py: Rework bytecode and .mpy file format to be mostly static data.

Background: .mpy files are precompiled .py files, built using mpy-cross,
that contain compiled bytecode functions (and can also contain machine
code). The benefit of using an .mpy file over a .py file is that they are
faster to import and take less memory when importing.  They are also
smaller on disk.

But the real benefit of .mpy files comes when they are frozen into the
firmware.  This is done by loading the .mpy file during compilation of the
firmware and turning it into a set of big C data structures (the job of
mpy-tool.py), which are then compiled and downloaded into the ROM of a
device.  These C data structures can be executed in-place, ie directly from
ROM.  This makes importing even faster because there is very little to do,
and also means such frozen modules take up much less RAM (because their
bytecode stays in ROM).

The downside of frozen code is that it requires recompiling and reflashing
the entire firmware.  This can be a big barrier to entry, slows down
development time, and makes it harder to do OTA updates of frozen code
(because the whole firmware must be updated).

This commit attempts to solve this problem by providing a solution that
sits between loading .mpy files into RAM and freezing them into the
firmware.  The .mpy file format has been reworked so that it consists of
data and bytecode which is mostly static and ready to run in-place.  If
these new .mpy files are located in flash/ROM which is memory addressable,
the .mpy file can be executed (mostly) in-place.

With this approach there is still a small amount of unpacking and linking
of the .mpy file that needs to be done when it's imported, but it's still
much better than loading an .mpy from disk into RAM (although not as good
as freezing .mpy files into the firmware).

The main trick to make static .mpy files is to adjust the bytecode so any
qstrs that it references now go through a lookup table to convert from
local qstr number in the module to global qstr number in the firmware.
That means the bytecode does not need linking/rewriting of qstrs when it's
loaded.  Instead only a small qstr table needs to be built (and put in RAM)
at import time.  This means the bytecode itself is static/constant and can
be used directly if it's in addressable memory.  Also the qstr string data
in the .mpy file, and some constant object data, can be used directly.
Note that the qstr table is global to the module (ie not per function).

In more detail, in the VM what used to be (schematically):

    qst = DECODE_QSTR_VALUE;

is now (schematically):

    idx = DECODE_QSTR_INDEX;
    qst = qstr_table[idx];

That allows the bytecode to be fixed at compile time and not need
relinking/rewriting of the qstr values.  Only qstr_table needs to be linked
when the .mpy is loaded.

Incidentally, this helps to reduce the size of bytecode because what used
to be 2-byte qstr values in the bytecode are now (mostly) 1-byte indices.
If the module uses the same qstr more than two times then the bytecode is
smaller than before.

The following changes are measured for this commit compared to the
previous (the baseline):
- average 7%-9% reduction in size of .mpy files
- frozen code size is reduced by about 5%-7%
- importing .py files uses about 5% less RAM in total
- importing .mpy files uses about 4% less RAM in total
- importing .py and .mpy files takes about the same time as before

The qstr indirection in the bytecode has only a small impact on VM
performance.  For stm32 on PYBv1.0 the performance change of this commit
is:

