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, 42 insertions, 8 deletions

diff --git a/py/emit.h b/py/emit.h
index 13bd3e9b2..6f3593a0e 100644
--- a/py/emit.h
+++ b/py/emit.h
@@ -92,6 +92,18 @@ typedef enum {
 
 typedef struct _emit_t emit_t;
 
+typedef struct _mp_emit_common_t {
+    pass_kind_t pass;
+    uint16_t ct_cur_obj_base;
+    uint16_t ct_cur_obj;
+    uint16_t ct_cur_child;
+    mp_uint_t *const_table;
+    mp_raw_code_t **children;
+    #if MICROPY_EMIT_BYTECODE_USES_QSTR_TABLE
+    mp_map_t qstr_map;
+    #endif
+} mp_emit_common_t;
+
 typedef struct _mp_emit_method_table_id_ops_t {
     void (*local)(emit_t *emit, qstr qst, mp_uint_t local_num, int kind);
     void (*global)(emit_t *emit, qstr qst, int kind);
@@ -99,7 +111,7 @@ typedef struct _mp_emit_method_table_id_ops_t {
 
 typedef struct _emit_method_table_t {
     #if MICROPY_DYNAMIC_COMPILER
-    emit_t *(*emit_new)(mp_obj_t * error_slot, uint *label_slot, mp_uint_t max_num_labels);
+    emit_t *(*emit_new)(mp_emit_common_t * emit_common, mp_obj_t *error_slot, uint *label_slot, mp_uint_t max_num_labels);
     void (*emit_free)(emit_t *emit);
     #endif
 
@@ -161,6 +173,28 @@ typedef struct _emit_method_table_t {
     void (*end_except_handler)(emit_t *emit);
 } emit_method_table_t;
 
+#if MICROPY_EMIT_BYTECODE_USES_QSTR_TABLE
+qstr_short_t mp_emit_common_use_qstr(mp_emit_common_t *emit, qstr qst);
+#else
+static inline qstr_short_t mp_emit_common_use_qstr(mp_emit_common_t *emit, qstr qst) {
+    return qst;
+}
+#endif
+
+static inline size_t mp_emit_common_alloc_const_obj(mp_emit_common_t *emit, mp_obj_t obj) {
+    if (emit->pass == MP_PASS_EMIT) {
+        emit->const_table[emit->ct_cur_obj] = (mp_uint_t)obj;
+    }
+    return emit->ct_cur_obj++;
+}
+
+static inline size_t mp_emit_common_alloc_const_child(mp_emit_common_t *emit, mp_raw_code_t *rc) {
+    if (emit->pass == MP_PASS_EMIT) {
+        emit->children[emit->ct_cur_child] = rc;
+    }
+    return emit->ct_cur_child++;
+}
+
 static inline void mp_emit_common_get_id_for_load(scope_t *scope, qstr qst) {
     scope_find_or_add_id(scope, qst, ID_INFO_KIND_GLOBAL_IMPLICIT);
 }
@@ -180,13 +214,13 @@ extern const mp_emit_method_table_id_ops_t mp_emit_bc_method_table_load_id_ops;
 extern const mp_emit_method_table_id_ops_t mp_emit_bc_method_table_store_id_ops;
 extern const mp_emit_method_table_id_ops_t mp_emit_bc_method_table_delete_id_ops;
 
-emit_t *emit_bc_new(void);
-emit_t *emit_native_x64_new(mp_obj_t *error_slot, uint *label_slot, mp_uint_t max_num_labels);
-emit_t *emit_native_x86_new(mp_obj_t *error_slot, uint *label_slot, mp_uint_t max_num_labels);
-emit_t *emit_native_thumb_new(mp_obj_t *error_slot, uint *label_slot, mp_uint_t max_num_labels);
-emit_t *emit_native_arm_new(mp_obj_t *error_slot, uint *label_slot, mp_uint_t max_num_labels);
-emit_t *emit_native_xtensa_new(mp_obj_t *error_slot, uint *label_slot, mp_uint_t max_num_labels);
-emit_t *emit_native_xtensawin_new(mp_obj_t *error_slot, uint *label_slot, mp_uint_t max_num_labels);
+emit_t *emit_bc_new(mp_emit_common_t *emit_common);
+emit_t *emit_native_x64_new(mp_emit_common_t *emit_common, mp_obj_t *error_slot, uint *label_slot, mp_uint_t max_num_labels);
+emit_t *emit_native_x86_new(mp_emit_common_t *emit_common, mp_obj_t *error_slot, uint *label_slot, mp_uint_t max_num_labels);
+emit_t *emit_native_thumb_new(mp_emit_common_t *emit_common, mp_obj_t *error_slot, uint *label_slot, mp_uint_t max_num_labels);
+emit_t *emit_native_arm_new(mp_emit_common_t *emit_common, mp_obj_t *error_slot, uint *label_slot, mp_uint_t max_num_labels);
+emit_t *emit_native_xtensa_new(mp_emit_common_t *emit_common, mp_obj_t *error_slot, uint *label_slot, mp_uint_t max_num_labels);
+emit_t *emit_native_xtensawin_new(mp_emit_common_t *emit_common, mp_obj_t *error_slot, uint *label_slot, mp_uint_t max_num_labels);
 
 void emit_bc_set_max_num_labels(emit_t *emit, mp_uint_t max_num_labels);