[Bug c/99880] New: ICE in maybe_set_vectorized_backedge_value, at tree-vect-loop.c:9161

[haoxintu at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99880

            Bug ID: 99880
           Summary: ICE in maybe_set_vectorized_backedge_value, at
                    tree-vect-loop.c:9161
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: ice-on-valid-code
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: haoxintu at gmail dot com
  Target Milestone: ---

Hi all.

This valid program makes GCC crash at -O3. GCC-10 also incurs an ICE with a
Segmentation fault, while GCC-10 downwards versions handle this case well.

$cat small.c
int a, b, c, d, e;
void f() {
  b = 5;
  for (; b <= 51; b++)
    ;
  int g = -8;
  while (g) {
    g += 5;
    int h = 10;
    do {
      h -= a = 1;
      for (; a; a++)
        ;
      c *= c >= d >= b;
    } while (h);
    c -= e;
  }
}

$gcc -w -O3 small.c
during GIMPLE pass: vect
small.c: In function ‘f’:
small.c:2:6: internal compiler error: in maybe_set_vectorized_backedge_value,
at tree-vect-loop.c:9161
    2 | void f() {
      |      ^
0x66093f maybe_set_vectorized_backedge_value
        ../../gcc/tree-vect-loop.c:9161
0xdc466a vect_transform_loop(_loop_vec_info*, gimple*)
        ../../gcc/tree-vect-loop.c:9650
0xdebbcc try_vectorize_loop_1
        ../../gcc/tree-vectorizer.c:1104
0xdec7b1 vectorize_loops()
        ../../gcc/tree-vectorizer.c:1243
Please submit a full bug report,
with preprocessed source if appropriate.
Please include the complete backtrace with any bug report.
See <https://gcc.gnu.org/bugs/> for instructions.

$gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/home/tuhaoxin/compilers/gcc/build-20210330/libexec/gcc/x86_64-pc-linux-gnu/11.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../configure
--prefix=/home/tuhaoxin/compilers/gcc/build-20210330/ --enable-bootstrap
--enable-checking=release --enable-languages=c,c++ -disable-multilib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 11.0.1 20210330 (experimental) (GCC)

Reproduced on Godbolt: https://godbolt.org/z/WTje44Wen

Thanks,
Haoxin