py/emitnative: Ensure load_subscr does not clobber existing REG_RET.

Fixes issue #7782, and part of issue #6314.

Signed-off-by: Damien George <damien@micropython.org>

py/emitnative.c

@@ -1560,6 +1560,7 @@ STATIC void emit_native_load_subscr(emit_t *emit) {
             int reg_base = REG_ARG_1;
             int reg_index = REG_ARG_2;
             emit_pre_pop_reg_flexible(emit, &vtype_base, &reg_base, reg_index, reg_index);
+            need_reg_single(emit, REG_RET, 0);
             switch (vtype_base) {
                 case VTYPE_PTR8: {
                     // pointer to 8-bit memory
@@ -1623,6 +1624,7 @@ STATIC void emit_native_load_subscr(emit_t *emit) {
             int reg_index = REG_ARG_2;
             emit_pre_pop_reg_flexible(emit, &vtype_index, &reg_index, REG_ARG_1, REG_ARG_1);
             emit_pre_pop_reg(emit, &vtype_base, REG_ARG_1);
+            need_reg_single(emit, REG_RET, 0);
             if (vtype_index != VTYPE_INT && vtype_index != VTYPE_UINT) {
                 EMIT_NATIVE_VIPER_TYPE_ERROR(emit,
                     MP_ERROR_TEXT("can't load with '%q' index"), vtype_to_qstr(vtype_index));

tests/micropython/viper_subscr_multi.py

@@ -0,0 +1,20 @@
+# test viper with multiple subscripts in a single expression
+
+
+@micropython.viper
+def f1(b: ptr8):
+    b[0] += b[1]
+
+
+@micropython.viper
+def f2(b: ptr8, i: int):
+    b[0] += b[i]
+
+
+b = bytearray(b"\x01\x02")
+f1(b)
+print(b)
+
+b = bytearray(b"\x01\x02")
+f2(b, 1)
+print(b)

tests/micropython/viper_subscr_multi.py.exp

@@ -0,0 +1,2 @@
+bytearray(b'\x03\x02')
+bytearray(b'\x03\x02')