[Bug tree-optimization/100578] New: [12 Regression] ICE Segmentation fault during GIMPLE pass: fre (under -g -O2/O3/Os)

[haoxintu at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100578

            Bug ID: 100578
           Summary: [12 Regression] ICE Segmentation fault during GIMPLE
                    pass: fre (under -g -O2/O3/Os)
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: haoxintu at gmail dot com
  Target Milestone: ---

Hi all.

$cat small.c
#include <stdint.h>
void a() {
  int b;
c:
  int8_t d = 3;
  if (b |= d ^= a >= 5)
    ;
  else
    goto c;
}


$gcc -w -g -O2 (same as -O3 and -Os)
during GIMPLE pass: fre
small.c: In function ‘a’:
small.c:2:6: internal compiler error: Segmentation fault
    2 | void a() {
      |      ^
0xba009f crash_signal
        ../../gcc/toplev.c:327
0xd1a01e eliminate_dom_walker::eliminate_stmt(basic_block_def*,
gimple_stmt_iterator*)
        ../../gcc/tree-ssa-sccvn.c:6336
0xd1b64d eliminate_dom_walker::before_dom_children(basic_block_def*)
        ../../gcc/tree-ssa-sccvn.c:6579
0xd1b64d eliminate_dom_walker::before_dom_children(basic_block_def*)
        ../../gcc/tree-ssa-sccvn.c:6512
0x144e967 dom_walker::walk(basic_block_def*)
        ../../gcc/domwalk.c:309
0xd12f10 eliminate_with_rpo_vn(bitmap_head*)
        ../../gcc/tree-ssa-sccvn.c:6759
0xd21d03 do_rpo_vn
        ../../gcc/tree-ssa-sccvn.c:7872
0xd22a90 execute
        ../../gcc/tree-ssa-sccvn.c:7954
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.


Note that this only crashes on the current trunk version of GCC, and I don't
see any crash on released GCC versions.

The output in Godbolt (https://godbolt.org/z/bbjnbGYb4) is:
```
#1 with x86-64 gcc (trunk)
during GIMPLE pass: evrp
<source>: In function 'a':
<source>:10:1: internal compiler error: tree check: expected class 'type', have
'exceptional' (error_mark) in useless_type_conversion_p, at gimple-expr.c:87
   10 | }
```

Another interesting behavior is that GCC-11 accepts this code while GCC-10
downwards versions reject it. GCC-10 output is (in
https://godbolt.org/z/W34EnfhqM):
```
<source>5:3: error: a label can only be part of a statement and a declaration
is not a statement
    5 |   int8_t d = 3;
      |   ^~~~~~
```

GCC version in my machine is:
$gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/home/tuhaoxin/compilers/gcc/build/libexec/gcc/x86_64-pc-linux-gnu/12.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../configure --prefix=/home/tuhaoxin/compilers/gcc/build/
--enable-bootstrap --enable-checking=release --enable-languages=c,c++
-disable-multilib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 12.0.0 20210510 (experimental) (GCC) 


Thanks,
Haoxin