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pl_mpeg: rewrite AC decoding to improve performance (~10/20%)
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@rasky
rasky committed Sep 22, 2023
1 parent 2baae4f commit 5f36b25
Showing 1 changed file with 47 additions and 31 deletions.
78 changes: 47 additions & 31 deletions src/video/pl_mpeg/pl_mpeg.h
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Expand Up @@ -3473,51 +3473,67 @@ void plm_video_decode_block(plm_video_t *self, int block) {
// Decode AC coefficients (+DC for non-intra)
PROFILE_START(PS_MPEG_MB_DECODE_AC, 0);
plm_buffer_has(self->buffer, 64*24);
int level = 0;
while (TRUE) {
static const uint16_t qtable0[128] __attribute__((aligned(16))) = { 0,0,0,0,65535,65535,65535,65535,49666,49666,51457,51457,49156,49156,51201,51201,42753,42753,42753,42753,42497,42497,42497,42497,41218,41218,41218,41218,42241,42241,42241,42241,60673,57350,60417,60161,58114,57603,57349,59905,32771,32771,32771,32771,32771,32771,32771,32771,33793,33793,33793,33793,33793,33793,33793,33793,33537,33537,33537,33537,33537,33537,33537,33537,24578,24578,24578,24578,24578,24578,24578,24578,24578,24578,24578,24578,24578,24578,24578,24578,25089,25089,25089,25089,25089,25089,25089,25089,25089,25089,25089,25089,25089,25089,25089,25089,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641,16641 };
static const uint16_t qtables1[256] __attribute__((aligned(16))) = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 57618, 57617, 57616, 57615, 58883, 61442, 61186, 60930, 60674, 60418, 60162, 65281, 65025, 64769, 64513, 64257, 49192, 49192, 49191, 49191, 49190, 49190, 49189, 49189, 49188, 49188, 49187, 49187, 49186, 49186, 49185, 49185, 49184, 49184, 49422, 49422, 49421, 49421, 49420, 49420, 49419, 49419, 49418, 49418, 49417, 49417, 49416, 49416, 40991, 40991, 40991, 40991, 40990, 40990, 40990, 40990, 40989, 40989, 40989, 40989, 40988, 40988, 40988, 40988, 40987, 40987, 40987, 40987, 40986, 40986, 40986, 40986, 40985, 40985, 40985, 40985, 40984, 40984, 40984, 40984, 40983, 40983, 40983, 40983, 40982, 40982, 40982, 40982, 40981, 40981, 40981, 40981, 40980, 40980, 40980, 40980, 40979, 40979, 40979, 40979, 40978, 40978, 40978, 40978, 40977, 40977, 40977, 40977, 40976, 40976, 40976, 40976, 35330, 35330, 35330, 35330, 35330, 35330, 35330, 35330, 35074, 35074, 35074, 35074, 35074, 35074, 35074, 35074, 34051, 34051, 34051, 34051, 34051, 34051, 34051, 34051, 33540, 33540, 33540, 33540, 33540, 33540, 33540, 33540, 33285, 33285, 33285, 33285, 33285, 33285, 33285, 33285, 33031, 33031, 33031, 33031, 33031, 33031, 33031, 33031, 33030, 33030, 33030, 33030, 33030, 33030, 33030, 33030, 32783, 32783, 32783, 32783, 32783, 32783, 32783, 32783, 32782, 32782, 32782, 32782, 32782, 32782, 32782, 32782, 32781, 32781, 32781, 32781, 32781, 32781, 32781, 32781, 32780, 32780, 32780, 32780, 32780, 32780, 32780, 32780, 39425, 39425, 39425, 39425, 39425, 39425, 39425, 39425, 39169, 39169, 39169, 39169, 39169, 39169, 39169, 39169, 38913, 38913, 38913, 38913, 38913, 38913, 38913, 38913, 38657, 38657, 38657, 38657, 38657, 38657, 38657, 38657, 38401, 38401, 38401, 38401, 38401, 38401, 38401, 38401 };
static const uint16_t qtables2[16] __attribute__((aligned(16))) = { 24587, 26626, 25603, 24586, 25092, 26370, 29953, 29697, 24585, 29441, 29185, 24837, 25347, 24584, 26114, 28929 };
static const uint16_t qtables34[2][4] __attribute__((aligned(16))) = { {12289, 9474, 8199, 8707}, {8452, 12033, 11777, 9218} };
static const uint16_t *tbl[4] = { qtables1, qtables2, qtables34[0], qtables34[1] };
static const uint8_t shift[4] = { 0,4,6,6 };

