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py: Implement "common word" compression scheme for error messages.

The idea here is that there's a moderate amount of ROM used up by exception
text.  Obviously we try to keep the messages short, and the code can enable
terse errors, but it still adds up.  Listed below is the total string data
size for various ports:

    bare-arm 2860
    minimal 2876
    stm32 8926  (PYBV11)
    cc3200 3751
    esp32 5721

This commit implements compression of these strings.  It takes advantage of
the fact that these strings are all 7-bit ascii and extracts the top 128
frequently used words from the messages and stores them packed (dropping
their null-terminator), then uses (0x80 | index) inside strings to refer to
these common words.  Spaces are automatically added around words, saving
more bytes.  This happens transparently in the build process, mirroring the
steps that are used to generate the QSTR data.  The MP_COMPRESSED_ROM_TEXT
macro wraps any literal string that should compressed, and it's
automatically decompressed in mp_decompress_rom_string.

There are many schemes that could be used for the compression, and some are
included in py/makecompresseddata.py for reference (space, Huffman, ngram,
common word).  Results showed that the common-word compression gets better
results.  This is before counting the increased cost of the Huffman
decoder.  This might be slightly counter-intuitive, but this data is
extremely repetitive at a word-level, and the byte-level entropy coder
can't quite exploit that as efficiently.  Ideally one would combine both
approaches, but for now the common-word approach is the one that is used.

For additional comparison, the size of the raw data compressed with gzip
and zlib is calculated, as a sort of proxy for a lower entropy bound.  With
this scheme we come within 15% on stm32, and 30% on bare-arm (i.e. we use
x% more bytes than the data compressed with gzip -- not counting the code
overhead of a decoder, and how this would be hypothetically implemented).

The feature is disabled by default and can be enabled by setting
MICROPY_ROM_TEXT_COMPRESSION at the Makefile-level.
pull/5861/head
Jim Mussared 5 years ago
committed by Damien George
parent
commit
154b4eb354
  1. 200
      py/makecompresseddata.py
  2. 47
      py/makeqstrdefs.py
  3. 54
      py/misc.h
  4. 38
      py/mkrules.mk
  5. 4
      py/py.mk
  6. 75
      py/qstr.c
  7. 5
      py/qstr.h

