py: Use variable length encoded uints in more places in bytecode.

Code-info size, block name, source name, n_state and n_exc_stack now use
variable length encoded uints.  This saves 7-9 bytes per bytecode
function for most functions.

8 files changed, 329 insertions, 244 deletions

diff --git a/py/bc.c b/py/bc.c
new file mode 100644
index 0000000000..e9cba3823d
--- /dev/null
+++ b/py/bc.c
@@ -0,0 +1,239 @@
+/*
+ * This file is part of the Micro Python project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2014 Damien P. George
+ * Copyright (c) 2014 Paul Sokolovsky
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include <stdbool.h>
+#include <string.h>
+#include <assert.h>
+
+#include "mpconfig.h"
+#include "nlr.h"
+#include "misc.h"
+#include "qstr.h"
+#include "obj.h"
+#include "objtuple.h"
+#include "objfun.h"
+#include "runtime0.h"
+#include "runtime.h"
+#include "bc.h"
+#include "stackctrl.h"
+
+#if 0 // print debugging info
+#define DEBUG_PRINT (1)
+#else // don't print debugging info
+#define DEBUG_printf(...) (void)0
+#endif
+
+mp_uint_t mp_decode_uint(const byte **ptr) {
+    mp_uint_t unum = 0;
+    byte val;
+    const byte *p = *ptr;
+    do {
+        val = *p++;
+        unum = (unum << 7) | (val & 0x7f);
+    } while ((val & 0x80) != 0);
+    *ptr = p;
+    return unum;
+}
+
+STATIC NORETURN void fun_pos_args_mismatch(mp_obj_fun_bc_t *f, mp_uint_t expected, mp_uint_t given) {
+#if MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_TERSE
+    // Generic message, to be reused for other argument issues
+    nlr_raise(mp_obj_new_exception_msg(&mp_type_TypeError,
+        "argument num/types mismatch"));
+#elif MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_NORMAL
+    nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError,
+        "function takes %d positional arguments but %d were given", expected, given));
+#elif MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_DETAILED
+    nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError,
+        "%s() takes %d positional arguments but %d were given",
+        mp_obj_fun_get_name(f), expected, given));
+#endif
+}
+
+#if DEBUG_PRINT
+STATIC void dump_args(const mp_obj_t *a, int sz) {
+    DEBUG_printf("%p: ", a);
+    for (int i = 0; i < sz; i++) {
+        DEBUG_printf("%p ", a[i]);
+    }
+    DEBUG_printf("\n");
+}
+#else
+#define dump_args(...) (void)0
+#endif
+
+// code_state should have ->ip filled in (pointing past code info block),
+// as well as ->n_state.
+void mp_setup_code_state(mp_code_state *code_state, mp_obj_t self_in, mp_uint_t n_args, mp_uint_t n_kw, const mp_obj_t *args) {
+    // This function is pretty complicated.  It's main aim is to be efficient in speed and RAM
+    // usage for the common case of positional only args.
+    mp_obj_fun_bc_t *self = self_in;
+    mp_uint_t n_state = code_state->n_state;
+    const byte *ip = code_state->ip;
+
+    code_state->code_info = self->bytecode;
+    code_state->sp = &code_state->state[0] - 1;
+    code_state->exc_sp = (mp_exc_stack_t*)(code_state->state + n_state) - 1;
+
+    // zero out the local stack to begin with
+    memset(code_state->state, 0, n_state * sizeof(*code_state->state));
+
+    const mp_obj_t *kwargs = args + n_args;
+
+    // var_pos_kw_args points to the stack where the var-args tuple, and var-kw dict, should go (if they are needed)
+    mp_obj_t *var_pos_kw_args = &code_state->state[n_state - 1 - self->n_pos_args - self->n_kwonly_args];
+
+    // check positional arguments
+
+    if (n_args > self->n_pos_args) {
+        // given more than enough arguments
+        if (!self->takes_var_args) {
+            fun_pos_args_mismatch(self, self->n_pos_args, n_args);
+        }
+        // put extra arguments in varargs tuple
+        *var_pos_kw_args-- = mp_obj_new_tuple(n_args - self->n_pos_args, args + self->n_pos_args);
+        n_args = self->n_pos_args;
+    } else {
+        if (self->takes_var_args) {
+            DEBUG_printf("passing empty tuple as *args\n");
+            *var_pos_kw_args-- = mp_const_empty_tuple;
+        }
+        // Apply processing and check below only if we don't have kwargs,
+        // otherwise, kw handling code below has own extensive checks.
+        if (n_kw == 0 && !self->has_def_kw_args) {
+            if (n_args >= self->n_pos_args - self->n_def_args) {
+                // given enough arguments, but may need to use some default arguments
+                for (mp_uint_t i = n_args; i < self->n_pos_args; i++) {
+                    code_state->state[n_state - 1 - i] = self->extra_args[i - (self->n_pos_args - self->n_def_args)];
+                }
+            } else {
+                fun_pos_args_mismatch(self, self->n_pos_args - self->n_def_args, n_args);
+            }
+        }
+    }
+
+    // copy positional args into state
