Some asm() correction.

[fjl] / dehuff.h

diff --git a/dehuff.h b/dehuff.h
index 728c62315c5570c9b3fb6f1771368d219330a2a3..2669034554c25fb16362ace9879f5fa280be9592 100644 (file)
--- a/dehuff.h
+++ b/dehuff.h
@@ -48,12 +48,11 @@ 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];
@@ -61,9 +60,45 @@ 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 << 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, 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) */