Introduce Virtual Register abstraction to improve register allocation

src/arm-codegen.c

@@ -485,16 +485,18 @@ void code_generate(void)
         emit_ph2_ir(ph2_ir);
 
     /* prepare 'argc' and 'argv', then proceed to 'main' function */
-    emit(__movw(__AL, __r8, GLOBAL_FUNC->stack_size));
-    emit(__movt(__AL, __r8, GLOBAL_FUNC->stack_size));
-    emit(__add_r(__AL, __r8, __r12, __r8));
-    emit(__lw(__AL, __r0, __r8, 0));
-    emit(__add_i(__AL, __r1, __r8, 4));
-    emit(__bl(__AL, MAIN_BB->elf_offset - elf_code->size));
+    if (MAIN_BB) {
+        emit(__movw(__AL, __r8, GLOBAL_FUNC->stack_size));
+        emit(__movt(__AL, __r8, GLOBAL_FUNC->stack_size));
+        emit(__add_r(__AL, __r8, __r12, __r8));
+        emit(__lw(__AL, __r0, __r8, 0));
+        emit(__add_i(__AL, __r1, __r8, 4));
+        emit(__bl(__AL, MAIN_BB->elf_offset - elf_code->size));
 
-    /* exit with main's return value - r0 already has the return value */
-    emit(__mov_i(__AL, __r7, 1));
-    emit(__svc());
+        /* exit with main's return value - r0 already has the return value */
+        emit(__mov_i(__AL, __r7, 1));
+        emit(__svc());
+    }
 
     for (int i = 0; i < ph2_ir_idx; i++) {
         ph2_ir = PH2_IR_FLATTEN[i];

src/defs.h

@@ -371,7 +371,14 @@ struct var {
     int consumed;
     bool is_ternary_ret;
     bool is_logical_ret;
-    bool is_const; /* whether a constant representaion or not */
+    bool is_const;  /* whether a constant representaion or not */
+    int vreg_id;    /* Virtual register ID */
+    int phys_reg;   /* Physical register assignment (-1 if unassigned) */
+    int vreg_flags; /* VReg flags */
+    int first_use;  /* First instruction index where variable is used */
+    int last_use;   /* Last instruction index where variable is used */
+    int loop_depth; /* Nesting depth if variable is in a loop */
+    int use_count;  /* Number of times variable is used */
 };
 
 typedef struct {

src/parser.c

@@ -63,6 +63,11 @@ var_t *require_var(block_t *blk)
     var_t *var = arena_calloc(BLOCK_ARENA, 1, sizeof(var_t));
     var_list->elements[var_list->size++] = var;
     var->consumed = -1;
+    var->phys_reg = -1;
+    var->first_use = -1;
+    var->last_use = -1;
+    var->loop_depth = 0;
+    var->use_count = 0;
     var->base = var;
     var->type = TY_int;
     return var;