+    for (mp_uint_t i = 0; i < n_args; i++) {
+        code_state->state[n_state - 1 - i] = args[i];
+    }
+
+    // check keyword arguments
+
+    if (n_kw != 0 || self->has_def_kw_args) {
+        DEBUG_printf("Initial args: ");
+        dump_args(code_state->state + n_state - self->n_pos_args - self->n_kwonly_args, self->n_pos_args + self->n_kwonly_args);
+
+        mp_obj_t dict = MP_OBJ_NULL;
+        if (self->takes_kw_args) {
+            dict = mp_obj_new_dict(n_kw); // TODO: better go conservative with 0?
+            *var_pos_kw_args = dict;
+        }
+
+        for (mp_uint_t i = 0; i < n_kw; i++) {
+            qstr arg_name = MP_OBJ_QSTR_VALUE(kwargs[2 * i]);
+            for (mp_uint_t j = 0; j < self->n_pos_args + self->n_kwonly_args; j++) {
+                if (arg_name == self->args[j]) {
+                    if (code_state->state[n_state - 1 - j] != MP_OBJ_NULL) {
+                        nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError,
+                            "function got multiple values for argument '%s'", qstr_str(arg_name)));
+                    }
+                    code_state->state[n_state - 1 - j] = kwargs[2 * i + 1];
+                    goto continue2;
+                }
+            }
+            // Didn't find name match with positional args
+            if (!self->takes_kw_args) {
+                nlr_raise(mp_obj_new_exception_msg(&mp_type_TypeError, "function does not take keyword arguments"));
+            }
+            mp_obj_dict_store(dict, kwargs[2 * i], kwargs[2 * i + 1]);
+continue2:;
+        }
+
+        DEBUG_printf("Args with kws flattened: ");
+        dump_args(code_state->state + n_state - self->n_pos_args - self->n_kwonly_args, self->n_pos_args + self->n_kwonly_args);
+
+        // fill in defaults for positional args
+        mp_obj_t *d = &code_state->state[n_state - self->n_pos_args];
+        mp_obj_t *s = &self->extra_args[self->n_def_args - 1];
+        for (int i = self->n_def_args; i > 0; i--, d++, s--) {
+            if (*d == MP_OBJ_NULL) {
+                *d = *s;
+            }
+        }
+
+        DEBUG_printf("Args after filling default positional: ");
+        dump_args(code_state->state + n_state - self->n_pos_args - self->n_kwonly_args, self->n_pos_args + self->n_kwonly_args);
+
+        // Check that all mandatory positional args are specified
+        while (d < &code_state->state[n_state]) {
+            if (*d++ == MP_OBJ_NULL) {
+                nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError,
+                    "function missing required positional argument #%d", &code_state->state[n_state] - d));
+            }
+        }
+
+        // Check that all mandatory keyword args are specified
+        // Fill in default kw args if we have them
+        for (mp_uint_t i = 0; i < self->n_kwonly_args; i++) {
+            if (code_state->state[n_state - 1 - self->n_pos_args - i] == MP_OBJ_NULL) {
+                mp_map_elem_t *elem = NULL;
+                if (self->has_def_kw_args) {
+                    elem = mp_map_lookup(&((mp_obj_dict_t*)self->extra_args[self->n_def_args])->map, MP_OBJ_NEW_QSTR(self->args[self->n_pos_args + i]), MP_MAP_LOOKUP);
+                }
+                if (elem != NULL) {
+                    code_state->state[n_state - 1 - self->n_pos_args - i] = elem->value;
+                } else {
+                    nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError,
+                        "function missing required keyword argument '%s'", qstr_str(self->args[self->n_pos_args + i])));
+                }
+            }
+        }
+
+    } else {
+        // no keyword arguments given
+        if (self->n_kwonly_args != 0) {
+            nlr_raise(mp_obj_new_exception_msg(&mp_type_TypeError,
+                "function missing keyword-only argument"));
+        }
+        if (self->takes_kw_args) {
+            *var_pos_kw_args = mp_obj_new_dict(0);
+        }
+    }
+
+    // bytecode prelude: initialise closed over variables
+    for (mp_uint_t n_local = *ip++; n_local > 0; n_local--) {
+        mp_uint_t local_num = *ip++;
+        code_state->state[n_state - 1 - local_num] = mp_obj_new_cell(code_state->state[n_state - 1 - local_num]);
+    }
+
+    // now that we skipped over the prelude, set the ip for the VM
+    code_state->ip = ip;
+
+    DEBUG_printf("Calling: n_pos_args=%d, n_kwonly_args=%d\n", self->n_pos_args, self->n_kwonly_args);
+    dump_args(code_state->state + n_state - self->n_pos_args - self->n_kwonly_args, self->n_pos_args + self->n_kwonly_args);
+    dump_args(code_state->state, n_state);
+}
diff --git a/py/bc.h b/py/bc.h
index 4793174997..b92342d84d 100644
--- a/py/bc.h
+++ b/py/bc.h
@@ -49,6 +49,8 @@ typedef struct _mp_code_state {
     //mp_exc_stack_t exc_state[0];
 } mp_code_state;
 
