X-Git-Url: https://git.sesse.net/?p=fjl;a=blobdiff_plain;f=dehuff.h;h=c54f68a55bf38528c629131b7802e71c3b9eda5d;hp=69fc798ddd1f426186576e971126101734531b94;hb=HEAD;hpb=fc9f7affda0b0141099f8e91c1a36ddceed8319c

diff --git a/dehuff.h b/dehuff.h
index 69fc798..c54f68a 100644
--- a/dehuff.h
+++ b/dehuff.h
@@ -14,6 +14,13 @@
 #define DEHUF_TABLE_SIZE (1 << DEHUF_TABLE_BITS)
 static const int DEHUF_SLOW_PATH = -1;
 
+// About 98% of all AC coefficients (control byte + coefficient) are <= 10 bits
+// long; again, see codelen.txt. This will cost us about 6 kB of data to store
+// in L1 cache.
+#define DEHUF_AC_TABLE_BITS 10
+#define DEHUF_AC_TABLE_SIZE (1 << DEHUF_AC_TABLE_BITS)
+static const int AC_DEHUF_SLOW_PATH = 0xf0000000;
+
 struct huffman_table {
 	unsigned num_codes[17];     // BITS
 	unsigned char codes[256];   // HUFFVAL
@@ -33,6 +40,15 @@ struct huffman_table {
 	// the lookup tables is int to avoid extra zero extending. 
 	int lookup_table_codes[DEHUF_TABLE_SIZE]; 
 	int lookup_table_length[DEHUF_TABLE_SIZE]; 
+	
+	// Further lookup tables for decoding AC coefficients.
+	// (Generated but obviously not used for DC coefficients.)
+	// Maps from 10-bit lookahead values to the signed coeffient (_codes),
+	// number of bits to skip (_length) and the number of zero coefficients
+	// after this one (_skip).
+	int ac_table_codes[DEHUF_AC_TABLE_SIZE]; 
+	uint8_t ac_table_length[DEHUF_AC_TABLE_SIZE]; 
+	uint8_t ac_table_skip[DEHUF_AC_TABLE_SIZE]; 
 };
 
 enum coefficient_class {
@@ -48,12 +64,11 @@ void read_huffman_tables(huffman_tables_t* dst, input_func_t* input_func, void*
 unsigned read_huffman_symbol_slow_path(const struct huffman_table* table,
                                        struct bit_source* source);
 
-static inline unsigned read_huffman_symbol(const struct huffman_table* table,
-                                           struct bit_source* source)
+static inline unsigned read_huffman_symbol_no_refill(
+	const struct huffman_table* table,
+	struct bit_source* source)
 {
-	// FIXME: We can read past the end of the stream here in some edge
-	// cases. We need to define some guarantees in the layers above.
-	possibly_refill(source, DEHUF_TABLE_BITS);
+	assert(source->bits_available >= DEHUF_TABLE_BITS);
 	unsigned lookup = peek_bits(source, DEHUF_TABLE_BITS);
 	int code = table->lookup_table_codes[lookup];
 	int length = table->lookup_table_length[lookup];
@@ -61,27 +76,45 @@ static inline unsigned read_huffman_symbol(const struct huffman_table* table,
 	if (code == DEHUF_SLOW_PATH) {
 		return read_huffman_symbol_slow_path(table, source);
 	}
-
+		
 	read_bits(source, length);
 	return code;
 }
 
+static inline unsigned read_huffman_symbol(const struct huffman_table* table,
+                                           struct bit_source* source)
+{
+	possibly_refill(source, DEHUF_TABLE_BITS);
+	return read_huffman_symbol_no_refill(table, source);
+}
+
 // procedure EXTEND (figure F.12)
 
 // Fast lookup table for (1 << (bits - 1)).
 // The table actually helps, since the load can go in parallel with the shift
 // operation below.
 static const int bit_thresholds[16] = {
-	0, 1 << 0, 1 << 2, 1 << 3, 1 << 4, 1 << 5, 1 << 6, 1 << 7, 1 << 8, 1 << 9, 1 << 10, 1 << 11, 1 << 12, 1 << 13, 1 << 14, 1 << 15
+	0, 1 << 0, 1 << 1, 1 << 2, 1 << 3, 1 << 4, 1 << 5, 1 << 6, 1 << 7, 1 << 8, 1 << 9, 1 << 10, 1 << 11, 1 << 12, 1 << 13, 1 << 14
 };
 
-static inline unsigned extend(int val, int bits)
+static inline unsigned extend(int val, unsigned bits)
 {
+#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
+	// GCC should ideally be able to figure out that the conditional move is better, but
+	// it doesn't for various reasons, and this is pretty important for speed, so we hardcode.
+	asm("cmp %1, %0 ; cmovl %2, %0"
+		: "+r" (val)
+		: "g" (bit_thresholds[bits]),
+ 		  "r" (val + (-1 << bits) + 1)
+		: "cc");
+	return val;
+#else
 	if (val < bit_thresholds[bits]) {
 		return val + (-1 << bits) + 1;
 	} else {
 		return val;
 	}
+#endif
 }
 
 #endif /* !defined(_DEHUFF_H) */