200
py/makecompresseddata.py

@ -0,0 +1,200 @@
from __future__ import print_function
import collections
import re
import sys
import gzip
import zlib
_COMPRESSED_MARKER = 0xFF
def check_non_ascii(msg):
for c in msg:
if ord(c) >= 0x80:
print(
'Unable to generate compressed data: message "{}" contains a non-ascii character "{}".'.format(
msg, c
),
file=sys.stderr,
)
sys.exit(1)
# Replace <char><space> with <char | 0x80>.
# Trival scheme to demo/test.
def space_compression(error_strings):
for line in error_strings:
check_non_ascii(line)
result = ""
for i in range(len(line)):
if i > 0 and line[i] == " ":
result = result[:-1]
result += "\\{:03o}".format(ord(line[i - 1]))
else:
result += line[i]
error_strings[line] = result
return None
# Replace common words with <0x80 | index>.
# Index is into a table of words stored as aaaaa<0x80|a>bbb<0x80|b>...
# Replaced words are assumed to have spaces either side to avoid having to store the spaces in the compressed strings.
def word_compression(error_strings):
topn = collections.Counter()
for line in error_strings.keys():
check_non_ascii(line)
for word in line.split(" "):
topn[word] += 1
# Order not just by frequency, but by expected saving. i.e. prefer a longer string that is used less frequently.
def bytes_saved(item):
w, n = item
return -((len(w) + 1) * (n - 1))
top128 = sorted(topn.items(), key=bytes_saved)[:128]
index = [w for w, _ in top128]
index_lookup = {w: i for i, w in enumerate(index)}
for line in error_strings.keys():
result = ""
need_space = False
for word in line.split(" "):
if word in index_lookup:
result += "\\{:03o}".format(0b10000000 | index_lookup[word])
need_space = False
else:
if need_space:
result += " "
need_space = True
result += word
error_strings[line] = result.strip()
return "".join(w[:-1] + "\\{:03o}".format(0b10000000 | ord(w[-1])) for w in index)
# Replace chars in text with variable length bit sequence.
# For comparison only (the table is not emitted).
def huffman_compression(error_strings):
# https://github.com/tannewt/huffman
import huffman
all_strings = "".join(error_strings)
cb = huffman.codebook(collections.Counter(all_strings).items())
for line in error_strings:
b = "1"
for c in line:
b += cb[c]
n = len(b)
if n % 8 != 0:
n += 8 - (n % 8)
result = ""
for i in range(0, n, 8):
result += "\\{:03o}".format(int(b[i : i + 8], 2))
if len(result) > len(line) * 4:
result = line
error_strings[line] = result
# TODO: This would be the prefix lengths and the table ordering.
return "_" * (10 + len(cb))
# Replace common N-letter sequences with <0x80 | index>, where
# the common sequences are stored in a separate table.
# This isn't very useful, need a smarter way to find top-ngrams.
def ngram_compression(error_strings):
topn = collections.Counter()
N = 2
for line in error_strings.keys():
check_non_ascii(line)
if len(line) < N:
continue
for i in range(0, len(line) - N, N):
topn[line[i : i + N]] += 1
def bytes_saved(item):
w, n = item
return -(len(w) * (n - 1))
top128 = sorted(topn.items(), key=bytes_saved)[:128]
index = [w for w, _ in top128]
index_lookup = {w: i for i, w in enumerate(index)}
for line in error_strings.keys():
result = ""
for i in range(0, len(line) - N + 1, N):
word = line[i : i + N]
if word in index_lookup:
result += "\\{:03o}".format(0b10000000 | index_lookup[word])
else:
result += word
if len(line) % N != 0:
result += line[len(line) - len(line) % N :]
error_strings[line] = result.strip()
return "".join(index)
def main(collected_path, fn):
error_strings = {}
max_uncompressed_len = 0
num_uses = 0
# Read in all MP_ERROR_TEXT strings.
with open(collected_path, "r") as f:
for line in f:
line = line.strip()
if not line:
continue
num_uses += 1
error_strings[line] = None
max_uncompressed_len = max(max_uncompressed_len, len(line))
# So that objexcept.c can figure out how big the buffer needs to be.
print("#define MP_MAX_UNCOMPRESSED_TEXT_LEN ({})".format(max_uncompressed_len))
# Run the compression.
compressed_data = fn(error_strings)
# Print the data table.
print('MP_COMPRESSED_DATA("{}")'.format(compressed_data))
# Print the replacements.
for uncomp, comp in error_strings.items():
print('MP_MATCH_COMPRESSED("{}", "\\{:03o}{}")'.format(uncomp, _COMPRESSED_MARKER, comp))
# Used to calculate the "true" length of the (escaped) compressed strings.
def unescape(s):
return re.sub(r"\\\d\d\d", "!", s)
# Stats. Note this doesn't include the cost of the decompressor code.
uncomp_len = sum(len(s) + 1 for s in error_strings.keys())
comp_len = sum(1 + len(unescape(s)) + 1 for s in error_strings.values())
data_len = len(compressed_data) + 1 if compressed_data else 0
print("// Total input length: {}".format(uncomp_len))
print("// Total compressed length: {}".format(comp_len))
print("// Total data length: {}".format(data_len))
print("// Predicted saving: {}".format(uncomp_len - comp_len - data_len))
# Somewhat meaningless comparison to zlib/gzip.
all_input_bytes = "\\0".join(error_strings.keys()).encode()
print()
if hasattr(gzip, "compress"):
gzip_len = len(gzip.compress(all_input_bytes)) + num_uses * 4
print("// gzip length: {}".format(gzip_len))
print("// Percentage of gzip: {:.1f}%".format(100 * (comp_len + data_len) / gzip_len))
if hasattr(zlib, "compress"):
zlib_len = len(zlib.compress(all_input_bytes)) + num_uses * 4
print("// zlib length: {}".format(zlib_len))
print("// Percentage of zlib: {:.1f}%".format(100 * (comp_len + data_len) / zlib_len))
if __name__ == "__main__":
main(sys.argv[1], word_compression)