+mp_uint_t mp_decode_uint(const byte **ptr);
+
 mp_vm_return_kind_t mp_execute_bytecode(mp_code_state *code_state, volatile mp_obj_t inject_exc);
 void mp_setup_code_state(mp_code_state *code_state, mp_obj_t self_in, mp_uint_t n_args, mp_uint_t n_kw, const mp_obj_t *args);
 void mp_bytecode_print(const void *descr, const byte *code, int len);
diff --git a/py/emitbc.c b/py/emitbc.c
index a1eacb2986..eada190e6f 100644
--- a/py/emitbc.c
+++ b/py/emitbc.c
@@ -85,6 +85,22 @@ void emit_bc_free(emit_t *emit) {
     m_del_obj(emit_t, emit);
 }
 
+STATIC void emit_write_uint(emit_t* emit, byte*(*allocator)(emit_t*, int), mp_uint_t val) {
+    // We store each 7 bits in a separate byte, and that's how many bytes needed
+    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 {
+        *--p = val & 0x7f;
+        val >>= 7;
+    } while (val != 0);
+    byte* c = allocator(emit, buf + sizeof(buf) - p);
+    while (p != buf + sizeof(buf) - 1) {
+        *c++ = *p++ | 0x80;
+    }
+    *c = *p;
+}
+
 // all functions must go through this one to emit code info
 STATIC byte* emit_get_cur_to_write_code_info(emit_t* emit, int num_bytes_to_write) {
     //printf("emit %d\n", num_bytes_to_write);
@@ -103,13 +119,12 @@ STATIC void emit_align_code_info_to_machine_word(emit_t* emit) {
     emit->code_info_offset = (emit->code_info_offset + sizeof(mp_uint_t) - 1) & (~(sizeof(mp_uint_t) - 1));
 }
 
-STATIC void emit_write_code_info_qstr(emit_t* emit, qstr qstr) {
-    byte* c = emit_get_cur_to_write_code_info(emit, 4);
-    // TODO variable length encoding for qstr
-    c[0] = qstr & 0xff;
-    c[1] = (qstr >> 8) & 0xff;
-    c[2] = (qstr >> 16) & 0xff;
-    c[3] = (qstr >> 24) & 0xff;
+STATIC void emit_write_code_info_uint(emit_t* emit, mp_uint_t val) {
+    emit_write_uint(emit, emit_get_cur_to_write_code_info, val);
+}
+
+STATIC void emit_write_code_info_qstr(emit_t* emit, qstr qst) {
+    emit_write_uint(emit, emit_get_cur_to_write_code_info, qst);
 }
 
 #if MICROPY_ENABLE_SOURCE_LINE
@@ -160,6 +175,10 @@ STATIC void emit_write_bytecode_byte(emit_t* emit, byte b1) {
     c[0] = b1;
 }
 
