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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug debug/99830] [11 Regression] ICE: in lra_eliminate_regs_1, at
 lra-eliminations.c:659 with -O2 -fno-expensive-optimizations
 -fno-split-wide-types -g
Date: Fri, 09 Apr 2021 19:10:03 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99830

Jakub Jelinek <jakub at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Assignee|jakub at gcc dot gnu.org           |unassigned at gcc d=
ot gnu.org

--- Comment #13 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Seems the exact spot where the clobber is optimized away is e.g. when
simplify_and_const_int_1 (SImode, (ashift:SI (subreg:SI (and:TI (clobber:TI
(const_int 0 [0])) (const_int 255 [0xff])) 0) (const_int 16 [0x10])), 255);
is called.
It calls nonzero_bits, nonzero_bits sees VARYING << 16 and so returns
0xffff0000,
  /* Turn off all bits in the constant that are known to already be zero.
     Thus, if the AND isn't needed at all, we will have CONSTOP =3D=3D NONZ=
ERO_BITS
     which is tested below.  */

  constop &=3D nonzero;

  /* If we don't have any bits left, return zero.  */
  if (constop =3D=3D 0)
    return const0_rtx;

So, are you suggesting that in all such spots we need to test side_effects_p
and punt?

Note, simplify_and_const_int_1 already starts with:
  if (GET_CODE (varop) =3D=3D CLOBBER)
    return NULL_RTX;
so it would need to use
  if (side_effects_p (varop))
    return NULL_RTX;
instead.=