47
py/makeqstrdefs.py

@ -13,17 +13,27 @@ import io
import os
# Extract MP_QSTR_FOO macros.
_MODE_QSTR = "qstr"
# Extract MP_COMPRESSED_ROM_TEXT("") macros. (Which come from MP_ERROR_TEXT)
_MODE_COMPRESS = "compress"
def write_out(fname, output):
if output:
for m, r in [("/", "__"), ("\\", "__"), (":", "@"), ("..", "@@")]:
fname = fname.replace(m, r)
with open(args.output_dir + "/" + fname + ".qstr", "w") as f:
with open(args.output_dir + "/" + fname + "." + args.mode, "w") as f:
f.write("\n".join(output) + "\n")
def process_file(f):
re_line = re.compile(r"#[line]*\s\d+\s\"([^\"]+)\"")
re_qstr = re.compile(r"MP_QSTR_[_a-zA-Z0-9]+")
if args.mode == _MODE_QSTR:
re_match = re.compile(r"MP_QSTR_[_a-zA-Z0-9]+")
elif args.mode == _MODE_COMPRESS:
re_match = re.compile(r'MP_COMPRESSED_ROM_TEXT\("([^"]*)"\)')
output = []
last_fname = None
for line in f:
@ -41,9 +51,12 @@ def process_file(f):
output = []
last_fname = fname
continue
for match in re_qstr.findall(line):
name = match.replace("MP_QSTR_", "")
output.append("Q(" + name + ")")
for match in re_match.findall(line):
if args.mode == _MODE_QSTR:
name = match.replace("MP_QSTR_", "")
output.append("Q(" + name + ")")
elif args.mode == _MODE_COMPRESS:
output.append(match)
write_out(last_fname, output)
return ""
@ -56,7 +69,7 @@ def cat_together():
hasher = hashlib.md5()
all_lines = []
outf = open(args.output_dir + "/out", "wb")
for fname in glob.glob(args.output_dir + "/*.qstr"):
for fname in glob.glob(args.output_dir + "/*." + args.mode):
with open(fname, "rb") as f:
lines = f.readlines()
all_lines += lines
@ -73,8 +86,11 @@ def cat_together():
old_hash = f.read()
except IOError:
pass
mode_full = "QSTR"
if args.mode == _MODE_COMPRESS:
mode_full = "Compressed data"
if old_hash != new_hash:
print("QSTR updated")
print(mode_full, "updated")
try:
# rename below might fail if file exists
os.remove(args.output_file)
@ -84,12 +100,12 @@ def cat_together():
with open(args.output_file + ".hash", "w") as f:
f.write(new_hash)
else:
print("QSTR not updated")
print(mode_full, "not updated")
if __name__ == "__main__":
if len(sys.argv) != 5:
print("usage: %s command input_filename output_dir output_file" % sys.argv[0])
if len(sys.argv) != 6:
print("usage: %s command mode input_filename output_dir output_file" % sys.argv[0])
sys.exit(2)
class Args:
@ -97,9 +113,14 @@ if __name__ == "__main__":
args = Args()
args.command = sys.argv[1]
args.input_filename = sys.argv[2]
args.output_dir = sys.argv[3]
args.output_file = sys.argv[4]
args.mode = sys.argv[2]
args.input_filename = sys.argv[3] # Unused for command=cat
args.output_dir = sys.argv[4]
args.output_file = None if len(sys.argv) == 5 else sys.argv[5] # Unused for command=split
if args.mode not in (_MODE_QSTR, _MODE_COMPRESS):
print("error: mode %s unrecognised" % sys.argv[2])
sys.exit(2)
try:
os.makedirs(args.output_dir)

