On Mon, 18 Nov 2019, Richard Biener wrote:
> On Mon, 18 Nov 2019, Jakub Jelinek wrote:
>
> > On Mon, Nov 18, 2019 at 10:44:14AM +0100, Richard Biener wrote:
> > > The following adjusts the find_base_{term,value} heuristic when
> > > looking through ANDs to the case where it is obviously not
> > > aligning a base (when the LSB is set).
> >
> > What is specific about the LSB? I mean & 2 is obviously not an aligning
> > AND either.
>
> It aligns 0x1 and 0x3 ;)
Oh, and of course known-zero-bits magic could have turned a
& 0x7 mask to a & 0x6 one because LSB is known to be clear
already ... :/
So any of these heuristics are - well - just heuristics.
> > Shouldn't we recurse only if the CONST_INT_P operand has
> > some set bits followed by clear bits, i.e. after the != 0 check
> > compute ctz_hwi and verify that INTVAL >> ctz == -1?
>
> I thought of more advanced heuristics but I guess that
> any contiguous set of bits with LSB unset might be aligning
> if the programmer knows upper bits are zero anyways? So I fear
> the == -1 test would not work reliable?
>
> But since find_base_term/value are broken anyways I guess we
> can live with that...
>
> Richard.
