liblzma: Range decoder: Add x86-64 inline assembly.

It's compatible with GCC and Clang.
This commit is contained in:
Lasse Collin 2024-02-12 17:09:10 +02:00
parent cba2edc991
commit 3182a330c1
1 changed files with 491 additions and 0 deletions

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@ -416,4 +416,495 @@ do { \
t_offset &= ~t_match_bit ^ rc_mask)
*/
////////////
// x86-64 //
////////////
#if defined(__x86_64__) && (defined(__GNUC__) || defined(__clang__))
// rc_asm_y and rc_asm_n are used as arguments to macros to control which
// strings to include or omit.
#define rc_asm_y(str) str
#define rc_asm_n(str)
// There are a few possible variations for normalization.
// This is the smallest variant which is also used by LZMA SDK.
//
// - This has partial register write (the MOV from (%[in_ptr])).
//
// - INC saves one byte in code size over ADD. False dependency on
// partial flags from INC shouldn't become a problem on any processor
// because the instructions after normalization don't read the flags
// until SUB which sets all flags.
//
#define rc_asm_normalize \
"cmp %[top_value], %[range]\n\t" \
"jae 1f\n\t" \
"shl %[shift_bits], %[code]\n\t" \
"mov (%[in_ptr]), %b[code]\n\t" \
"shl %[shift_bits], %[range]\n\t" \
"inc %[in_ptr]\n" \
"1:\n"
// rc_asm_calc(prob) is roughly equivalent to the C version of rc_if_0(prob)...
//
// rc_bound = (rc.range >> RC_BIT_MODEL_TOTAL_BITS) * (prob);
// if (rc.code < rc_bound)
//
// ...but the bound is stored in "range":
//
// t0 = range;
// range = (range >> RC_BIT_MODEL_TOTAL_BITS) * (prob);
// t0 -= range;
// t1 = code;
// code -= range;
//
// The carry flag (CF) from the last subtraction holds the negation of
// the decoded bit (if CF==0 then the decoded bit is 1).
// The values in t0 and t1 are needed for rc_update_0(prob) and
// rc_update_1(prob). If the bit is 0, rc_update_0(prob)...
//
// rc.range = rc_bound;
//
// ...has already been done but the "code -= range" has to be reverted using
// the old value stored in t1. (Also, prob needs to be updated.)
//
// If the bit is 1, rc_update_1(prob)...
//
// rc.range -= rc_bound;
// rc.code -= rc_bound;
//
// ...is already done for "code" but the value for "range" needs to be taken
// from t0. (Also, prob needs to be updated here as well.)
//
// The assignments from t0 and t1 can be done in a branchless manner with CMOV
// after the instructions from this macro. The CF from SUB tells which moves
// are needed.
#define rc_asm_calc(prob) \
"mov %[range], %[t0]\n\t" \
"shr %[bit_model_total_bits], %[range]\n\t" \
"imul %[" prob "], %[range]\n\t" \
"sub %[range], %[t0]\n\t" \
"mov %[code], %[t1]\n\t" \
"sub %[range], %[code]\n\t"
// Also, prob needs to be updated: The update math depends on the decoded bit.
// It can be expressed in a few slightly different ways but this is fairly
// convenient here:
//
// prob -= (prob + (bit ? 0 : RC_BIT_MODEL_OFFSET)) >> RC_MOVE_BITS;
//
// To do it in branchless way when the negation of the decoded bit is in CF,
// both "prob" and "prob + RC_BIT_MODEL_OFFSET" are needed. Then the desired
// value can be picked with CMOV. The addition can be done using LEA without
// affecting CF.
//
// (This prob update method is a tiny bit different from LZMA SDK 23.01.
// In the LZMA SDK a single register is reserved solely for a constant to
// be used with CMOV when updating prob. That is fine since there are enough
// free registers to do so. The method used here uses one fewer register,
// which is valuable with inline assembly.)
//
// * * *
//
// In bittree decoding, each (unrolled) loop iteration decodes one bit
// and needs one prob variable. To make it faster, the prob variable of
// the iteration N+1 is loaded during iteration N. There are two possible
// prob variables to choose from for N+1. Both are loaded from memory and
// the correct one is chosen with CMOV using the same CF as is used for
// other things described above.
