Comment 1 for bug 1026634

bisect result (have not investigated further, but certainly seems plausible):

52b1041d3a14eaa4e45f6d8edfbdc0dec4292239 is the first bad commit
commit 52b1041d3a14eaa4e45f6d8edfbdc0dec4292239
Author: Christophe Rhodes <email address hidden>
Date: Thu Apr 5 19:55:05 2012 +0100

    Fix bug in unsigned modular arithmetic using a signed implementation

    If we aim to be clever by implementing an unsigned modular arithmetic
    computation using signed arithmetic, we need to make sure that we
    don't accidentally contaminate the computation with any extraneous
    high bits. This means that we must be sure to cut constants to the
    appropriate width, as well as computations, so do so; this fixes
    bug #974406 from Paul Dietz. (In addition the change from cutting
    to the requested width to the implementation width fixes #903821,
    so Go Team!)

    Test cases. Minimally horrible test case for #903821; far worse
    suggestions were made on #sbcl IRC...

:100644 100644 509ca7c5043b162d723f6a688119439e774bd065 328934f43ae6875bab69a443ce7567722a07114f M NEWS
:040000 040000 b57dbe6c0b63fc1dd48a5840188001d35f048e0f a0fbdf585fe5f186ae594a8730c34b9ebadf25da M src
:040000 040000 c9d018018bdd0753fe721f2575e5dbec4f0de6e0 a196a935833031531cb583eb9297499ec65f7b8f M tests
bisect run success