Introduce ablation study feature

Block chaining and macro-operation (MOP) fusion are accelerator
techniques designed to enhance performance. To conduct further ablation
studies, two feature options, MOP_FUSION and BLOCK_CHAINING, are
introduced. These features can be freely combined with the other
emulator features.

Ablation study of these two techniques in system emulation:
| Metric             | Block Chaining & | Block Chaining | MOP fusion |
|                    | MOP fusion       |                |            |
|--------------------|------------------|----------------|------------|
| Dhrystone (DMIPS)  | 175              | 123            | 175        |
| (1000000 runs)     |                  |                |            |
|                    |                  |                |            |
| Coremark  (Iter/s) | 222              | 204            | 222        |

Makefile

@@ -18,6 +18,14 @@ CFLAGS += -include src/common.h -Isrc/
 ENABLE_ELF_LOADER ?= 0
 $(call set-feature, ELF_LOADER)
 
+# Enable MOP fusion, easier for ablation study
+ENABLE_MOP_FUSION ?= 1
+$(call set-feature, MOP_FUSION)
+
+# Enable block chaining, easier for ablation study
+ENABLE_BLOCK_CHAINING ?= 1
+$(call set-feature, BLOCK_CHAINING)
+
 # Enable system emulation
 ENABLE_SYSTEM ?= 0
 $(call set-feature, SYSTEM)

src/emulate.c

@@ -865,8 +865,10 @@ static block_t *block_find_or_translate(riscv_t *rv)
 #if RV32_HAS(GDBSTUB)
     if (likely(!rv->debug_mode))
 #endif
+#if RV32_HAS(MOP_FUSION)
         /* macro operation fusion */
         match_pattern(rv, next_blk);
+#endif
 
 #if !RV32_HAS(JIT)
     /* insert the block into block map */
@@ -1027,6 +1029,7 @@ void rv_step(void *arg)
          * the previous block.
          */
 
+#if RV32_HAS(BLOCK_CHAINING)
         if (prev) {
             rv_insn_t *last_ir = prev->ir_tail;
             /* chain block */
@@ -1042,6 +1045,7 @@ void rv_step(void *arg)
                 }
             }
         }
+#endif
         last_pc = rv->PC;
 #if RV32_HAS(JIT)
 #if RV32_HAS(T2C)

src/feature.h

@@ -73,5 +73,15 @@
 #define RV32_FEATURE_ELF_LOADER 0
 #endif
 
+/* MOP fusion */
+#ifndef RV32_FEATURE_MOP_FUSION
+#define RV32_FEATURE_MOP_FUSION 1
+#endif
+
+/* Block chaining */
+#ifndef RV32_FEATURE_BLOCK_CHAINING
+#define RV32_FEATURE_BLOCK_CHAINING 1
+#endif
+
 /* Feature test macro */
 #define RV32_HAS(x) RV32_FEATURE_##x