Problem with spuriously failing gcc tests with dg-output.

Hi, when I am running gcc test suite

$ make check-gcc RUNTESTFLAGS='-all ubsan.exp=float-cast-overflow-2.c'

                 === gcc tests ===

Schedule of variations:
     unix

Running target unix
Using /usr/share/dejagnu/baseboards/unix.exp as board description file 
for target.
Using /usr/share/dejagnu/config/unix.exp as generic interface file for 
target.
Using /home/georg/gnu/gcc.gnu.org/trunk/gcc/testsuite/config/default.exp 
as tool-and-target-specific interface file.
Running 
/home/georg/gnu/gcc.gnu.org/trunk/gcc/testsuite/gcc.dg/ubsan/ubsan.exp ...
PASS: c-c++-common/ubsan/float-cast-overflow-2.c   -O0  (test for excess 
errors)
PASS: c-c++-common/ubsan/float-cast-overflow-2.c   -O0  execution test
FAIL: c-c++-common/ubsan/float-cast-overflow-2.c   -O0  output pattern test
PASS: c-c++-common/ubsan/float-cast-overflow-2.c   -O1  (test for
...

I am getting spurious FAILs.  Rerunning the same tests might come up 
with more or less FAILs / PASSes.

The tests have "lots" of dg-output directives

$ cat gcc/testsuite/c-c++-common/ubsan/float-cast-overflow-2.c | tail -n 3
/* { dg-output "\[^\n\r]*runtime error: value -?nan is outside the range 
of representable values of type '__int128 
unsigned'\[^\n\r]*(\n|\r\n|\r)" } */
/* { dg-output "\[^\n\r]*runtime error: value inf is outside the range 
of representable values of type '__int128 
unsigned'\[^\n\r]*(\n|\r\n|\r)" } */
/* { dg-output "\[^\n\r]*runtime error: value -inf is outside the range 
of representable values of type '__int128 unsigned'" } */

https://gcc.gnu.org/viewcvs/gcc/trunk/gcc/testsuite/c-c%2B%2B-common/ubsan/float-cast-overflow-2.c?revision=212755&view=markup#l1

In the according log file, some of the "PASS..." output is messed up 
with the output from dg-output, making the test fail:

$ cat testsuite/gcc/gcc.log
PASS: c-c++-common/ubsan/float-cast-overflow-2.c   -O0  (test for excess 
errors)
Setting LD_LIBRARY_PATH to : [cut]
spawn [open ...]

/home/georg/gnu/gcc.gnu.org/trunk/gcc/testsuite/c-c++-common/ubsan/float-cast-overflow-2.c:20:3: 
runtime error: value 1.70141e+38 is outside the range of representable 
values of type '__int128'
/home/georg/gnu/gcc.gnu.org/trunk/gcc/testsuite/c-c++-common/ubsan/float-cast-overflow-2.c:20:3: 
runtime error: value 1.70141e+38 is outside the range of representable 
values of type '__int128'
[...]
/home/georg/gnu/gcc.gnu.org/trunk/gcc/testsuite/c-c++-common/ubsan/float-cast-overflow-2.c:25:3: 
runtime error: value 3.40282e+38 is outside the range of representable 
values of type '__int128 unsigned'
/home/georg/gnu/gcc.gnu.org/trunk/gcc/testsuite/c-c++-common/ubsan/float-cast-overflow-2.c:25:3: 
runtime error: value 3.40282e+38 is outside the range of representable 
values of type '__int128 unsigned'
/home/georg/gnu/gcc.gnu.org/trunk/gcc/testsuite/c-c++-common/ubsan/float-cast-overflow-2.c:25:3: 
runtime error: PASS: c-c++-common/ubsan/float-cast-overflow-2.c   -O0 
execution test

BUMMER! The PASS output comes at the wrong place and messes up the dg-output

FAIL: c-c++-common/ubsan/float-cast-overflow-2.c   -O0  output pattern test
Output was:
c-c++-common/ubsan/float-cast-overflow-2.c:20:3: runtime error: value 
1.70141e+38 is outside the range of representable values of type '__int128'
c-c++-common/ubsan/float-cast-overflow-2.c:20:3: runtime error: value 
1.70141e+38 is outside the range of representable values of type '__int128'
[...]
c-c++-common/ubsan/float-cast-overflow-2.c:25:3: runtime error: value 
3.40282e+38 is outside the range of representable values of type 
'__int128 unsigned'
c-c++-common/ubsan/float-cast-overflow-2.c:25:3: runtime error: value 
3.40282e+38 is outside the range of representable values of type 
'__int128 unsigned'
c-c++-common/ubsan/float-cast-overflow-2.c:25:3: runtime error:
Should match:

Output is missing after "runtime error: " which is the place where the 
"PASS" message was emit.

Any ideas what can cause such fails?  It's on a 8-core x86_64 machine, 
but the testsuite is not started in parallel mode.

$ runtest -version
Expect version is       5.45
Tcl version is          8.6
Framework version is    1.5.3

Does this ring a bell somewhere?  What could shred the output?

Thanks in advance,

Johann