+STATIC void emit_write_bytecode_uint(emit_t* emit, mp_uint_t val) {
+    emit_write_uint(emit, emit_get_cur_to_write_bytecode, val);
+}
+
 STATIC void emit_write_bytecode_byte_byte(emit_t* emit, byte b1, uint b2) {
     assert((b2 & (~0xff)) == 0);
     byte* c = emit_get_cur_to_write_bytecode(emit, 2);
@@ -167,22 +186,6 @@ STATIC void emit_write_bytecode_byte_byte(emit_t* emit, byte b1, uint b2) {
     c[1] = 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_FOR_INT];
-    byte *p = buf + sizeof(buf);
-    // We encode in little-ending order, but store in big-endian, to help decoding
-    do {
-        *--p = num & 0x7f;
-        num >>= 7;
-    } while (num != 0);
-    byte* c = emit_get_cur_to_write_bytecode(emit, buf + sizeof(buf) - p);
-    while (p != buf + sizeof(buf) - 1) {
-        *c++ = *p++ | 0x80;
-    }
-    *c = *p;
-}
-
 // Similar to emit_write_bytecode_uint(), just some extra handling to encode sign
 STATIC void emit_write_bytecode_byte_int(emit_t* emit, byte b1, mp_int_t num) {
     emit_write_bytecode_byte(emit, b1);
@@ -210,9 +213,9 @@ STATIC void emit_write_bytecode_byte_int(emit_t* emit, byte b1, mp_int_t num) {
     *c = *p;
 }
 
-STATIC void emit_write_bytecode_byte_uint(emit_t* emit, byte b, uint num) {
+STATIC void emit_write_bytecode_byte_uint(emit_t* emit, byte b, mp_uint_t val) {
     emit_write_bytecode_byte(emit, b);
-    emit_write_bytecode_uint(emit, num);
+    emit_write_uint(emit, emit_get_cur_to_write_bytecode, val);
 }
 
 // aligns the pointer so it is friendly to GC
@@ -281,23 +284,18 @@ STATIC void emit_bc_start_pass(emit_t *emit, pass_kind_t pass, scope_t *scope) {
     emit->bytecode_offset = 0;
     emit->code_info_offset = 0;
 
-    // write code info size; use maximum space (4 bytes) to write it; TODO possible optimise this
-    {
-        byte* c = emit_get_cur_to_write_code_info(emit, 4);
-        mp_uint_t s = emit->code_info_size;
-        c[0] = s & 0xff;
-        c[1] = (s >> 8) & 0xff;
-        c[2] = (s >> 16) & 0xff;
-        c[3] = (s >> 24) & 0xff;
+    // Write code info size as compressed uint.  If we are not in the final pass
+    // then space for this uint is reserved in emit_bc_end_pass.
+    if (pass == MP_PASS_EMIT) {
+        emit_write_code_info_uint(emit, emit->code_info_size);
     }
 
-    // code info
-    emit_write_code_info_qstr(emit, scope->source_file);
+    // write the name and source file of this function
     emit_write_code_info_qstr(emit, scope->simple_name);
+    emit_write_code_info_qstr(emit, scope->source_file);
 
     // bytecode prelude: local state size and exception stack size; 16 bit uints for now
     {
-        byte* c = emit_get_cur_to_write_bytecode(emit, 4);
         uint n_state = scope->num_locals + scope->stack_size;
         if (n_state == 0) {
             // Need at least 1 entry in the state, in the case an exception is
@@ -305,10 +303,8 @@ STATIC void emit_bc_start_pass(emit_t *emit, pass_kind_t pass, scope_t *scope) {
             // the highest slot in the state (fastn[0], see vm.c).
             n_state = 1;
         }
-        c[0] = n_state & 0xff;
-        c[1] = (n_state >> 8) & 0xff;
-        c[2] = scope->exc_stack_size & 0xff;
-        c[3] = (scope->exc_stack_size >> 8) & 0xff;
+        emit_write_bytecode_uint(emit, n_state);
+        emit_write_bytecode_uint(emit, scope->exc_stack_size);
     }
 
     // bytecode prelude: initialise closed over variables
@@ -336,10 +332,27 @@ STATIC void emit_bc_end_pass(emit_t *emit) {
     }
 
     *emit_get_cur_to_write_code_info(emit, 1) = 0; // end of line number info
-    emit_align_code_info_to_machine_word(emit); // align so that following bytecode is aligned
 
     if (emit->pass == MP_PASS_CODE_SIZE) {
-        // calculate size of code in bytes
+        // Need to make sure we have enough room in the code-info block to write
+        // the size of the code-info block.  Since the size is written as a
+        // compressed uint, we don't know its size until we write it!  Thus, we
+        // take the biggest possible value it could be and write that here.
+        // Then there will be enough room to write the value, and any leftover
+        // space will be absorbed in the alignment at the end of the code-info
+        // block.
+        mp_uint_t max_code_info_size =
+            emit->code_info_offset  // current code-info size
+            + BYTES_FOR_INT         // maximum space for compressed uint
+            + BYTES_PER_WORD - 1;   // maximum space for alignment padding
+        emit_write_code_info_uint(emit, max_code_info_size);
+
+        // Align code-info so that following bytecode is aligned on a machine word.
+        // We don't need to write anything here, it's just dead space between the
+        // code-info block and the bytecode block that follows it.
+        emit_align_code_info_to_machine_word(emit);
+
+        // calculate size of total code-info + bytecode, in bytes
         emit->code_info_size = emit->code_info_offset;
         emit->bytecode_size = emit->bytecode_offset;
         emit->code_base = m_new0(byte, emit->code_info_size + emit->bytecode_size);
diff --git a/py/objfun.c b/py/objfun.c
index 2a86033dad..8c08ce78c2 100644
--- a/py/objfun.c
+++ b/py/objfun.c
@@ -143,8 +143,8 @@ mp_obj_t mp_make_function_var_between(int n_args_min, int n_args_max, mp_fun_var
 /* byte code functions                                                        */
 