int level = 0;
int run = 0;
PROFILE_START(PS_MPEG_MB_DECODE_AC_VLC, 0);
uint16_t coeff = plm_video_decode_dct_coeff(self->buffer);
// uint16_t coeff = plm_buffer_read_vlc_uint(self->buffer, PLM_VIDEO_DCT_COEFF);
PROFILE_STOP(PS_MPEG_MB_DECODE_AC_VLC, 0);
unsigned int coeff;

PROFILE_START(PS_MPEG_MB_DECODE_AC_CODE, 0);
uint64_t bits = plm_buffer_showbits(self->buffer);
uint64_t bits = plm_buffer_showbits2(self->buffer);
#define readbits(n) ({ uint64_t val = bits>>(64-n); bits <<= n; self->buffer->bit_index += n; val; })

if ((coeff == 0x0001) && (n > 0) && (readbits(1) == 0)) {
// end_of_block
break;
}
if (coeff == 0xffff) {
// escape
run = readbits(6);
level = readbits(8);
if (level == 0) {
level = readbits(8);
}
else if (level == 128) {
level = readbits(8) - 256;
}
else if (level > 128) {
level = level - 256;
}
}
else {
run = coeff >> 8;
level = coeff & 0xff;
if (bits>>63 == 1) {
readbits(1);
if ((n > 0) && (readbits(1) == 0))
break;
run = 0;
level = 1;
if (readbits(1)) {
level = -level;
}
} else {
unsigned int bit0 = (bits>>56) & 0xff;
if (bit0 >= 4) {
coeff = qtable0[bit0];
} else {
unsigned int bit1 = (bits >> 48) & 0xff;
coeff = tbl[bit0][bit1 >> shift[bit0]];
readbits(8);
if (coeff == 0xffff) __builtin_unreachable();
}
if (coeff == 0xffff) {

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@pcercuei

pcercuei Sep 22, 2023

Random idea, but this case looks very unlikely, so you could:

#define likely(x) __builtin_expect(!!(x), 1)
#define unlikely(x) __builtin_expect(!!(x), 1)

Then use:

if (unlikely(coeff == 0xffff)) { ... }

GCC will then make sure the code path that's the most likely will be the one without the branch penalty.

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@rasky

rasky Mar 27, 2024

Author Collaborator

Sorry for the late response, I found this today.

It’s a bit unlikely but not that much, it’s a special marker. N64 doesn’t have a branch predictor so the unlikely clause is useful only to move cold code out of the icache (at the end of the function). For instance, our assert macro does that. In this case, I expect all of this block of code to be in icache anyway during a I-frame decoding, so I don’t think it matters much speed wise.
readbits(6);
run = readbits(6);
level = readbits(8);
if (level == 0) {
level = readbits(8);
} else if (level == 128) {
level = readbits(8) - 256;
} else if (level > 128) {
level = level - 256;
}
} else {
readbits((coeff >> 13) + 1);
coeff &= 0x1fff;
run = coeff >> 8;
level = coeff & 0xff;
if (readbits(1)) {
level = -level;
}
}
}

n += run;
if (n < 0 || n >= 64) {
fprintf(stderr, "INVALID AC COEFF\n");
return; // invalid
}
PROFILE_STOP(PS_MPEG_MB_DECODE_AC_CODE, 0);
PROFILE_START(PS_MPEG_MB_DECODE_AC_DEQUANT, 0);

if (RSP_MODE < 2) {
int de_zig_zagged = PLM_VIDEO_ZIG_ZAG[n];
Expand Down Expand Up @@ -3546,7 +3562,7 @@ void plm_video_decode_block(plm_video_t *self, int block) {
rsp_mpeg1_block_coeff(n, level);
}
n++;
PROFILE_STOP(PS_MPEG_MB_DECODE_AC_DEQUANT, 0);
// PROFILE_STOP(PS_MPEG_MB_DECODE_AC_DEQUANT, 0);
}
PROFILE_STOP(PS_MPEG_MB_DECODE_AC, 0);

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