diff of scores (higher is better)
N=100 M=100             baseline -> this-commit  diff      diff% (error%)
bm_chaos.py               371.07 ->  357.39 :  -13.68 =  -3.687% (+/-0.02%)
bm_fannkuch.py             78.72 ->   77.49 :   -1.23 =  -1.563% (+/-0.01%)
bm_fft.py                2591.73 -> 2539.28 :  -52.45 =  -2.024% (+/-0.00%)
bm_float.py              6034.93 -> 5908.30 : -126.63 =  -2.098% (+/-0.01%)
bm_hexiom.py               48.96 ->   47.93 :   -1.03 =  -2.104% (+/-0.00%)
bm_nqueens.py            4510.63 -> 4459.94 :  -50.69 =  -1.124% (+/-0.00%)
bm_pidigits.py            650.28 ->  644.96 :   -5.32 =  -0.818% (+/-0.23%)
core_import_mpy_multi.py  564.77 ->  581.49 :  +16.72 =  +2.960% (+/-0.01%)
core_import_mpy_single.py  68.67 ->   67.16 :   -1.51 =  -2.199% (+/-0.01%)
core_qstr.py               64.16 ->   64.12 :   -0.04 =  -0.062% (+/-0.00%)
core_yield_from.py        362.58 ->  354.50 :   -8.08 =  -2.228% (+/-0.00%)
misc_aes.py               429.69 ->  405.59 :  -24.10 =  -5.609% (+/-0.01%)
misc_mandel.py           3485.13 -> 3416.51 :  -68.62 =  -1.969% (+/-0.00%)
misc_pystone.py          2496.53 -> 2405.56 :  -90.97 =  -3.644% (+/-0.01%)
misc_raytrace.py          381.47 ->  374.01 :   -7.46 =  -1.956% (+/-0.01%)
viper_call0.py            576.73 ->  572.49 :   -4.24 =  -0.735% (+/-0.04%)
viper_call1a.py           550.37 ->  546.21 :   -4.16 =  -0.756% (+/-0.09%)
viper_call1b.py           438.23 ->  435.68 :   -2.55 =  -0.582% (+/-0.06%)
viper_call1c.py           442.84 ->  440.04 :   -2.80 =  -0.632% (+/-0.08%)
viper_call2a.py           536.31 ->  532.35 :   -3.96 =  -0.738% (+/-0.06%)
viper_call2b.py           382.34 ->  377.07 :   -5.27 =  -1.378% (+/-0.03%)

And for unix on x64:

diff of scores (higher is better)
N=2000 M=2000        baseline -> this-commit     diff      diff% (error%)
bm_chaos.py          13594.20 ->  13073.84 :  -520.36 =  -3.828% (+/-5.44%)
bm_fannkuch.py          60.63 ->     59.58 :    -1.05 =  -1.732% (+/-3.01%)
bm_fft.py           112009.15 -> 111603.32 :  -405.83 =  -0.362% (+/-4.03%)
bm_float.py         246202.55 -> 247923.81 : +1721.26 =  +0.699% (+/-2.79%)
bm_hexiom.py           615.65 ->    617.21 :    +1.56 =  +0.253% (+/-1.64%)
bm_nqueens.py       215807.95 -> 215600.96 :  -206.99 =  -0.096% (+/-3.52%)
bm_pidigits.py        8246.74 ->   8422.82 :  +176.08 =  +2.135% (+/-3.64%)
misc_aes.py          16133.00 ->  16452.74 :  +319.74 =  +1.982% (+/-1.50%)
misc_mandel.py      128146.69 -> 130796.43 : +2649.74 =  +2.068% (+/-3.18%)
misc_pystone.py      83811.49 ->  83124.85 :  -686.64 =  -0.819% (+/-1.03%)
misc_raytrace.py     21688.02 ->  21385.10 :  -302.92 =  -1.397% (+/-3.20%)

The code size change is (firmware with a lot of frozen code benefits the
most):

       bare-arm:  +396 +0.697%
    minimal x86: +1595 +0.979% [incl +32(data)]
       unix x64: +2408 +0.470% [incl +800(data)]
    unix nanbox: +1396 +0.309% [incl -96(data)]
          stm32: -1256 -0.318% PYBV10
         cc3200:  +288 +0.157%
        esp8266:  -260 -0.037% GENERIC
          esp32:  -216 -0.014% GENERIC[incl -1072(data)]
            nrf:  +116 +0.067% pca10040
            rp2:  -664 -0.135% PICO
           samd:  +844 +0.607% ADAFRUIT_ITSYBITSY_M4_EXPRESS

As part of this change the .mpy file format version is bumped to version 6.
And mpy-tool.py has been improved to provide a good visualisation of the
contents of .mpy files.

In summary: this commit changes the bytecode to use qstr indirection, and
reworks the .mpy file format to be simpler and allow .mpy files to be
executed in-place.  Performance is not impacted too much.  Eventually it
will be possible to store such .mpy files in a linear, read-only, memory-
mappable filesystem so they can be executed from flash/ROM.  This will
essentially be able to replace frozen code for most applications.