 const char *mp_obj_code_get_name(const byte *code_info) {
-    qstr block_name = code_info[8] | (code_info[9] << 8) | (code_info[10] << 16) | (code_info[11] << 24);
-    return qstr_str(block_name);
+    mp_decode_uint(&code_info); // skip code_info_size entry
+    return qstr_str(mp_decode_uint(&code_info));
 }
 
 const char *mp_obj_fun_get_name(mp_const_obj_t fun_in) {
@@ -172,21 +172,6 @@ STATIC void dump_args(const mp_obj_t *a, int sz) {
 #define dump_args(...) (void)0
 #endif
 
-STATIC NORETURN void fun_pos_args_mismatch(mp_obj_fun_bc_t *f, mp_uint_t expected, mp_uint_t given) {
-#if MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_TERSE
-    // Generic message, to be reused for other argument issues
-    nlr_raise(mp_obj_new_exception_msg(&mp_type_TypeError,
-        "argument num/types mismatch"));
-#elif MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_NORMAL
-    nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError,
-        "function takes %d positional arguments but %d were given", expected, given));
-#elif MICROPY_ERROR_REPORTING == MICROPY_ERROR_REPORTING_DETAILED
-    nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError,
-        "%s() takes %d positional arguments but %d were given",
-        mp_obj_fun_get_name(f), expected, given));
-#endif
-}
-
 // With this macro you can tune the maximum number of function state bytes
 // that will be allocated on the stack.  Any function that needs more
 // than this will use the heap.
@@ -195,159 +180,6 @@ STATIC NORETURN void fun_pos_args_mismatch(mp_obj_fun_bc_t *f, mp_uint_t expecte
 // Set this to enable a simple stack overflow check.
 #define VM_DETECT_STACK_OVERFLOW (0)
 
