+            }
+
+            // update lookahead tokens
+            {
+                for (int i = 0; i < W; i++) {
+                    tokens_j_prev[i] = tokens_j[0][i];
+                }
+
+                for (int j = 0; j < N - 2; j++) {
+                    tokens_j[j] = tokens_j[j + 1];
+                }
+
+                if (v == 0) {
+                    // sample from the last level
+                    for (int i = 0; i < W; i++) {
+                        tokens_j[N - 2][i] = llama_sampling_sample(ctx_sampling, ctx, NULL, ngrams_cur.size()*(N-1) + W*(N - 2) + i);
+                    }
+                } else {
+                    for (int i = 0; i < W; i++) {
+                        // there are different ways to init these tokens
+                        if (0) {
+                            // random init
+                            tokens_j[N - 2][i] = all[1 + rand() % (all.size() - 1)];
+                        } else {
+                            // init from the previous level
+                            tokens_j[N - 2][i] = tokens_j[0][i];
+                        }
+                    }
+                }
+            }
+
+            // update observed ngrams
+            if (v == 0) {
+                // the first token of the n-gram is determined by the index in the container so it is not stored
+                std::vector<llama_token> ngram(N - 1);
+
+                // n-gram generation
+                // ref: https://github.com/hao-ai-lab/LookaheadDecoding/issues/14#issuecomment-1826198518
+                for (int f = 0; f < W; ++f) {
+                    const int ft = tokens_j_prev[f]; // first token of the n-gram
+
+                    for (int j = 0; j < N - 1; ++j) {
+                        ngram[j] = tokens_j[j][f];
+                    }
+
+                    // filter-out repeating n-grams
+                    {
+                        bool is_unique = true;
+
+                        for (int k = 0; k < ngrams_observed.cnt[ft]; ++k) {
+                            const int idx = ft*(N - 1)*G + k*(N - 1);
+
+                            bool is_match = true;
+                            for (int j = 0; j < N - 1; ++j) {
+                                if (ngrams_observed.tokens[idx + j] != ngram[j]) {
+                                    is_match = false;
+                                    break;
+                                }
+                            }
+
+                            if (is_match) {
+                                is_unique = false;
+                                break;
+                            }
+                        }
+
+                        if (!is_unique) {
+                            continue;
+                        }
+                    }
+
+                    const int head = ngrams_observed.head[ft];
+                    const int idx  = ft*(N - 1)*G + head*(N - 1);
+
+                    for (int i = 0; i < N - 1; i++) {
+                        ngrams_observed.tokens[idx + i] = ngram[i];
+                    }
+
+                    ngrams_observed.cnt[ft]  = std::min(G, ngrams_observed.cnt[ft] + 1);
+                    ngrams_observed.head[ft] = (head + 1) % G;
+
+                    ngrams_observed.n_total++;
+                }
+            }
+        }
+
+        if (n_predict > params.n_predict || has_eos) {
+            break;
+        }
+
+        // KV cache management
+        // if no verification token matched, we simply remove all cells from this batch -> no fragmentation
+        llama_kv_cache_seq_rm(ctx, -1, n_past, -1);
+
+        if (seq_id_best != 0) {
+            // if a verification token matched, we keep the best sequence and remove the rest
+            // this leads to some KV cache fragmentation
+            llama_kv_cache_seq_keep(ctx, seq_id_best);
+            llama_kv_cache_seq_cp  (ctx, seq_id_best, 0, -1, -1);
+            llama_kv_cache_seq_rm  (ctx, seq_id_best,    -1, -1);
+
+            for (int s = 1; s < W + G + 1; ++s) {
+                llama_kv_cache_seq_cp(ctx, 0, s, -1, -1);
+            }
+        }
+    }
+
+    auto t_dec_end = ggml_time_us();
+
+    LOG_TEE("\n\n");
+
+    LOG_TEE("encoded %4d tokens in %8.3f seconds, speed: %8.3f t/s\n", n_input,   (t_enc_end - t_enc_start) / 1e6f, inp.size() / ((t_enc_end - t_enc_start) / 1e6f));
+    LOG_TEE("decoded %4d tokens in %8.3f seconds, speed: %8.3f t/s\n", n_predict, (t_dec_end - t_dec_start) / 1e6f, n_predict  / ((t_dec_end - t_dec_start) / 1e6f));
+
+    LOG_TEE("\n");
+    LOG_TEE("W = %2d\n", W);
+    LOG_TEE("N = %2d\n", N);
+    LOG_TEE("G = %2d\n", G);
+    LOG_TEE("\n");
+    LOG_TEE("n_predict = %d\n", n_predict);
+    LOG_TEE("n_accept  = %d\n", n_accept);
+
+    llama_print_timings(ctx);
+
+    llama_kv_cache_view_free(&kvc_view);
+    llama_sampling_free(ctx_sampling);
+
+    llama_batch_free(batch);
+
+    llama_free(ctx);
+    llama_free_model(model);
+
+    llama_backend_free();
+
+    fprintf(stderr, "\n\n");
+
+    return 0;
+}