Building valgrind from git @ 43543527a293e626e601202ca4eeb2216f40815d
on s390x with gcc-5.real (Ubuntu 5.4.0-6ubuntu1~16.04.12) 5.4.0 20160609
produces this warning for callgrind/main.c

main.c: In function ‘vgCallgrind_post_syscalltime’:
main.c:1779:25: warning: ‘*((void *)&ts_now+8)’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     struct vki_timespec ts_now;
main.c:1779:25: warning: ‘ts_now’ may be used uninitialized in this function [-Wmaybe-uninitialized]

In function collect_time the conditional expression in the switch statement
has type int (after integral promotions). GCC assumes that it may have
values other than the ones listed in the enumerated type it was promoted from.
In that case the memory pointed to by its 1st argument remains unintialised.
Later on vki_timespec_diff will read the contents of ts_now undoditionally.
Hence the warning.
The patch below removes the warning and makes the code more robust should
another enumerator ever be added to Collect_Systime.

diff --git a/callgrind/main.c b/callgrind/main.c
index 904eb42..bda10dc 100644
--- a/callgrind/main.c
+++ b/callgrind/main.c
@@ -1711,7 +1711,7 @@ static
 void collect_time (struct vki_timespec *systime, struct vki_timespec *syscputime)
 {
   switch (CLG_(clo).collect_systime) {
-    case systime_no: tl_assert (0);
+    default: tl_assert (0);
     case systime_msec: {
       UInt ms_timer = VG_(read_millisecond_timer)();
       systime->tv_sec = ms_timer / 1000;