py/emitnative: Generate shorter RV32 code for exception handling.

This commit lets the native emitter generate shorter code when clearing
exception objects on RV32.

Since there are no direct generic ASM functions to set a specific
immediate to a local variable, the native emitter usually generates an
immediate assignment to a temporary register and then a store of that
register into the chosen local variable.  This pattern is also followed
when clearing certain local variables related to exception handling,
using MP_OBJ_NULL as the immediate value to set.

Given that at the moment MP_OBJ_NULL is defined to be 0 (with some other
spots in the native emitter that leverage that fact when checking the
state of the variables mentioned earlier), and that the RV32 CPU has a
dedicated register that is hardwired to read 0, a new method to set
local variables to MP_OBJ_NULL is introduced.

When generating RV32 code, the new macro will skip the intermediate
register assignment and directly uses the X0/ZERO register to set the
chosen local variable to MP_OBJ_NULL.  Other platforms will still
generate the same code sequence as before this change.

This is a followup to 40585ea.

Signed-off-by: Alessandro Gatti <[email protected]>

Author: agatti

py/emitnative.c

@@ -287,12 +287,27 @@ struct _emit_t {
 #define ASM_CLR_REG(state, rd) ASM_XOR_REG_REG(state, rd, rd)
 #endif
 
+#if N_RV32
+#define ASM_MOV_LOCAL_MP_OBJ_NULL(as, local_num, reg_temp) \
+    ASM_MOV_LOCAL_REG(as, local_num, REG_ZERO)
+#else
+#define ASM_MOV_LOCAL_MP_OBJ_NULL(as, local_num, reg_temp) \
+    ASM_MOV_REG_IMM(as, reg_temp, (mp_uint_t)MP_OBJ_NULL); \
+    ASM_MOV_LOCAL_REG(as, local_num, reg_temp)
+#endif
+
 static void emit_load_reg_with_object(emit_t *emit, int reg, mp_obj_t obj);
 static void emit_native_global_exc_entry(emit_t *emit);
 static void emit_native_global_exc_exit(emit_t *emit);
 static void emit_native_load_const_obj(emit_t *emit, mp_obj_t obj);
 
 emit_t *EXPORT_FUN(new)(mp_emit_common_t * emit_common, mp_obj_t *error_slot, uint *label_slot, mp_uint_t max_num_labels) {
+    // Generated code performing exception handling assumes that MP_OBJ_NULL
+    // equals to 0 to simplify some checks, leveraging dedicated opcodes for
+    // comparisons against 0.  If this assumption does not hold true anymore
+    // then generated code won't work correctly.
+    MP_STATIC_ASSERT(MP_OBJ_NULL == 0);
+
     emit_t *emit = m_new0(emit_t, 1);
     emit->emit_common = emit_common;
     emit->error_slot = error_slot;
@@ -1291,8 +1306,7 @@ static void emit_native_global_exc_entry(emit_t *emit) {
 
             // Check LOCAL_IDX_THROW_VAL for any injected value
             ASM_MOV_REG_LOCAL(emit->as, REG_ARG_1, LOCAL_IDX_THROW_VAL(emit));
-            ASM_MOV_REG_IMM(emit->as, REG_ARG_2, (mp_uint_t)MP_OBJ_NULL);
-            ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_THROW_VAL(emit), REG_ARG_2);
+            ASM_MOV_LOCAL_MP_OBJ_NULL(emit->as, LOCAL_IDX_THROW_VAL(emit), REG_ARG_2);
             emit_call(emit, MP_F_NATIVE_RAISE);
         }
     }
@@ -2208,8 +2222,7 @@ static void emit_native_with_cleanup(emit_t *emit, mp_uint_t label) {
 
     // Replace exception with MP_OBJ_NULL.
     emit_native_label_assign(emit, *emit->label_slot);
-    ASM_MOV_REG_IMM(emit->as, REG_TEMP0, (mp_uint_t)MP_OBJ_NULL);
-    ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_EXC_VAL(emit), REG_TEMP0);
+    ASM_MOV_LOCAL_MP_OBJ_NULL(emit->as, LOCAL_IDX_EXC_VAL(emit), REG_TEMP0);
 
     // end of with cleanup nlr_catch block
     emit_native_label_assign(emit, *emit->label_slot + 1);
@@ -2316,8 +2329,7 @@ static void emit_native_for_iter_end(emit_t *emit) {
 static void emit_native_pop_except_jump(emit_t *emit, mp_uint_t label, bool within_exc_handler) {
     if (within_exc_handler) {
         // Cancel any active exception so subsequent handlers don't see it
-        ASM_MOV_REG_IMM(emit->as, REG_TEMP0, (mp_uint_t)MP_OBJ_NULL);
-        ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_EXC_VAL(emit), REG_TEMP0);
+        ASM_MOV_LOCAL_MP_OBJ_NULL(emit->as, LOCAL_IDX_EXC_VAL(emit), REG_TEMP0);
     } else {
         emit_native_leave_exc_stack(emit, false);
     }
@@ -3002,8 +3014,7 @@ static void emit_native_yield(emit_t *emit, int kind) {
     if (kind == MP_EMIT_YIELD_VALUE) {
         // Check LOCAL_IDX_THROW_VAL for any injected value
         ASM_MOV_REG_LOCAL(emit->as, REG_ARG_1, LOCAL_IDX_THROW_VAL(emit));
-        ASM_MOV_REG_IMM(emit->as, REG_ARG_2, (mp_uint_t)MP_OBJ_NULL);
-        ASM_MOV_LOCAL_REG(emit->as, LOCAL_IDX_THROW_VAL(emit), REG_ARG_2);
+        ASM_MOV_LOCAL_MP_OBJ_NULL(emit->as, LOCAL_IDX_THROW_VAL(emit), REG_ARG_2);
         emit_call(emit, MP_F_NATIVE_RAISE);
     } else {
         // Label loop entry