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

1 files changed, 43 insertions, 26 deletions

diff --git a/py/bc.c b/py/bc.c
index 69b6739e8f..b98df39e28 100644
--- a/py/bc.c
+++ b/py/bc.c
@@ -29,9 +29,9 @@
 #include <string.h>
 #include <assert.h>
 
-#include "py/runtime.h"
 #include "py/bc0.h"
 #include "py/bc.h"
+#include "py/objfun.h"
 
 #if MICROPY_DEBUG_VERBOSE // print debugging info
 #define DEBUG_PRINT (1)
@@ -40,7 +40,23 @@
 #define DEBUG_printf(...) (void)0
 #endif
 
-#if !MICROPY_PERSISTENT_CODE
+void mp_encode_uint(void *env, mp_encode_uint_allocator_t allocator, mp_uint_t val) {
+    // We store each 7 bits in a separate byte, and that's how many bytes needed
+    byte buf[MP_ENCODE_UINT_MAX_BYTES];
+    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(env, buf + sizeof(buf) - p);
+    if (c != NULL) {
+        while (p != buf + sizeof(buf) - 1) {
+            *c++ = *p++ | 0x80;
+        }
+        *c = *p;
+    }
+}
 
 mp_uint_t mp_decode_uint(const byte **ptr) {
     mp_uint_t unum = 0;
@@ -72,8 +88,6 @@ const byte *mp_decode_uint_skip(const byte *ptr) {
     return ptr;
 }
 
-#endif
-
 STATIC NORETURN void fun_pos_args_mismatch(mp_obj_fun_bc_t *f, size_t expected, size_t given) {
     #if MICROPY_ERROR_REPORTING <= MICROPY_ERROR_REPORTING_TERSE
     // generic message, used also for other argument issues
@@ -107,8 +121,8 @@ STATIC void dump_args(const mp_obj_t *a, size_t sz) {
 // On entry code_state should be allocated somewhere (stack/heap) and
 // contain the following valid entries:
 //    - code_state->fun_bc should contain a pointer to the function object
-//    - code_state->ip should contain the offset in bytes from the pointer
-//      code_state->fun_bc->bytecode to the entry n_state (0 for bytecode, non-zero for native)
+//    - code_state->ip should contain a pointer to the beginning of the prelude
+//    - code_state->n_state should be the number of objects in the local state
 void mp_setup_code_state(mp_code_state_t *code_state, size_t n_args, size_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.
@@ -116,9 +130,6 @@ void mp_setup_code_state(mp_code_state_t *code_state, size_t n_args, size_t n_kw
     // get the function object that we want to set up (could be bytecode or native code)
     mp_obj_fun_bc_t *self = code_state->fun_bc;
 
-    // ip comes in as an offset into bytecode, so turn it into a true pointer
-    code_state->ip = self->bytecode + (size_t)code_state->ip;
-
     #if MICROPY_STACKLESS
     code_state->prev = NULL;
     #endif
@@ -134,6 +145,7 @@ void mp_setup_code_state(mp_code_state_t *code_state, size_t n_args, size_t n_kw
     // Decode prelude
     size_t n_state_unused, n_exc_stack_unused, scope_flags, n_pos_args, n_kwonly_args, n_def_pos_args;
     MP_BC_PRELUDE_SIG_DECODE_INTO(code_state->ip, n_state_unused, n_exc_stack_unused, scope_flags, n_pos_args, n_kwonly_args, n_def_pos_args);
+    MP_BC_PRELUDE_SIZE_DECODE(code_state->ip);
     (void)n_state_unused;
     (void)n_exc_stack_unused;
 
@@ -194,14 +206,20 @@ void mp_setup_code_state(mp_code_state_t *code_state, size_t n_args, size_t n_kw
             *var_pos_kw_args = dict;
         }
 