-// code_state should have ->ip filled in (pointing past code info block),
-// as well as ->n_state.
-void mp_setup_code_state(mp_code_state *code_state, mp_obj_t self_in, mp_uint_t n_args, mp_uint_t n_kw, const mp_obj_t *args) {
-    // This function is pretty complicated.  It's main aim is to be efficient in speed and RAM
-    // usage for the common case of positional only args.
-    mp_obj_fun_bc_t *self = self_in;
-    mp_uint_t n_state = code_state->n_state;
-    const byte *ip = code_state->ip;
-
-    code_state->code_info = self->bytecode;
-    code_state->sp = &code_state->state[0] - 1;
-    code_state->exc_sp = (mp_exc_stack_t*)(code_state->state + n_state) - 1;
-
-    // zero out the local stack to begin with
-    memset(code_state->state, 0, n_state * sizeof(*code_state->state));
-
-    const mp_obj_t *kwargs = args + n_args;
-
-    // var_pos_kw_args points to the stack where the var-args tuple, and var-kw dict, should go (if they are needed)
-    mp_obj_t *var_pos_kw_args = &code_state->state[n_state - 1 - self->n_pos_args - self->n_kwonly_args];
-
-    // check positional arguments
-
-    if (n_args > self->n_pos_args) {
-        // given more than enough arguments
-        if (!self->takes_var_args) {
-            fun_pos_args_mismatch(self, self->n_pos_args, n_args);
-        }
-        // put extra arguments in varargs tuple
-        *var_pos_kw_args-- = mp_obj_new_tuple(n_args - self->n_pos_args, args + self->n_pos_args);
-        n_args = self->n_pos_args;
-    } else {
-        if (self->takes_var_args) {
-            DEBUG_printf("passing empty tuple as *args\n");
-            *var_pos_kw_args-- = mp_const_empty_tuple;
-        }
-        // Apply processing and check below only if we don't have kwargs,
-        // otherwise, kw handling code below has own extensive checks.
-        if (n_kw == 0 && !self->has_def_kw_args) {
-            if (n_args >= self->n_pos_args - self->n_def_args) {
-                // given enough arguments, but may need to use some default arguments
-                for (mp_uint_t i = n_args; i < self->n_pos_args; i++) {
-                    code_state->state[n_state - 1 - i] = self->extra_args[i - (self->n_pos_args - self->n_def_args)];
-                }
-            } else {
-                fun_pos_args_mismatch(self, self->n_pos_args - self->n_def_args, n_args);
-            }
-        }
-    }
-
-    // copy positional args into state
-    for (mp_uint_t i = 0; i < n_args; i++) {
-        code_state->state[n_state - 1 - i] = args[i];
-    }
-
-    // check keyword arguments
-
-    if (n_kw != 0 || self->has_def_kw_args) {
-        DEBUG_printf("Initial args: ");
-        dump_args(code_state->state + n_state - self->n_pos_args - self->n_kwonly_args, self->n_pos_args + self->n_kwonly_args);
-
-        mp_obj_t dict = MP_OBJ_NULL;
-        if (self->takes_kw_args) {
-            dict = mp_obj_new_dict(n_kw); // TODO: better go conservative with 0?
-            *var_pos_kw_args = dict;
-        }
-
-        for (mp_uint_t i = 0; i < n_kw; i++) {
-            qstr arg_name = MP_OBJ_QSTR_VALUE(kwargs[2 * i]);
-            for (mp_uint_t j = 0; j < self->n_pos_args + self->n_kwonly_args; j++) {
-                if (arg_name == self->args[j]) {
-                    if (code_state->state[n_state - 1 - j] != MP_OBJ_NULL) {
-                        nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError,
-                            "function got multiple values for argument '%s'", qstr_str(arg_name)));
-                    }
-                    code_state->state[n_state - 1 - j] = kwargs[2 * i + 1];
-                    goto continue2;
-                }
-            }
-            // Didn't find name match with positional args
-            if (!self->takes_kw_args) {
-                nlr_raise(mp_obj_new_exception_msg(&mp_type_TypeError, "function does not take keyword arguments"));
-            }
-            mp_obj_dict_store(dict, kwargs[2 * i], kwargs[2 * i + 1]);
-continue2:;
-        }
-
-        DEBUG_printf("Args with kws flattened: ");
-        dump_args(code_state->state + n_state - self->n_pos_args - self->n_kwonly_args, self->n_pos_args + self->n_kwonly_args);
-
-        // fill in defaults for positional args
-        mp_obj_t *d = &code_state->state[n_state - self->n_pos_args];
-        mp_obj_t *s = &self->extra_args[self->n_def_args - 1];
-        for (int i = self->n_def_args; i > 0; i--, d++, s--) {
-            if (*d == MP_OBJ_NULL) {
-                *d = *s;
-            }
-        }
-
-        DEBUG_printf("Args after filling default positional: ");
-        dump_args(code_state->state + n_state - self->n_pos_args - self->n_kwonly_args, self->n_pos_args + self->n_kwonly_args);
-
-        // Check that all mandatory positional args are specified
-        while (d < &code_state->state[n_state]) {
-            if (*d++ == MP_OBJ_NULL) {
-                nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError,
-                    "function missing required positional argument #%d", &code_state->state[n_state] - d));
-            }
-        }
-
-        // Check that all mandatory keyword args are specified
-        // Fill in default kw args if we have them
-        for (mp_uint_t i = 0; i < self->n_kwonly_args; i++) {
-            if (code_state->state[n_state - 1 - self->n_pos_args - i] == MP_OBJ_NULL) {
-                mp_map_elem_t *elem = NULL;
-                if (self->has_def_kw_args) {
-                    elem = mp_map_lookup(&((mp_obj_dict_t*)self->extra_args[self->n_def_args])->map, MP_OBJ_NEW_QSTR(self->args[self->n_pos_args + i]), MP_MAP_LOOKUP);
-                }
-                if (elem != NULL) {
-                    code_state->state[n_state - 1 - self->n_pos_args - i] = elem->value;
-                } else {
-                    nlr_raise(mp_obj_new_exception_msg_varg(&mp_type_TypeError,
-                        "function missing required keyword argument '%s'", qstr_str(self->args[self->n_pos_args + i])));
-                }
-            }
-        }
-
-    } else {
-        // no keyword arguments given
-        if (self->n_kwonly_args != 0) {
-            nlr_raise(mp_obj_new_exception_msg(&mp_type_TypeError,
-                "function missing keyword-only argument"));
-        }
-        if (self->takes_kw_args) {
-            *var_pos_kw_args = mp_obj_new_dict(0);
-        }
-    }
-
-    // bytecode prelude: initialise closed over variables
-    for (mp_uint_t n_local = *ip++; n_local > 0; n_local--) {
-        mp_uint_t local_num = *ip++;
-        code_state->state[n_state - 1 - local_num] = mp_obj_new_cell(code_state->state[n_state - 1 - local_num]);
-    }
-
-    // now that we skipped over the prelude, set the ip for the VM
-    code_state->ip = ip;
-
-    DEBUG_printf("Calling: n_pos_args=%d, n_kwonly_args=%d\n", self->n_pos_args, self->n_kwonly_args);
-    dump_args(code_state->state + n_state - self->n_pos_args - self->n_kwonly_args, self->n_pos_args + self->n_kwonly_args);
-    dump_args(code_state->state, n_state);
-}
-
-
 STATIC mp_obj_t fun_bc_call(mp_obj_t self_in, mp_uint_t n_args, mp_uint_t n_kw, const mp_obj_t *args) {
     MP_STACK_CHECK();
 