//
// This preloading/prefetching requires an extra register. To avoid
// useless moves from "preloaded prob register" to "current prob register",
// the macros swap between the two registers for odd and even iterations.
//
// * * *
//
// Finally, the decoded bit has to be stored in "symbol". Since the negation
// of the bit is in CF, this can be done with SBB: symbol -= CF - 1. That is,
// if the decoded bit is 0 (CF==1) the operation is a no-op "symbol -= 0"
// and when bit is 1 (CF==0) the operation is "symbol -= 0 - 1" which is
// the same as "symbol += 1".
//
// The instructions for all things are intertwined for a few reasons:
// - freeing temporary registers for new use
// - not modifying CF too early
// - instruction scheduling
//
// The first and last iterations can cheat a little. For example,
// on the first iteration "symbol" is known to start from 1 so it
// doesn't need to be read; it can even be immediately initialized
// to 2 to prepare for the second iteration of the loop.
//
// * * *
//
// a = number of the current prob variable (0 or 1)
// b = number of the next prob variable (1 or 0)
// *_only = rc_asm_y or _n to include or exclude code marked with them
#define rc_asm_bittree(a, b, first_only, middle_only, last_only) \
first_only( \
"movzw 2(%[probs_base]), %[prob" #a "]\n\t" \
"mov $2, %[symbol]\n\t" \
"movzw 4(%[probs_base]), %[prob" #b "]\n\t" \
) \
middle_only( \
/* Note the scaling of 4 instead of 2: */ \
"movzw (%[probs_base], %q[symbol], 4), %[prob" #b "]\n\t" \
) \
last_only( \
"add %[symbol], %[symbol]\n\t" \
) \
\
rc_asm_normalize \
rc_asm_calc("prob" #a) \
\
"cmovae %[t0], %[range]\n\t" \
\
first_only( \
"movzw 6(%[probs_base]), %[t0]\n\t" \
"cmovae %[t0], %[prob" #b "]\n\t" \
) \
middle_only( \
"movzw 2(%[probs_base], %q[symbol], 4), %[t0]\n\t" \
"lea (%q[symbol], %q[symbol]), %[symbol]\n\t" \
"cmovae %[t0], %[prob" #b "]\n\t" \
) \
last_only( \
/*"lea (%q[symbol], %q[symbol]), %[symbol]\n\t"*/ \
) \
\
"lea %c[bit_model_offset](%q[prob" #a "]), %[t0]\n\t" \
"cmovb %[t1], %[code]\n\t" \
"mov %[symbol], %[t1]\n\t" \
"cmovae %[prob" #a "], %[t0]\n\t" \
\
first_only( \
"sbb $-1, %[symbol]\n\t" \
) \
middle_only( \
"sbb $-1, %[symbol]\n\t" \
) \
last_only( \
"sbb %[last_sbb], %[symbol]\n\t" \
) \
\
"shr %[move_bits], %[t0]\n\t" \
"sub %[t0], %[prob" #a "]\n\t" \
/* Scaling of 1 instead of 2 because symbol <<= 1. */ \
"mov %w[prob" #a "], (%[probs_base], %q[t1], 1)\n\t"
// NOTE: The order of variables in __asm__ can affect speed and code size.