-        // get pointer to arg_names array
-        const mp_obj_t *arg_names = (const mp_obj_t *)self->const_table;
-
         for (size_t i = 0; i < n_kw; i++) {
             // the keys in kwargs are expected to be qstr objects
             mp_obj_t wanted_arg_name = kwargs[2 * i];
+
+            // get pointer to arg_names array
+            const uint8_t *arg_names = code_state->ip;
+            arg_names = mp_decode_uint_skip(arg_names);
+
             for (size_t j = 0; j < n_pos_args + n_kwonly_args; j++) {
-                if (wanted_arg_name == arg_names[j]) {
+                qstr arg_qstr = mp_decode_uint(&arg_names);
+                #if MICROPY_EMIT_BYTECODE_USES_QSTR_TABLE
+                arg_qstr = self->context->constants.qstr_table[arg_qstr];
+                #endif
+                if (wanted_arg_name == MP_OBJ_NEW_QSTR(arg_qstr)) {
                     if (code_state->state[n_state - 1 - j] != MP_OBJ_NULL) {
                         mp_raise_msg_varg(&mp_type_TypeError,
                             MP_ERROR_TEXT("function got multiple values for argument '%q'"), MP_OBJ_QSTR_VALUE(wanted_arg_name));
@@ -248,17 +266,25 @@ void mp_setup_code_state(mp_code_state_t *code_state, size_t n_args, size_t n_kw
 
         // Check that all mandatory keyword args are specified
         // Fill in default kw args if we have them
+        const uint8_t *arg_names = mp_decode_uint_skip(code_state->ip);
+        for (size_t i = 0; i < n_pos_args; i++) {
+            arg_names = mp_decode_uint_skip(arg_names);
+        }
         for (size_t i = 0; i < n_kwonly_args; i++) {
+            qstr arg_qstr = mp_decode_uint(&arg_names);
+            #if MICROPY_EMIT_BYTECODE_USES_QSTR_TABLE
+            arg_qstr = self->context->constants.qstr_table[arg_qstr];
+            #endif
             if (code_state->state[n_state - 1 - n_pos_args - i] == MP_OBJ_NULL) {
                 mp_map_elem_t *elem = NULL;
                 if ((scope_flags & MP_SCOPE_FLAG_DEFKWARGS) != 0) {
-                    elem = mp_map_lookup(&((mp_obj_dict_t *)MP_OBJ_TO_PTR(self->extra_args[n_def_pos_args]))->map, arg_names[n_pos_args + i], MP_MAP_LOOKUP);
+                    elem = mp_map_lookup(&((mp_obj_dict_t *)MP_OBJ_TO_PTR(self->extra_args[n_def_pos_args]))->map, MP_OBJ_NEW_QSTR(arg_qstr), MP_MAP_LOOKUP);
                 }
                 if (elem != NULL) {
                     code_state->state[n_state - 1 - n_pos_args - i] = elem->value;
                 } else {
                     mp_raise_msg_varg(&mp_type_TypeError,
-                        MP_ERROR_TEXT("function missing required keyword argument '%q'"), MP_OBJ_QSTR_VALUE(arg_names[n_pos_args + i]));
+                        MP_ERROR_TEXT("function missing required keyword argument '%q'"), arg_qstr);
                 }
             }
         }
@@ -273,12 +299,8 @@ void mp_setup_code_state(mp_code_state_t *code_state, size_t n_args, size_t n_kw
         }
     }
 
-    // read the size part of the prelude
-    const byte *ip = code_state->ip;
-    MP_BC_PRELUDE_SIZE_DECODE(ip);
-
-    // jump over code info (source file and line-number mapping)
-    ip += n_info;
+    // jump over code info (source file, argument names and line-number mapping)
+    const uint8_t *ip = code_state->ip + n_info;
 
     // bytecode prelude: initialise closed over variables
     for (; n_cell; --n_cell) {
@@ -287,11 +309,6 @@ void mp_setup_code_state(mp_code_state_t *code_state, size_t n_args, size_t n_kw
             mp_obj_new_cell(code_state->state[n_state - 1 - local_num]);
     }
 
-    #if !MICROPY_PERSISTENT_CODE
-    // so bytecode is aligned
-    ip = MP_ALIGN(ip, sizeof(mp_uint_t));
-    #endif
-
     // now that we skipped over the prelude, set the ip for the VM
     code_state->ip = ip;