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, 68 insertions, 16 deletions

diff --git a/py/bc.h b/py/bc.h
index ef5afeae1..7d761e30e 100644
--- a/py/bc.h
+++ b/py/bc.h
@@ -28,7 +28,6 @@
 #define MICROPY_INCLUDED_PY_BC_H
 
 #include "py/runtime.h"
-#include "py/objfun.h"
 
 // bytecode layout:
 //
@@ -50,7 +49,9 @@
 //
 //  source info section:
 //      simple_name : var qstr
-//      source_file : var qstr
+//      argname0    : var qstr
+//      ...         : var qstr
+//      argnameN    : var qstr      N = num_pos_args + num_kwonly_args - 1
 //      <line number info>
 //
 //  closure section:
@@ -58,19 +59,16 @@
 //      ...         : byte
 //      local_numN  : byte          N = n_cells-1
 //
-//  <word alignment padding>        only needed if bytecode contains pointers
-//
 //  <bytecode>
 //
 //
 // constant table layout:
 //
-//  argname0        : obj (qstr)
-//  ...             : obj (qstr)
-//  argnameN        : obj (qstr)    N = num_pos_args + num_kwonly_args
 //  const0          : obj
 //  constN          : obj
 
+#define MP_ENCODE_UINT_MAX_BYTES ((MP_BYTES_PER_OBJ_WORD * 8 + 6) / 7)
+
 #define MP_BC_PRELUDE_SIG_ENCODE(S, E, scope, out_byte, out_env) \
     do {                                                            \
         /*// Get values to store in prelude */                      \
@@ -182,9 +180,9 @@ typedef struct _mp_bytecode_prelude_t {
     uint n_pos_args;
     uint n_kwonly_args;
     uint n_def_pos_args;
-    qstr qstr_block_name;
-    qstr qstr_source_file;
+    qstr qstr_block_name_idx;
     const byte *line_info;
+    const byte *line_info_top;
     const byte *opcodes;
 } mp_bytecode_prelude_t;
 
@@ -198,12 +196,46 @@ typedef struct _mp_exc_stack_t {
     mp_obj_base_t *prev_exc;
 } mp_exc_stack_t;
 
+// Constants associated with a module, to interface bytecode with runtime.
+typedef struct _mp_module_constants_t {
+    #if MICROPY_EMIT_BYTECODE_USES_QSTR_TABLE
+    qstr_short_t *qstr_table;
+    #else
+    qstr source_file;
+    #endif
+    mp_obj_t *obj_table;
+} mp_module_constants_t;
+
+// State associated with a module.
+typedef struct _mp_module_context_t {
+    mp_obj_module_t module;
+    mp_module_constants_t constants;
+} mp_module_context_t;
+
+// Outer level struct defining a compiled module.
+typedef struct _mp_compiled_module_t {
+    const mp_module_context_t *context;
+    const struct _mp_raw_code_t *rc;
+    #if MICROPY_PERSISTENT_CODE_SAVE
+    bool has_native;
+    size_t n_qstr;
+    size_t n_obj;
+    #endif
+} mp_compiled_module_t;
+
+// Outer level struct defining a frozen module.
+typedef struct _mp_frozen_module_t {
+    const mp_module_constants_t constants;
+    const struct _mp_raw_code_t *rc;
+} mp_frozen_module_t;
+
+// State for an executing function.
 typedef struct _mp_code_state_t {
     // The fun_bc entry points to the underlying function object that is being executed.
     // It is needed to access the start of bytecode and the const_table.
     // It is also needed to prevent the GC from reclaiming the bytecode during execution,
     // because the ip pointer below will always point to the interior of the bytecode.
-    mp_obj_fun_bc_t *fun_bc;
+    struct _mp_obj_fun_bc_t *fun_bc;
     const byte *ip;
     mp_obj_t *sp;
     uint16_t n_state;
@@ -222,6 +254,10 @@ typedef struct _mp_code_state_t {
     // mp_exc_stack_t exc_state[0];
 } mp_code_state_t;
 
+// Allocator may return NULL, in which case data is not stored (can be used to compute size).
+typedef uint8_t *(*mp_encode_uint_allocator_t)(void *env, size_t nbytes);
+
+void mp_encode_uint(void *env, mp_encode_uint_allocator_t allocator, mp_uint_t val);
 mp_uint_t mp_decode_uint(const byte **ptr);
 mp_uint_t mp_decode_uint_value(const byte *ptr);
 const byte *mp_decode_uint_skip(const byte *ptr);
@@ -229,10 +265,10 @@ const byte *mp_decode_uint_skip(const byte *ptr);
 mp_vm_return_kind_t mp_execute_bytecode(mp_code_state_t *code_state, volatile mp_obj_t inject_exc);
 mp_code_state_t *mp_obj_fun_bc_prepare_codestate(mp_obj_t func, size_t n_args, size_t n_kw, const mp_obj_t *args);
 void mp_setup_code_state(mp_code_state_t *code_state, size_t n_args, size_t n_kw, const mp_obj_t *args);
-void mp_bytecode_print(const mp_print_t *print, const void *descr, const byte *code, mp_uint_t len, const mp_uint_t *const_table);
-void mp_bytecode_print2(const mp_print_t *print, const byte *code, size_t len, const mp_uint_t *const_table);
+void mp_bytecode_print(const mp_print_t *print, const void *descr, const byte *code, mp_uint_t len, const mp_module_constants_t *cm);
+void mp_bytecode_print2(const mp_print_t *print, const byte *code, size_t len, const mp_module_constants_t *cm);
 const byte *mp_bytecode_print_str(const mp_print_t *print, const byte *ip);
-#define mp_bytecode_print_inst(print, code, const_table) mp_bytecode_print2(print, code, 1, const_table)
+#define mp_bytecode_print_inst(print, code, x_table) mp_bytecode_print2(print, code, 1, x_table)
 
 // Helper macros to access pointer with least significant bits holding flags
 #define MP_TAGPTR_PTR(x) ((void *)((uintptr_t)(x) & ~((uintptr_t)3)))
@@ -246,10 +282,26 @@ uint mp_opcode_format(const byte *ip, size_t *opcode_size, bool count_var_uint);
 
 #endif
 
-static inline size_t mp_bytecode_get_source_line(const byte *line_info, size_t bc_offset) {
+static inline void mp_module_context_alloc_tables(mp_module_context_t *context, size_t n_qstr, size_t n_obj) {
+    #if MICROPY_EMIT_BYTECODE_USES_QSTR_TABLE
+    size_t nq = (n_qstr * sizeof(qstr_short_t) + sizeof(mp_uint_t) - 1) / sizeof(mp_uint_t);
+    size_t no = n_obj;
+    mp_uint_t *mem = m_new(mp_uint_t, nq + no);
+    context->constants.qstr_table = (void *)(mem);
+    context->constants.obj_table = (void *)(mem + nq);
+    #else
+    if (n_obj == 0) {
+        context->constants.obj_table = NULL;
+    } else {
+        context->constants.obj_table = m_new(mp_obj_t, n_obj);
+    }
+    #endif
+}
+
+static inline size_t mp_bytecode_get_source_line(const byte *line_info, const byte *line_info_top, size_t bc_offset) {
     size_t source_line = 1;
-    size_t c;
-    while ((c = *line_info)) {
+    while (line_info < line_info_top) {
+        size_t c = *line_info;
         size_t b, l;
         if ((c & 0x80) == 0) {
             // 0b0LLBBBBB encoding