[Bug rtl-optimization/101188] [postreload] Uses content of a clobbered register

["gjl at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101188

--- Comment #9 from Georg-Johann Lay <gjl at gcc dot gnu.org> ---
The bug works as follows:

postreload.cc::reload_cse_move2add() loops over all insns, and at some point it
encounters

(insn 44 14 15 2 (set (reg/f:HI 14 r14 [58])
        (reg/v/f:HI 16 r16 [orig:51 self ] [51])) "fail1.c":28:5 101
{*movhi_split}
     (nil))

During the analysis for that insn, it executes

          rtx_insn *next = next_nonnote_nondebug_insn (insn);
          rtx set = NULL_RTX;
          if (next)
            set = single_set (next);

where next is

(insn 15 44 16 2 (parallel [
            (set (reg/f:HI 14 r14 [58])
                (plus:HI (reg/f:HI 14 r14 [58])
                    (const_int 68 [0x44])))
            (clobber (reg:QI 31 r31))
        ]) "fail1.c":28:5 175 {addhi3_clobber}
     (nil))

Further down, it continues with success = 0:

      if (success)
        delete_insn (insn);
      changed |= success;
      insn = next;
      [...]
      continue;

The scan then continues with NEXT_INSN (insn), which is the insn AFTER insn 15,
so the CLOBBER of QI:31 in insn 15 is bypassed, and note_stores or similar is
never executed on insn 15.  The "set = single_set (next)" also bypasses that
insn 15 is a PARALLEL with a CLOBBER of a general purpose register.

Appears the code is in postreload since 2003, when postreload.c was split out
of reload1.c.