Created attachment 39583 [details]
reduced testcase

Compiler output:
$ x86_64-pc-linux-gnu-gcc -Os -fno-forward-propagate -fno-gcse -fno-rerun-cse-after-loop -mstringop-strategy=byte_loop testcase.c -Wno-psabi
testcase.c: In function 'foo':
testcase.c:9:1: error: dominator of 5 should be 4, not 2
 }
 ^
testcase.c:9:1: internal compiler error: in verify_dominators, at dominance.c:1039
0x7bd965 verify_dominators(cdi_direction)
        /repo/gcc-trunk/gcc/dominance.c:1039
0x7bdb8f checking_verify_dominators
        /repo/gcc-trunk/gcc/dominance.h:71
0x7bdb8f calculate_dominance_info(cdi_direction)
        /repo/gcc-trunk/gcc/dominance.c:664
0x74acdd flow_loops_find(loops*)
        /repo/gcc-trunk/gcc/cfgloop.c:419
0x9ff122 loop_optimizer_init(unsigned int)
        /repo/gcc-trunk/gcc/loop-init.c:93
0x14dd398 if_convert
        /repo/gcc-trunk/gcc/ifcvt.c:5407
0x14df20d execute
        /repo/gcc-trunk/gcc/ifcvt.c:5628
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <http://gcc.gnu.org/bugs.html> for instructions.

$ x86_64-pc-linux-gnu-gcc -v
Using built-in specs.
COLLECT_GCC=/repo/gcc-trunk/binary-latest-amd64/bin/x86_64-pc-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/repo/gcc-trunk/binary-trunk-240036-checking-yes-rtl-df-extra-nographite-amd64/bin/../libexec/gcc/x86_64-pc-linux-gnu/7.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /repo/gcc-trunk//configure --enable-languages=c,c++ --enable-valgrind-annotations --disable-nls --enable-checking=yes,rtl,df,extra --without-cloog --without-ppl --without-isl --build=x86_64-pc-linux-gnu --host=x86_64-pc-linux-gnu --target=x86_64-pc-linux-gnu --with-ld=/usr/bin/x86_64-pc-linux-gnu-ld --with-as=/usr/bin/x86_64-pc-linux-gnu-as --disable-libstdcxx-pch --prefix=/repo/gcc-trunk//binary-trunk-240036-checking-yes-rtl-df-extra-nographite-amd64
Thread model: posix
gcc version 7.0.0 20160908 (experimental) (GCC) 

Tested revisions:
r240036 - ICE
6-branch r239849 - OK