[Bug tree-optimization/93586] New: wrong code at -O1 on x86_64-linux-gnu

[qrzhang at gatech dot edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93586

            Bug ID: 93586
           Summary: wrong code at -O1 on x86_64-linux-gnu
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: qrzhang at gatech dot edu
  Target Milestone: ---

The master branch version of gcc miscompiles the code at -O1 and above.

It seems to be a recent regression. gcc-9 works fine.

Bisection points to:

commit ede31f6ffe73357705e95016046e77c7e3d6ad13
Author: Jan Hubicka <hubi...@ucw.cz>
Date:   Tue Oct 1 21:46:09 2019 +0200

$ gcc-trunk -v
Using built-in specs.
COLLECT_GCC=gcc-trunk
COLLECT_LTO_WRAPPER=/home/absozero/trunk/root-gcc/libexec/gcc/x86_64-pc-linux-gnu/10.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc/configure --prefix=/home/absozero/trunk/root-gcc
--enable-languages=c,c++ --disable-werror --enable-multilib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 10.0.1 20200204 (experimental) [master revision
5f0653a8b75:037d7906159:5bc9d2f5ed4c39a7cad74db34e2bb125e012fa60] (GCC)


$ gcc-trunk abc.c && ./a.out
0

$ gcc-9 -O1 abc.c && ./a.out
0

$ gcc-trunk -O1 abc.c && ./a.out
-1


$ cat abc.c
int printf(const char *, ...);
short a;
long b;
int main() {
  char c[][10][1] = {7, 0, 5, 5};
  for (; b <= 3; b++) {
    a = 0;
    for (; a >= 0; a--)
      if (c[a][b][a])
        break;
  }
  printf("%d\n", a);
}