On Thu, Aug 24, 2023 at 11:37 PM Richard Biener via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > On Thu, Aug 24, 2023 at 9:16 PM Andrew Pinski via Gcc-patches > <gcc-patches@gcc.gnu.org> wrote: > > > > Even though this is handled by other code inside both VRP and CCP, > > sometimes we want to optimize this outside of VRP and CCP. > > An example is given in PR 106677 where phiopt will happen > > after VRP (which removes a cast for a comparison) and then > > phiopt will optimize the phi to be `a | 1` which can then > > be optimized to `1` due to this patch. > > Also works for xor, no?
No, because IOR is a saturation operation while XOR is not. So if you know that x and C are full subsets (nonzero(x) & ~nonzero(C) == 0) then A^C is not the constant C but rather just (~A)&C which we already a pattern for that to turn it back in to A^C: /* Simplify (~X & Y) to X ^ Y if we know that (X & ~Y) is 0. */ The only thing you can do for XOR is that if you have `A ^ B` and A and B are known not to share any bits in common (that is nonzero(A) & nonzero(B) == 0), you can convert it to `A | B` (that is what simplify-rtx.cc does). Which looks like we don't do on the gimple level. > > > OK? Bootstrapped and tested on x86_64-linux-gnu with no regressions. > > OK with or without adding XOR. Ok. Thanks, Andrew > > Richard. > > > Note Similar code already exists in simplify_rtx for the RTL level; > > it was moved from combine to simplify_rtx in r0-72539-gbd1ef757767f6d. > > gcc/ChangeLog: > > > > * match.pd (`a | C -> C`): New pattern. > > --- > > gcc/match.pd | 6 ++++++ > > 1 file changed, 6 insertions(+) > > > > diff --git a/gcc/match.pd b/gcc/match.pd > > index c87a0795667..3bbeceb37b4 100644 > > --- a/gcc/match.pd > > +++ b/gcc/match.pd > > @@ -1456,6 +1456,12 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT) > > (if (INTEGRAL_TYPE_P (TREE_TYPE (@0)) > > && wi::bit_and_not (get_nonzero_bits (@0), wi::to_wide (@1)) == 0) > > @0)) > > +/* x | C -> C if we know that x & ~C == 0. */ > > +(simplify > > + (bit_ior SSA_NAME@0 INTEGER_CST@1) > > + (if (INTEGRAL_TYPE_P (TREE_TYPE (@0)) > > + && wi::bit_and_not (get_nonzero_bits (@0), wi::to_wide (@1)) == 0) > > + @1)) > > #endif > > > > /* ~(~X - Y) -> X + Y and ~(~X + Y) -> X - Y. */ > > -- > > 2.31.1 > >