#define rc_asm_bittree_n(probs_base_var, final_add, asm_str) \
do { \
uint32_t t0; \
uint32_t t1; \
uint32_t t_prob0; \
uint32_t t_prob1; \
\
__asm__( \
asm_str \
: \
[range] "+&r"(rc.range), \
[code] "+&r"(rc.code), \
[t0] "=&r"(t0), \
[t1] "=&r"(t1), \
[prob0] "=&r"(t_prob0), \
[prob1] "=&r"(t_prob1), \
[symbol] "=&r"(symbol), \
[in_ptr] "+&r"(rc_in_ptr) \
: \
[probs_base] "r"(probs_base_var), \
[last_sbb] "n"(-1 - (final_add)), \
[top_value] "n"(RC_TOP_VALUE), \
[shift_bits] "n"(RC_SHIFT_BITS), \
[bit_model_total_bits] "n"(RC_BIT_MODEL_TOTAL_BITS), \
[bit_model_offset] "n"(RC_BIT_MODEL_OFFSET), \
[move_bits] "n"(RC_MOVE_BITS) \
: \
"cc", "memory"); \
} while (0)
#undef rc_bittree3
#define rc_bittree3(probs_base_var, final_add) \
rc_asm_bittree_n(probs_base_var, final_add, \
rc_asm_bittree(0, 1, rc_asm_y, rc_asm_n, rc_asm_n) \
rc_asm_bittree(1, 0, rc_asm_n, rc_asm_y, rc_asm_n) \
rc_asm_bittree(0, 1, rc_asm_n, rc_asm_n, rc_asm_y) \
)
#undef rc_bittree6
#define rc_bittree6(probs_base_var, final_add) \
rc_asm_bittree_n(probs_base_var, final_add, \
rc_asm_bittree(0, 1, rc_asm_y, rc_asm_n, rc_asm_n) \
rc_asm_bittree(1, 0, rc_asm_n, rc_asm_y, rc_asm_n) \
rc_asm_bittree(0, 1, rc_asm_n, rc_asm_y, rc_asm_n) \
rc_asm_bittree(1, 0, rc_asm_n, rc_asm_y, rc_asm_n) \
rc_asm_bittree(0, 1, rc_asm_n, rc_asm_y, rc_asm_n) \
rc_asm_bittree(1, 0, rc_asm_n, rc_asm_n, rc_asm_y) \
)
#undef rc_bittree8
#define rc_bittree8(probs_base_var, final_add) \
rc_asm_bittree_n(probs_base_var, final_add, \
rc_asm_bittree(0, 1, rc_asm_y, rc_asm_n, rc_asm_n) \
rc_asm_bittree(1, 0, rc_asm_n, rc_asm_y, rc_asm_n) \
rc_asm_bittree(0, 1, rc_asm_n, rc_asm_y, rc_asm_n) \
rc_asm_bittree(1, 0, rc_asm_n, rc_asm_y, rc_asm_n) \
rc_asm_bittree(0, 1, rc_asm_n, rc_asm_y, rc_asm_n) \
rc_asm_bittree(1, 0, rc_asm_n, rc_asm_y, rc_asm_n) \
rc_asm_bittree(0, 1, rc_asm_n, rc_asm_y, rc_asm_n) \
rc_asm_bittree(1, 0, rc_asm_n, rc_asm_n, rc_asm_y) \
)
// Fixed-sized reverse bittree
//
// This uses the indexing that constructs the final value in symbol directly.
// add = 1, 2, 4, 8
// dcur = -, 4, 8, 16
// dnext0 = 4, 8, 16, -
// dnext0 = 6, 12, 24, -
#define rc_asm_bittree_rev(a, b, add, dcur, dnext0, dnext1, \
first_only, middle_only, last_only) \
first_only( \
"movzw 2(%[probs_base]), %[prob" #a "]\n\t" \
"xor %[symbol], %[symbol]\n\t" \
"movzw 4(%[probs_base]), %[prob" #b "]\n\t" \
) \
middle_only( \
"movzw " #dnext0 "(%[probs_base], %q[symbol], 2), " \
"%[prob" #b "]\n\t" \
) \
\
rc_asm_normalize \
rc_asm_calc("prob" #a) \
\
"cmovae %[t0], %[range]\n\t" \
\
first_only( \
"movzw 6(%[probs_base]), %[t0]\n\t" \
"cmovae %[t0], %[prob" #b "]\n\t" \
) \
middle_only( \
