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| author | Damien George <damien.p.george@gmail.com> | 2019-09-16 22:12:59 +1000 |
|---|---|---|
| committer | Damien George <damien.p.george@gmail.com> | 2019-10-01 12:26:22 +1000 |
| commit | b5ebfadbd615de42c43851f27a062bacd9147996 (patch) | |
| tree | e4602e96a0eaf9ee0c30913dbabfe9013dda617a /py/bc.h | |
| parent | 81d04a0200e0d4038c011e4946bfae5707ef9d9c (diff) | |
py: Compress first part of bytecode prelude.
The start of the bytecode prelude contains 6 numbers telling the amount of
stack needed for the Python values and exceptions, and the signature of the
function. Prior to this patch these numbers were all encoded one after the
other (2x variable unsigned integers, then 4x bytes), but using so many
bytes is unnecessary.
An entropy analysis of around 150,000 bytecode functions from the CPython
standard library showed that the optimal Shannon coding would need about
7.1 bits on average to encode these 6 numbers, compared to the existing 48
bits.
This patch attempts to get close to this optimal value by packing the 6
numbers into a single, varible-length unsigned integer via bit-wise
interleaving. The interleaving scheme is chosen to minimise the average
number of bytes needed, and at the same time keep the scheme simple enough
so it can be implemented without too much overhead in code size or speed.
The scheme requires about 10.5 bits on average to store the 6 numbers.
As a result most functions which originally took 6 bytes to encode these 6
numbers now need only 1 byte (in 80% of cases).
Diffstat (limited to 'py/bc.h')
| -rw-r--r-- | py/bc.h | 74 |
1 files changed, 68 insertions, 6 deletions
@@ -32,12 +32,15 @@ // bytecode layout: // -// n_state : var uint -// n_exc_stack : var uint -// scope_flags : byte -// n_pos_args : byte number of arguments this function takes -// n_kwonly_args : byte number of keyword-only arguments this function takes -// n_def_pos_args : byte number of default positional arguments +// func signature : var uint +// contains six values interleaved bit-wise as: xSSSSEAA [xFSSKAED repeated] +// x = extension another byte follows +// S = n_state - 1 number of entries in Python value stack +// E = n_exc_stack number of entries in exception stack +// F = scope_flags four bits of flags, MP_SCOPE_FLAG_xxx +// A = n_pos_args number of arguments this function takes +// K = n_kwonly_args number of keyword-only arguments this function takes +// D = n_def_pos_args number of default positional arguments // // code_info_size : var uint | code_info_size counts bytes in this chunk // simple_name : var qstr | @@ -60,6 +63,65 @@ // const0 : obj // constN : obj +#define MP_BC_PRELUDE_SIG_ENCODE(S, E, scope, out_byte, out_env) \ +do { \ + /*// Get values to store in prelude */ \ + size_t F = scope->scope_flags & 0x0f; /* only need to store lower 4 flag bits */ \ + size_t A = scope->num_pos_args; \ + size_t K = scope->num_kwonly_args; \ + size_t D = scope->num_def_pos_args; \ + \ + /* Adjust S to shrink range, to compress better */ \ + S -= 1; \ + \ + /* Encode prelude */ \ + /* xSSSSEAA */ \ + uint8_t z = (S & 0xf) << 3 | (E & 1) << 2 | (A & 3); \ + S >>= 4; \ + E >>= 1; \ + A >>= 2; \ + while (S | E | F | A | K | D) { \ + out_byte(out_env, 0x80 | z); \ + /* xFSSKAED */ \ + z = (F & 1) << 6 | (S & 3) << 4 | (K & 1) << 3 \ + | (A & 1) << 2 | (E & 1) << 1 | (D & 1); \ + S >>= 2; \ + E >>= 1; \ + F >>= 1; \ + A >>= 1; \ + K >>= 1; \ + D >>= 1; \ + } \ + out_byte(out_env, z); \ +} while (0) + +#define MP_BC_PRELUDE_SIG_DECODE_INTO(ip, S, E, F, A, K, D) \ +do { \ + uint8_t z = *(ip)++; \ + /* xSSSSEAA */ \ + S = (z >> 3) & 0xf; \ + E = (z >> 2) & 0x1; \ + F = 0; \ + A = z & 0x3; \ + K = 0; \ + D = 0; \ + for (unsigned n = 0; z & 0x80; ++n) { \ + z = *(ip)++; \ + /* xFSSKAED */ \ + S |= (z & 0x30) << (2 * n); \ + E |= (z & 0x02) << n; \ + F |= ((z & 0x40) >> 6) << n; \ + A |= (z & 0x4) << n; \ + K |= ((z & 0x08) >> 3) << n; \ + D |= (z & 0x1) << n; \ + } \ + S += 1; \ +} while (0) + +#define MP_BC_PRELUDE_SIG_DECODE(ip) \ + size_t n_state, n_exc_stack, scope_flags, n_pos_args, n_kwonly_args, n_def_pos_args; \ + MP_BC_PRELUDE_SIG_DECODE_INTO(ip, n_state, n_exc_stack, scope_flags, n_pos_args, n_kwonly_args, n_def_pos_args) + // Sentinel value for mp_code_state_t.exc_sp_idx #define MP_CODE_STATE_EXC_SP_IDX_SENTINEL ((uint16_t)-1) |