54
py/misc.h

@ -257,4 +257,58 @@ typedef union _mp_float_union_t {
#endif // MICROPY_PY_BUILTINS_FLOAT
/** ROM string compression *************/
#ifdef NO_QSTR
// QSTR extraction sets NO_QSTR.
// So leave MP_COMPRESSED_ROM_TEXT in place for makeqstrdefs.py / makecompresseddata.py to find them.
// However, dynamic native modules also set NO_QSTR, so provide a dummy implementation.
#if MICROPY_ENABLE_DYNRUNTIME
typedef const char *mp_rom_error_text_t;
#define MP_COMPRESSED_ROM_TEXT(x) x
#endif
#else
#if MICROPY_ROM_TEXT_COMPRESSION
// Force usage of the MP_ERROR_TEXT macro by requiring an opaque type.
typedef struct {} *mp_rom_error_text_t;
// Regular build -- map MP_COMPRESSED_ROM_TEXT to the compressed strings.
#include <string.h>
inline __attribute__((always_inline)) const char *MP_COMPRESSED_ROM_TEXT(const char *msg) {
// "genhdr/compressed.data.h" contains an invocation of the MP_MATCH_COMPRESSED macro for each compressed string.
// The giant if(strcmp) tree is optimized by the compiler, which turns this into a direct return of the compressed data.
#define MP_MATCH_COMPRESSED(a, b) if (strcmp(msg, a) == 0) { return b; } else
// It also contains a single invocation of the MP_COMPRESSED_DATA macro, we don't need that here.
#define MP_COMPRESSED_DATA(x)
#include "genhdr/compressed.data.h"
#undef MP_COMPRESSED_DATA
#undef MP_MATCH_COMPRESSED
return msg;
}
#else
// Compression not enabled, just make it a no-op.
typedef const char *mp_rom_error_text_t;
#define MP_COMPRESSED_ROM_TEXT(x) x
#endif // MICROPY_ROM_TEXT_COMPRESSION
#endif // NO_QSTR
// Might add more types of compressed text in the future.
// For now, forward directly to MP_COMPRESSED_ROM_TEXT.
#define MP_ERROR_TEXT(x) (mp_rom_error_text_t)MP_COMPRESSED_ROM_TEXT(x)
#endif // MICROPY_INCLUDED_PY_MISC_H

