py/emitnative: Ensure load_subscr does not clobber existing REG_ARG_2.

Follow up from a similar fix in 426785a19e

Fixes issue #6314.

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

py/emitnative.c

@@ -1527,6 +1527,7 @@ STATIC void emit_native_load_subscr(emit_t *emit) {
                             break;
                         }
                         #endif
+                        need_reg_single(emit, reg_index, 0);
                         ASM_MOV_REG_IMM(emit->as, reg_index, index_value);
                         ASM_ADD_REG_REG(emit->as, reg_index, reg_base); // add index to base
                         reg_base = reg_index;
@@ -1544,6 +1545,7 @@ STATIC void emit_native_load_subscr(emit_t *emit) {
                             break;
                         }
                         #endif
+                        need_reg_single(emit, reg_index, 0);
                         ASM_MOV_REG_IMM(emit->as, reg_index, index_value << 1);
                         ASM_ADD_REG_REG(emit->as, reg_index, reg_base); // add 2*index to base
                         reg_base = reg_index;
@@ -1561,6 +1563,7 @@ STATIC void emit_native_load_subscr(emit_t *emit) {
                             break;
                         }
                         #endif
+                        need_reg_single(emit, reg_index, 0);
                         ASM_MOV_REG_IMM(emit->as, reg_index, index_value << 2);
                         ASM_ADD_REG_REG(emit->as, reg_index, reg_base); // add 4*index to base
                         reg_base = reg_index;

tests/micropython/viper_subscr_multi.py

@@ -6,15 +6,24 @@ def f1(b: ptr8):
     b[0] += b[1]
 
 
+b = bytearray(b"\x01\x02")
+f1(b)
+print(b)
+
+
 @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)
+
+
+@micropython.viper
+def f3(b: ptr8) -> int:
+    return b[0] << 24 | b[1] << 16 | b[2] << 8 | b[3]
+
+
+print(hex(f3(b"\x01\x02\x03\x04")))

tests/micropython/viper_subscr_multi.py.exp

@@ -1,2 +1,3 @@
 bytearray(b'\x03\x02')
 bytearray(b'\x03\x02')
+0x1020304