Bug 9491

commit b5ee04c4142c1e4841d2a8a2badcec3128e18f57
Author: Denys Vlasenko <vda.linux@googlemail.com>
Date:   Mon Jan 9 13:55:11 2017 +0100

    unlzma: fix erroneous "while" instead of "if"

    These parts of the code essentially check whether
    stepping back by rep0 goes negative or not.

    LZMA SDK from lzma1604.7z has the following in the corresponding places:

    ... = dic[dicPos - rep0 + (dicPos < rep0 ? dicBufSize : 0)]

    Clearly, not loop here.

    Technically, "while" here works: if condition is false (because pos
    underflowed), it iterates once, adds header.dict_size (a.k.a. dicBufSize),
    this makes pos positive but smaller than header.dict_size, and loop exits.

    Now we'll just check for negative result of subtraction, which is less code:

    function                                             old     new   delta
    unpack_lzma_stream                                  2659    2641     -18

    (I hope 2 Gbyte+ dictionaries won't be in use soon).
...
...

                                uint32_t pos = buffer_pos - rep0;
-                               while (pos >= header.dict_size)
+                               pos = buffer_pos - rep0;
+                               if ((int32_t)pos < 0)
                                        pos += header.dict_size;