@@ -359,16 +191,14 @@ STATIC mp_obj_t fun_bc_call(mp_obj_t self_in, mp_uint_t n_args, mp_uint_t n_kw,
     mp_obj_fun_bc_t *self = self_in;
     DEBUG_printf("Func n_def_args: %d\n", self->n_def_args);
 
-    const byte *ip = self->bytecode;
-
-    // get code info size, and skip line number table
-    mp_uint_t code_info_size = ip[0] | (ip[1] << 8) | (ip[2] << 16) | (ip[3] << 24);
-    ip += code_info_size;
+    // skip code-info block
+    const byte *code_info = self->bytecode;
+    mp_uint_t code_info_size = mp_decode_uint(&code_info);
+    const byte *ip = self->bytecode + code_info_size;
 
     // bytecode prelude: state size and exception stack size; 16 bit uints
-    mp_uint_t n_state = ip[0] | (ip[1] << 8);
-    mp_uint_t n_exc_stack = ip[2] | (ip[3] << 8);
-    ip += 4;
+    mp_uint_t n_state = mp_decode_uint(&ip);
+    mp_uint_t n_exc_stack = mp_decode_uint(&ip);
 
 #if VM_DETECT_STACK_OVERFLOW
     n_state += 1;
diff --git a/py/objgenerator.c b/py/objgenerator.c
index 7c364a7b2a..9d16d2bee0 100644
--- a/py/objgenerator.c
+++ b/py/objgenerator.c
@@ -57,15 +57,14 @@ STATIC mp_obj_t gen_wrap_call(mp_obj_t self_in, mp_uint_t n_args, mp_uint_t n_kw
     mp_obj_fun_bc_t *self_fun = (mp_obj_fun_bc_t*)self->fun;
     assert(MP_OBJ_IS_TYPE(self_fun, &mp_type_fun_bc));
 
-    const byte *bytecode = self_fun->bytecode;
-    // get code info size, and skip the line number table
-    mp_uint_t code_info_size = bytecode[0] | (bytecode[1] << 8) | (bytecode[2] << 16) | (bytecode[3] << 24);
-    bytecode += code_info_size;
+    // skip code-info block
+    const byte *code_info = self_fun->bytecode;
+    mp_uint_t code_info_size = mp_decode_uint(&code_info);
+    const byte *ip = self_fun->bytecode + code_info_size;
 
     // bytecode prelude: get state size and exception stack size
-    mp_uint_t n_state = bytecode[0] | (bytecode[1] << 8);
-    mp_uint_t n_exc_stack = bytecode[2] | (bytecode[3] << 8);
-    bytecode += 4;
+    mp_uint_t n_state = mp_decode_uint(&ip);
+    mp_uint_t n_exc_stack = mp_decode_uint(&ip);
 
     // allocate the generator object, with room for local stack and exception stack
     mp_obj_gen_instance_t *o = m_new_obj_var(mp_obj_gen_instance_t, byte,
@@ -74,7 +73,7 @@ STATIC mp_obj_t gen_wrap_call(mp_obj_t self_in, mp_uint_t n_args, mp_uint_t n_kw
 
