Introduce background compilation

Given the significant runtime compilation overhead associated with
performing aggressive optimizations, we have implemented a background
compilation mechanism to mitigate this issue. When the runtime profiler
identifies a strong hotspot, it adds a T2C compilation request to the
wait queue. A background thread, which continuously monitors this queue,
triggers T2C to process the requests and notifies the main thread upon
completion by updating a flag.

This mechanism defers the execution of T2C-generated machine code,
leading to more frequent use of T1C-generated code. Despite this, the
approach effectively minimizes runtime compilation delays by eliminating
the need for the main thread to wait for T2C compilation to complete,
thereby improving overall performance.

Close: sysprog21#239

.github/workflows/main.yml

@@ -75,7 +75,7 @@ jobs:
     - name: undefined behavior test
       run: |
             make clean && make ENABLE_UBSAN=1 check -j$(nproc)
-            make ENABLE_JIT=1 clean clean && make ENABLE_JIT=1 ENABLE_UBSAN=1 check -j$(nproc)
+            make ENABLE_JIT=1 clean && make ENABLE_JIT=1 ENABLE_UBSAN=1 check -j$(nproc)
 
   host-arm64:
     needs: [detect-code-related-file-changes]
@@ -134,6 +134,9 @@ jobs:
       run: |
             sudo apt-get update -q -y
             sudo apt-get install -q -y clang clang-tools libsdl2-dev libsdl2-mixer-dev
+            wget https://apt.llvm.org/llvm.sh
+            chmod +x ./llvm.sh
+            sudo ./llvm.sh 17
       shell: bash
     - name: run scan-build without JIT
       run: make distclean && scan-build -v -o ~/scan-build --status-bugs --use-cc=clang --force-analyze-debug-code --show-description -analyzer-config stable-report-filename=true -enable-checker valist,nullability make ENABLE_EXT_F=0 ENABLE_SDL=0 ENABLE_JIT=0

src/emulate.c

@@ -308,6 +308,9 @@ static block_t *block_alloc(riscv_t *rv)
     block->has_loops = false;
     block->n_invoke = 0;
     INIT_LIST_HEAD(&block->list);
+#if RV32_HAS(T2C)
+    block->compiled = false;
+#endif
 #endif
     return block;
 }
@@ -993,13 +996,14 @@ void rv_step(void *arg)
             ((exec_t2c_func_t) block->func)(rv);
             prev = NULL;
             continue;
-        } /* check if the execution path is strong hotspot */
-        if (block->n_invoke >= THRESHOLD) {
-            t2c_compile(block,
-                        (uint64_t) ((memory_t *) PRIV(rv)->mem)->mem_base);
-            ((exec_t2c_func_t) block->func)(rv);
-            prev = NULL;
-            continue;
+        } /* check if invoking times of t1 generated code exceed threshold */
+        else if (!block->compiled && block->n_invoke >= THRESHOLD) {
+            block->compiled = true;
+            queue_entry_t *entry = malloc(sizeof(queue_entry_t));
+            entry->block = block;
+            pthread_mutex_lock(&rv->wait_queue_lock);
+            list_add(&entry->list, &rv->wait_queue);
+            pthread_mutex_unlock(&rv->wait_queue_lock);
         }
 #endif
         /* executed through the tier-1 JIT compiler */

src/feature.h

@@ -50,12 +50,11 @@
 /* Experimental just-in-time compiler */
 #ifndef RV32_FEATURE_JIT
 #define RV32_FEATURE_JIT 0
-#endif
-
 /* Experimental tier-2 just-in-time compiler */
 #ifndef RV32_FEATURE_T2C
 #define RV32_FEATURE_T2C 0
 #endif
+#endif
 
 /* Feature test macro */
 #define RV32_HAS(x) RV32_FEATURE_##x

src/riscv.c

@@ -28,6 +28,9 @@
 #include "riscv_private.h"
 #include "utils.h"
 #if RV32_HAS(JIT)
+#if RV32_HAS(T2C)
+#include <pthread.h>
+#endif
 #include "cache.h"
 #include "jit.h"
 #define CODE_CACHE_SIZE (4 * 1024 * 1024)
@@ -184,6 +187,31 @@ IO_HANDLER_IMPL(byte, write_b, W)
 #undef R
 #undef W
 
