[Bug tree-optimization/83668] New: [8 Regression] wrong code with -O -fno-tree-dominator-opts -fgraphite-identity

[zsojka at seznam dot cz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83668

            Bug ID: 83668
           Summary: [8 Regression] wrong code with -O
                    -fno-tree-dominator-opts -fgraphite-identity
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Keywords: wrong-code
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zsojka at seznam dot cz
  Target Milestone: ---
              Host: x86_64-pc-linux-gnu
            Target: x86_64-pc-linux-gnu

Created attachment 43015
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43015&action=edit
reduced testcase

I have encountered several miscompilations with graphite, the minimum set of
flags seems to be the one reported in this PR. -fno-tree-dominator-opts seems
to be always present in the flags.

Output:
$ x86_64-pc-linux-gnu-gcc -O -fno-tree-dominator-opts -fgraphite-identity
testcase.c
$ ./a.out 
Aborted

Here, a == 0.

$ 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-256069-checking-yes-rtl-df-extra-nobootstrap-amd64/bin/../libexec/gcc/x86_64-pc-linux-gnu/8.0.0/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
--disable-bootstrap --with-cloog --with-ppl --with-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-256069-checking-yes-rtl-df-extra-nobootstrap-amd64
Thread model: posix
gcc version 8.0.0 20180102 (experimental) (GCC)