"movzw " #dnext1 "(%[probs_base], %q[symbol], 2), %[t0]\n\t" \
"cmovae %[t0], %[prob" #b "]\n\t" \
) \
\
"lea " #add "(%q[symbol]), %[t0]\n\t" \
"cmovb %[t1], %[code]\n\t" \
middle_only( \
"mov %[symbol], %[t1]\n\t" \
) \
last_only( \
"mov %[symbol], %[t1]\n\t" \
) \
"cmovae %[t0], %[symbol]\n\t" \
"lea %c[bit_model_offset](%q[prob" #a "]), %[t0]\n\t" \
"cmovae %[prob" #a "], %[t0]\n\t" \
\
"shr %[move_bits], %[t0]\n\t" \
"sub %[t0], %[prob" #a "]\n\t" \
first_only( \
"mov %w[prob" #a "], 2(%[probs_base])\n\t" \
) \
middle_only( \
"mov %w[prob" #a "], " \
#dcur "(%[probs_base], %q[t1], 2)\n\t" \
) \
last_only( \
"mov %w[prob" #a "], " \
#dcur "(%[probs_base], %q[t1], 2)\n\t" \
)
#undef rc_bittree_rev4
#define rc_bittree_rev4(probs_base_var) \
rc_asm_bittree_n(probs_base_var, 4, \
rc_asm_bittree_rev(0, 1, 1, -, 4, 6, rc_asm_y, rc_asm_n, rc_asm_n) \
rc_asm_bittree_rev(1, 0, 2, 4, 8, 12, rc_asm_n, rc_asm_y, rc_asm_n) \
rc_asm_bittree_rev(0, 1, 4, 8, 16, 24, rc_asm_n, rc_asm_y, rc_asm_n) \
rc_asm_bittree_rev(1, 0, 8, 16, -, -, rc_asm_n, rc_asm_n, rc_asm_y) \
)
#undef rc_bit_add_if_1
#define rc_bit_add_if_1(probs_base_var, dest_var, value_to_add_if_1) \
do { \
uint32_t t0; \
uint32_t t1; \
uint32_t t2 = (value_to_add_if_1); \
uint32_t t_prob; \
uint32_t t_index; \
\
__asm__( \
"movzw (%[probs_base], %q[symbol], 2), %[prob]\n\t" \
"mov %[symbol], %[index]\n\t" \
\
"add %[dest], %[t2]\n\t" \
"add %[symbol], %[symbol]\n\t" \
\
rc_asm_normalize \
rc_asm_calc("prob") \
\
"cmovae %[t0], %[range]\n\t" \
"lea %c[bit_model_offset](%q[prob]), %[t0]\n\t" \
"cmovb %[t1], %[code]\n\t" \
"cmovae %[prob], %[t0]\n\t" \
\
"cmovae %[t2], %[dest]\n\t" \
"sbb $-1, %[symbol]\n\t" \
\
"sar %[move_bits], %[t0]\n\t" \
"sub %[t0], %[prob]\n\t" \
"mov %w[prob], (%[probs_base], %q[index], 2)" \
: \
[range] "+&r"(rc.range), \
[code] "+&r"(rc.code), \
[t0] "=&r"(t0), \
[t1] "=&r"(t1), \
[prob] "=&r"(t_prob), \
[index] "=&r"(t_index), \
[symbol] "+&r"(symbol), \
[t2] "+&r"(t2), \
[dest] "+&r"(dest_var), \
[in_ptr] "+&r"(rc_in_ptr) \
: \
[probs_base] "r"(probs_base_var), \
[top_value] "n"(RC_TOP_VALUE), \
[shift_bits] "n"(RC_SHIFT_BITS), \
[bit_model_total_bits] "n"(RC_BIT_MODEL_TOTAL_BITS), \
[bit_model_offset] "n"(RC_BIT_MODEL_OFFSET), \
[move_bits] "n"(RC_MOVE_BITS) \
: \
"cc", "memory"); \
} while (0)
// Literal decoding uses a normal 8-bit bittree but literal with match byte
// is more complex in picking the probability variable from the correct
// subtree. This doesn't use preloading/prefetching of the next prob because
// there are four choices instead of two.
//
// FIXME? The first iteration starts with symbol = 1 so it could be optimized
// by a tiny amount.
