https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99830
--- Comment #14 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #13)
> 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 ==
> NONZERO_BITS
> which is tested below. */
>
> constop &= nonzero;
>
> /* If we don't have any bits left, return zero. */
> if (constop == 0)
> return const0_rtx;
>
> So, are you suggesting that in all such spots we need to test side_effects_p
> and punt?
Yes, you need to do check side_effects_p *everywhere* you can potentially
remove a side effect. This is not specific to combine, even.
> Note, simplify_and_const_int_1 already starts with:
> if (GET_CODE (varop) == CLOBBER)
> return NULL_RTX;
> so it would need to use
> if (side_effects_p (varop))
> return NULL_RTX;
> instead.
Yeah. This no longer disallows a VOIDmode clobber, but we should not see
those here anyway.
You'll need the same change a few lines later, btw:
varop = force_to_mode (varop, mode, constop, 0);
/* If VAROP is a CLOBBER, we will fail so return it. */
if (GET_CODE (varop) == CLOBBER)
return varop;
(you only need that second one, even, force_to_mode immediately returns
its arg if it is a clobber).
