rework afs_random() yet again

clang 7 notes that ~0 is signed and that left-shifting into the sign
bit is undefined behvaior.  Use a new construction to clear the low
byte of tv_usec with only bitwise operations that are independent of
the width of tv_usec and stay within the realm of C's defined behavior.

Change-Id: I3e4f0fa4a8b8b72df23ef0c8ad7c4a229ac942f3
Reviewed-on: https://gerrit.openafs.org/13474
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Cheyenne Wills <cwills@sinenomine.net>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>

diff --git a/src/afs/afs_server.c b/src/afs/afs_server.c
index 5977604..d8414d6 100644 (file)
--- a/src/afs/afs_server.c
+++ b/src/afs/afs_server.c
@@ -826,10 +826,12 @@ afs_random(void)
        osi_timeval_t t;
        osi_GetTime(&t);
        /*
-        * 0xfffffff0 was changed to (~0 << 4) since it works no matter how many
-        * bits are in a tv_usec
+        * Clear the low byte of tv_usec in a size-independent manner before adding
+        * in the rest of the state.
         */
-       state = (t.tv_usec & (~0 << 4)) + (rxi_getaddr() & 0xff);
+       state = t.tv_usec;
+       state ^= (state & 0xff);
+       state += rxi_getaddr() & 0xff;
        state += (t.tv_sec & 0xff);
        for (i = 0; i < 30; i++) {
            ranstage(state);