#define rc_asm_matched_literal(nonlast_only) \
"add %[offset], %[symbol]\n\t" \
"and %[offset], %[match_bit]\n\t" \
"add %[match_bit], %[symbol]\n\t" \
\
"movzw (%[probs_base], %q[symbol], 2), %[prob]\n\t" \
\
"add %[symbol], %[symbol]\n\t" \
\
nonlast_only( \
"xor %[match_bit], %[offset]\n\t" \
"add %[match_byte], %[match_byte]\n\t" \
) \
\
rc_asm_normalize \
rc_asm_calc("prob") \
\
"cmovae %[t0], %[range]\n\t" \
"lea %c[bit_model_offset](%q[prob]), %[t0]\n\t" \
"cmovb %[t1], %[code]\n\t" \
"mov %[symbol], %[t1]\n\t" \
"cmovae %[prob], %[t0]\n\t" \
\
nonlast_only( \
"cmovae %[match_bit], %[offset]\n\t" \
"mov %[match_byte], %[match_bit]\n\t" \
) \
\
"sbb $-1, %[symbol]\n\t" \
\
"shr %[move_bits], %[t0]\n\t" \
/* Undo symbol += match_bit + offset: */ \
"and $0x1FF, %[symbol]\n\t" \
"sub %[t0], %[prob]\n\t" \
\
/* Scaling of 1 instead of 2 because symbol <<= 1. */ \
"mov %w[prob], (%[probs_base], %q[t1], 1)\n\t"
#undef rc_matched_literal
#define rc_matched_literal(probs_base_var, match_byte_value) \
do { \
uint32_t t0; \
uint32_t t1; \
uint32_t t_prob; \
uint32_t t_match_byte = (match_byte_value) << 1; \
uint32_t t_match_bit = t_match_byte; \
uint32_t t_offset = 0x100; \
symbol = 1; \
\
__asm__( \
rc_asm_matched_literal(rc_asm_y) \
rc_asm_matched_literal(rc_asm_y) \
rc_asm_matched_literal(rc_asm_y) \
rc_asm_matched_literal(rc_asm_y) \
rc_asm_matched_literal(rc_asm_y) \
rc_asm_matched_literal(rc_asm_y) \
rc_asm_matched_literal(rc_asm_y) \
rc_asm_matched_literal(rc_asm_n) \
: \
[range] "+&r"(rc.range), \
[code] "+&r"(rc.code), \
[t0] "=&r"(t0), \
[t1] "=&r"(t1), \
[prob] "=&r"(t_prob), \
[match_bit] "+&r"(t_match_bit), \
[symbol] "+&r"(symbol), \
[match_byte] "+&r"(t_match_byte), \
[offset] "+&r"(t_offset), \
[in_ptr] "+&r"(rc_in_ptr) \
: \
[probs_base] "r"(probs_base_var), \
[top_value] "n"(RC_TOP_VALUE), \
[shift_bits] "n"(RC_SHIFT_BITS), \
[bit_model_total_bits] "n"(RC_BIT_MODEL_TOTAL_BITS), \
[bit_model_offset] "n"(RC_BIT_MODEL_OFFSET), \
[move_bits] "n"(RC_MOVE_BITS) \
: \
"cc", "memory"); \
} while (0)
// Doing the loop in asm instead of C seems to help a little.
#undef rc_direct
#define rc_direct(dest_var, count_var) \
do { \
uint32_t t0; \
uint32_t t1; \
\
__asm__( \
"2:\n\t" \
"add %[dest], %[dest]\n\t" \
"lea 1(%q[dest]), %[t1]\n\t" \
\
rc_asm_normalize \
\
"shr $1, %[range]\n\t" \
"mov %[code], %[t0]\n\t" \
"sub %[range], %[code]\n\t" \
"cmovns %[t1], %[dest]\n\t" \
"cmovs %[t0], %[code]\n\t" \
"dec %[count]\n\t" \
"jnz 2b\n\t" \
: \
[range] "+&r"(rc.range), \
[code] "+&r"(rc.code), \
[t0] "=&r"(t0), \
[t1] "=&r"(t1), \
[dest] "+&r"(dest_var), \
[count] "+&r"(count_var), \
[in_ptr] "+&r"(rc_in_ptr) \
: \
[top_value] "n"(RC_TOP_VALUE), \
[shift_bits] "n"(RC_SHIFT_BITS) \
: \
"cc", "memory"); \
} while (0)
#endif // x86_64
#endif