38
py/mkrules.mk

@ -4,6 +4,23 @@ THIS_MAKEFILE = $(lastword $(MAKEFILE_LIST))
include $(dir $(THIS_MAKEFILE))mkenv.mk
endif
# Extra deps that need to happen before object compilation.
OBJ_EXTRA_ORDER_DEPS =
# QSTR generation uses the same CFLAGS, with these modifications.
# Note: := to force evalulation immediately.
QSTR_GEN_CFLAGS := $(CFLAGS)
QSTR_GEN_CFLAGS += -DNO_QSTR
QSTR_GEN_CFLAGS += -I$(BUILD)/tmp
ifeq ($(MICROPY_ROM_TEXT_COMPRESSION),1)
# If compression is enabled, trigger the build of compressed.data.h...
OBJ_EXTRA_ORDER_DEPS += $(HEADER_BUILD)/compressed.data.h
# ...and enable the MP_COMPRESSED_ROM_TEXT macro (used by MP_ERROR_TEXT).
# Note, this doesn't get added to the QSTR_GEN_CFLAGS.
CFLAGS += -DMICROPY_ROM_TEXT_COMPRESSION=1
endif
# This file expects that OBJ contains a list of all of the object files.
# The directory portion of each object file is used to locate the source
# and should not contain any ..'s but rather be relative to the top of the
@ -46,9 +63,6 @@ vpath %.c . $(TOP) $(USER_C_MODULES)
$(BUILD)/%.o: %.c
$(call compile_c)
QSTR_GEN_EXTRA_CFLAGS += -DNO_QSTR
QSTR_GEN_EXTRA_CFLAGS += -I$(BUILD)/tmp
vpath %.c . $(TOP) $(USER_C_MODULES)
$(BUILD)/%.pp: %.c
@ -64,7 +78,7 @@ $(BUILD)/%.pp: %.c
# the right .o's to get recompiled if the generated.h file changes. Adding
# an order-only dependency to all of the .o's will cause the generated .h
# to get built before we try to compile any of them.
$(OBJ): | $(HEADER_BUILD)/qstrdefs.generated.h $(HEADER_BUILD)/mpversion.h
$(OBJ): | $(HEADER_BUILD)/qstrdefs.generated.h $(HEADER_BUILD)/mpversion.h $(OBJ_EXTRA_ORDER_DEPS)
# The logic for qstr regeneration is:
# - if anything in QSTR_GLOBAL_DEPENDENCIES is newer, then process all source files ($^)
@ -73,16 +87,26 @@ $(OBJ): | $(HEADER_BUILD)/qstrdefs.generated.h $(HEADER_BUILD)/mpversion.h
# See more information about this process in docs/develop/qstr.rst.
$(HEADER_BUILD)/qstr.i.last: $(SRC_QSTR) $(QSTR_GLOBAL_DEPENDENCIES) | $(QSTR_GLOBAL_REQUIREMENTS)
$(ECHO) "GEN $@"
$(Q)$(CPP) $(QSTR_GEN_EXTRA_CFLAGS) $(CFLAGS) $(if $(filter $?,$(QSTR_GLOBAL_DEPENDENCIES)),$^,$(if $?,$?,$^)) >$(HEADER_BUILD)/qstr.i.last
$(Q)$(CPP) $(QSTR_GEN_CFLAGS) $(if $(filter $?,$(QSTR_GLOBAL_DEPENDENCIES)),$^,$(if $?,$?,$^)) >$(HEADER_BUILD)/qstr.i.last
$(HEADER_BUILD)/qstr.split: $(HEADER_BUILD)/qstr.i.last
$(ECHO) "GEN $@"
$(Q)$(PYTHON) $(PY_SRC)/makeqstrdefs.py split $(HEADER_BUILD)/qstr.i.last $(HEADER_BUILD)/qstr $(QSTR_DEFS_COLLECTED)
$(Q)$(PYTHON) $(PY_SRC)/makeqstrdefs.py split qstr $< $(HEADER_BUILD)/qstr _
$(Q)$(TOUCH) $@
$(QSTR_DEFS_COLLECTED): $(HEADER_BUILD)/qstr.split
$(ECHO) "GEN $@"
$(Q)$(PYTHON) $(PY_SRC)/makeqstrdefs.py cat $(HEADER_BUILD)/qstr.i.last $(HEADER_BUILD)/qstr $(QSTR_DEFS_COLLECTED)
$(Q)$(PYTHON) $(PY_SRC)/makeqstrdefs.py cat qstr _ $(HEADER_BUILD)/qstr $@
# Compressed error strings.
$(HEADER_BUILD)/compressed.split: $(HEADER_BUILD)/qstr.i.last
$(ECHO) "GEN $@"
$(Q)$(PYTHON) $(PY_SRC)/makeqstrdefs.py split compress $< $(HEADER_BUILD)/compress _
$(Q)$(TOUCH) $@
$(HEADER_BUILD)/compressed.collected: $(HEADER_BUILD)/compressed.split
$(ECHO) "GEN $@"
$(Q)$(PYTHON) $(PY_SRC)/makeqstrdefs.py cat compress _ $(HEADER_BUILD)/compress $@
# $(sort $(var)) removes duplicates
#

