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.

1 files changed, 54 insertions, 0 deletions

diff --git a/py/misc.h b/py/misc.h
index eafcef9667..d1f8a65177 100644
--- a/py/misc.h
+++ b/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