     o->globals = self_fun->globals;
     o->code_state.n_state = n_state;
-    o->code_state.ip = bytecode;
+    o->code_state.ip = ip;
     mp_setup_code_state(&o->code_state, self_fun, n_args, n_kw, args);
     return o;
 }
diff --git a/py/py.mk b/py/py.mk
index 62a37ddbb7..0720e434b1 100644
--- a/py/py.mk
+++ b/py/py.mk
@@ -102,6 +102,7 @@ PY_O_BASENAME = \
 	modstruct.o \
 	modsys.o \
 	vm.o \
+	bc.o \
 	showbc.o \
 	repl.o \
 	smallint.o \
diff --git a/py/showbc.c b/py/showbc.c
index 6c10333c9f..67c5d7adea 100644
--- a/py/showbc.c
+++ b/py/showbc.c
@@ -30,7 +30,10 @@
 #include "mpconfig.h"
 #include "misc.h"
 #include "qstr.h"
+#include "obj.h"
+#include "runtime.h"
 #include "bc0.h"
+#include "bc.h"
 
 #if MICROPY_DEBUG_PRINTERS
 
@@ -60,20 +63,19 @@ void mp_bytecode_print(const void *descr, const byte *ip, int len) {
     const byte *ip_start = ip;
 
     // get code info size
-    mp_uint_t code_info_size = ip[0] | (ip[1] << 8) | (ip[2] << 16) | (ip[3] << 24);
     const byte *code_info = ip;
+    mp_uint_t code_info_size = mp_decode_uint(&code_info);
     ip += code_info_size;
 
-    qstr source_file = code_info[4] | (code_info[5] << 8) | (code_info[6] << 16) | (code_info[7] << 24);
-    qstr block_name = code_info[8] | (code_info[9] << 8) | (code_info[10] << 16) | (code_info[11] << 24);
+    qstr block_name = mp_decode_uint(&code_info);
+    qstr source_file = mp_decode_uint(&code_info);
     printf("File %s, code block '%s' (descriptor: %p, bytecode @%p %d bytes)\n",
         qstr_str(source_file), qstr_str(block_name), descr, code_info, len);
 
     // bytecode prelude: state size and exception stack size; 16 bit uints
     {
-        uint n_state = ip[0] | (ip[1] << 8);
-        uint n_exc_stack = ip[2] | (ip[3] << 8);
-        ip += 4;
+        uint n_state = mp_decode_uint(&ip);
+        uint n_exc_stack = mp_decode_uint(&ip);
         printf("(N_STATE %u)\n", n_state);
         printf("(N_EXC_STACK %u)\n", n_exc_stack);
     }
diff --git a/py/vm.c b/py/vm.c
index 8c59b9c3aa..d959880f12 100644
--- a/py/vm.c
+++ b/py/vm.c
@@ -921,26 +921,25 @@ exception_handler:
             // But consider how to handle nested exceptions.
             // TODO need a better way of not adding traceback to constant objects (right now, just GeneratorExit_obj and MemoryError_obj)
             if (mp_obj_is_exception_instance(nlr.ret_val) && nlr.ret_val != &mp_const_GeneratorExit_obj && nlr.ret_val != &mp_const_MemoryError_obj) {
-                const byte *code_info = code_state->code_info;
-                mp_uint_t code_info_size = code_info[0] | (code_info[1] << 8) | (code_info[2] << 16) | (code_info[3] << 24);
-                qstr source_file = code_info[4] | (code_info[5] << 8) | (code_info[6] << 16) | (code_info[7] << 24);
-                qstr block_name = code_info[8] | (code_info[9] << 8) | (code_info[10] << 16) | (code_info[11] << 24);
-                mp_uint_t bc = code_state->ip - code_info - code_info_size;
-                //printf("find %lu %d %d\n", bc, code_info[12], code_info[13]);
+                const byte *ip = code_state->code_info;
+                mp_uint_t code_info_size = mp_decode_uint(&ip);
+                qstr block_name = mp_decode_uint(&ip);
+                qstr source_file = mp_decode_uint(&ip);
+                mp_uint_t bc = code_state->ip - code_state->code_info - code_info_size;
                 mp_uint_t source_line = 1;
                 mp_uint_t c;
-                for (const byte *ci = code_info + 12; (c = *ci);) {
+                while ((c = *ip)) {
                     mp_uint_t b, l;
                     if ((c & 0x80) == 0) {
                         // 0b0LLBBBBB encoding
                         b = c & 0x1f;
                         l = c >> 5;
-                        ci += 1;
+                        ip += 1;
                     } else {
                         // 0b1LLLBBBB 0bLLLLLLLL encoding (l's LSB in second byte)
                         b = c & 0xf;
-                        l = ((c << 4) & 0x700) | ci[1];
-                        ci += 2;
+                        l = ((c << 4) & 0x700) | ip[1];
+                        ip += 2;
                     }
                     if (bc >= b) {
                         bc -= b;