+#if RV32_HAS(T2C)
+static pthread_t t2c_thread;
+static void *t2c_runloop(void *arg)
+{
+    riscv_t *rv = (riscv_t *) arg;
+    while (1) {
+        if (!list_empty(&rv->wait_queue)) {
+            queue_entry_t *entry =
+                list_last_entry(&rv->wait_queue, queue_entry_t, list);
+            pthread_mutex_lock(&rv->wait_queue_lock);
+            list_del_init(&entry->list);
+            pthread_mutex_unlock(&rv->wait_queue_lock);
+            t2c_compile(entry->block,
+                        (uint64_t) ((memory_t *) PRIV(rv)->mem)->mem_base);
+            free(entry);
+        }
+        /* Instead of writing while(rv->quit), placing the code here prevents
+         * rv->quit from being optimized by the compiler.*/
+        if (rv->quit)
+            break;
+    }
+    return NULL;
+}
+#endif
+
 riscv_t *rv_create(riscv_user_t rv_attr)
 {
     assert(rv_attr);
@@ -269,6 +297,14 @@ riscv_t *rv_create(riscv_user_t rv_attr)
     rv->jit_state = jit_state_init(CODE_CACHE_SIZE);
     rv->block_cache = cache_create(BLOCK_MAP_CAPACITY_BITS);
     assert(rv->block_cache);
+#if RV32_HAS(T2C)
+    rv->quit = false;
+    /* prepare wait queue. */
+    pthread_mutex_init(&rv->wait_queue_lock, NULL);
+    INIT_LIST_HEAD(&rv->wait_queue);
+    /* activate the background compilation thread. */
+    pthread_create(&t2c_thread, NULL, t2c_runloop, rv);
+#endif
 #endif
 
     return rv;
@@ -353,6 +389,11 @@ void rv_delete(riscv_t *rv)
     memory_delete(attr->mem);
     block_map_destroy(rv);
 #else
+#if RV32_HAS(T2C)
+    rv->quit = true;
+    pthread_join(t2c_thread, NULL);
+    pthread_mutex_destroy(&rv->wait_queue_lock);
+#endif
     mpool_destroy(rv->chain_entry_mp);
     jit_state_exit(rv->jit_state);
     cache_free(rv->block_cache);

src/riscv_private.h

@@ -14,6 +14,9 @@
 #include "riscv.h"
 #include "utils.h"
 #if RV32_HAS(JIT)
+#if RV32_HAS(T2C)
+#include <pthread.h>
+#endif
 #include "cache.h"
 #endif
 
@@ -70,7 +73,10 @@ typedef struct block {
     bool
         translatable; /**< Determine the block has RV32AF insturctions or not */
     bool has_loops;   /**< Determine the block has loop or not */
-    uint32_t offset;  /**< The machine code offset in T1 code cache */
+#if RV32_HAS(T2C)
+    bool compiled; /**< The T2C request is enqueued or not */
+#endif
+    uint32_t offset;   /**< The machine code offset in T1 code cache */
     uint32_t n_invoke; /**< The invoking times of T1 machine code */
     void *func;        /**< The function pointer of T2 machine code */
     struct list_head list;
@@ -82,6 +88,12 @@ typedef struct {
     block_t *block;
     struct list_head list;
 } chain_entry_t;
+#if RV32_HAS(T2C)
+typedef struct {
+    block_t *block;
+    struct list_head list;
+} queue_entry_t;
+#endif
 #endif
 
 typedef struct {
@@ -134,6 +146,11 @@ struct riscv_internal {
 #else
     struct cache *block_cache;
     struct mpool *chain_entry_mp;
+#if RV32_HAS(T2C)
+    struct list_head wait_queue;
+    pthread_mutex_t wait_queue_lock;
+    bool quit; /**< Determine the main thread is terminated or not */
+#endif
 #endif
     struct mpool *block_mp, *block_ir_mp;