4
py/py.mk

@ -255,6 +255,10 @@ $(HEADER_BUILD)/qstrdefs.generated.h: $(PY_QSTR_DEFS) $(QSTR_DEFS) $(QSTR_DEFS_C
$(Q)$(CAT) $(PY_QSTR_DEFS) $(QSTR_DEFS) $(QSTR_DEFS_COLLECTED) | $(SED) 's/^Q(.*)/"&"/' | $(CPP) $(CFLAGS) - | $(SED) 's/^\"\(Q(.*)\)\"/\1/' > $(HEADER_BUILD)/qstrdefs.preprocessed.h
$(Q)$(PYTHON) $(PY_SRC)/makeqstrdata.py $(HEADER_BUILD)/qstrdefs.preprocessed.h > $@
$(HEADER_BUILD)/compressed.data.h: $(HEADER_BUILD)/compressed.collected
$(ECHO) "GEN $@"
$(Q)$(PYTHON) $(PY_SRC)/makecompresseddata.py $< > $@
# build a list of registered modules for py/objmodule.c.
$(HEADER_BUILD)/moduledefs.h: $(SRC_QSTR) $(QSTR_GLOBAL_DEPENDENCIES) | $(HEADER_BUILD)/mpversion.h
@$(ECHO) "GEN $@"

75
py/qstr.c

@ -310,3 +310,78 @@ void qstr_dump_data(void) {
QSTR_EXIT();
}
#endif
#if MICROPY_ROM_TEXT_COMPRESSION
#ifdef NO_QSTR
// If NO_QSTR is set, it means we're doing QSTR extraction.
// So we won't yet have "genhdr/compressed.data.h"
#else
// Emit the compressed_string_data string.
#define MP_COMPRESSED_DATA(x) STATIC const char *compressed_string_data = x;
#define MP_MATCH_COMPRESSED(a, b)
#include "genhdr/compressed.data.h"
#undef MP_COMPRESSED_DATA
#undef MP_MATCH_COMPRESSED
#endif // NO_QSTR
// This implements the "common word" compression scheme (see makecompresseddata.py) where the most
// common 128 words in error messages are replaced by their index into the list of common words.
// The compressed string data is delimited by setting high bit in the final char of each word.
// e.g. aaaa<0x80|a>bbbbbb<0x80|b>....
// This method finds the n'th string.
STATIC const byte *find_uncompressed_string(uint8_t n) {
const byte *c = (byte *)compressed_string_data;
while (n > 0) {
while ((*c & 0x80) == 0) {
++c;
}
++c;
--n;
}
return c;
}
// Given a compressed string in src, decompresses it into dst.
// dst must be large enough (use MP_MAX_UNCOMPRESSED_TEXT_LEN+1).
void mp_decompress_rom_string(byte *dst, const mp_rom_error_text_t src_chr) {
// Skip past the 0xff marker.
const byte *src = (byte *)src_chr + 1;
// Need to add spaces around compressed words, except for the first (i.e. transition from 1<->2).
// 0 = start, 1 = compressed, 2 = regular.
int state = 0;
while (*src) {
if ((byte) * src >= 128) {
if (state != 0) {
*dst++ = ' ';
}
state = 1;
// High bit set, replace with common word.
const byte *word = find_uncompressed_string(*src & 0x7f);
// The word is terminated by the final char having its high bit set.
while ((*word & 0x80) == 0) {
*dst++ = *word++;
}
*dst++ = (*word & 0x7f);
} else {
// Otherwise just copy one char.
if (state == 1) {
*dst++ = ' ';
}
state = 2;
*dst++ = *src;
}
++src;
}
// Add null-terminator.
*dst = 0;
}
#endif // MICROPY_ROM_TEXT_COMPRESSION

5
py/qstr.h

@ -74,4 +74,9 @@ const byte *qstr_data(qstr q, size_t *len);
void qstr_pool_info(size_t *n_pool, size_t *n_qstr, size_t *n_str_data_bytes, size_t *n_total_bytes);
void qstr_dump_data(void);
#if MICROPY_ROM_TEXT_COMPRESSION
void mp_decompress_rom_string(byte *dst, mp_rom_error_text_t src);
#define MP_IS_COMPRESSED_ROM_STRING(s) (*(byte *)(s) == 0xff)
#endif
#endif // MICROPY_INCLUDED_PY_QSTR_H

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