py, emitters: Fix dummy_data size for bytecode and thumb.

Thumb uses a bit less RAM, bytecode uses a tiny bit more, to avoid
overflow of the dummy buffer in certain cases.

Addresses issue #599.

py/asmthumb.c

@@ -46,7 +46,7 @@ struct _asm_thumb_t {
     uint code_offset;
     uint code_size;
     byte *code_base;
-    byte dummy_data[8];
+    byte dummy_data[4];
 
     uint max_num_labels;
     int *label_offsets;
@@ -113,6 +113,7 @@ void asm_thumb_end_pass(asm_thumb_t *as) {
 }
 
 // all functions must go through this one to emit bytes
+// if as->pass < ASM_THUMB_PASS_EMIT, then this function only returns a buffer of 4 bytes length
 STATIC byte *asm_thumb_get_cur_to_write_bytes(asm_thumb_t *as, int num_bytes_to_write) {
     //printf("emit %d\n", num_bytes_to_write);
     if (as->pass < ASM_THUMB_PASS_EMIT) {
@@ -251,10 +252,13 @@ void asm_thumb_align(asm_thumb_t* as, uint align) {
 
 void asm_thumb_data(asm_thumb_t* as, uint bytesize, uint val) {
     byte *c = asm_thumb_get_cur_to_write_bytes(as, bytesize);
-    // little endian
-    for (uint i = 0; i < bytesize; i++) {
-        *c++ = val;
-        val >>= 8;
+    // only write to the buffer in the emit pass (otherwise we overflow dummy_data)
+    if (as->pass == ASM_THUMB_PASS_EMIT) {
+        // little endian
+        for (uint i = 0; i < bytesize; i++) {
+            *c++ = val;
+            val >>= 8;
+        }
     }
 }
 

py/emitbc.c

@@ -44,6 +44,9 @@
 
 #if !MICROPY_EMIT_CPYTHON
 
+#define BYTES_FOR_INT ((BYTES_PER_WORD * 8 + 6) / 7)
+#define DUMMY_DATA_SIZE (BYTES_FOR_INT)
+
 struct _emit_t {
     pass_kind_t pass;
     int stack_size;
@@ -62,7 +65,7 @@ struct _emit_t {
     uint bytecode_offset;
     uint bytecode_size;
     byte *code_base; // stores both byte code and code info
-    byte dummy_data[8];
+    byte dummy_data[DUMMY_DATA_SIZE];
 };
 
 STATIC void emit_bc_rot_two(emit_t *emit);
@@ -152,7 +155,7 @@ STATIC void emit_write_bytecode_byte_byte(emit_t* emit, byte b1, uint b2) {
 
 STATIC void emit_write_bytecode_uint(emit_t* emit, uint num) {
     // We store each 7 bits in a separate byte, and that's how many bytes needed
-    byte buf[(BYTES_PER_WORD * 8 + 6) / 7];
+    byte buf[BYTES_FOR_INT];
     byte *p = buf + sizeof(buf);
     // We encode in little-ending order, but store in big-endian, to help decoding
     do {
@@ -171,7 +174,7 @@ STATIC void emit_write_bytecode_byte_int(emit_t* emit, byte b1, machine_int_t nu
     emit_write_bytecode_byte(emit, b1);
 
     // We store each 7 bits in a separate byte, and that's how many bytes needed
-    byte buf[(BYTES_PER_WORD * 8 + 6) / 7];
+    byte buf[BYTES_FOR_INT];
     byte *p = buf + sizeof(buf);
     // We encode in little-ending order, but store in big-endian, to help decoding
     do {