Hi Marc,
Good points! I will rework the patch with your suggestions in mind.
Thanks!
Bill
On Mon, 2014-04-21 at 18:51 +0200, Marc Glisse wrote:
> On Mon, 21 Apr 2014, Bill Schmidt wrote:
>
> > Note that it would be possible to do a more general transformation here,
> > in which any vec_select feeding another could be replaced by a
> > vec_select performing the composite function of the other two. I have
> > not done this because I am unaware of this situation arising in
> > practice. If it's desirable, I can extend the patch in this direction.
>
> It does arise, but I think it isn't done because not all permutations are
> (optimally) supported by all targets.
>
> > Index: gcc/simplify-rtx.c
> > ===================================================================
> > --- gcc/simplify-rtx.c (revision 209516)
> > +++ gcc/simplify-rtx.c (working copy)
> > @@ -3673,6 +3673,34 @@ simplify_binary_operation_1 (enum rtx_code code, e
> > }
> > }
> >
> > + /* If we have two nested selects that are inverses of each
> > + other, replace them with the source operand. */
> > + if (GET_CODE (trueop0) == VEC_SELECT)
> > + {
> > + enum machine_mode reg_mode = GET_MODE (XEXP (trueop0, 0));
> > + rtx op0_subop1 = XEXP (trueop0, 1);
> > + gcc_assert (VECTOR_MODE_P (reg_mode));
> > + gcc_assert (GET_MODE_INNER (mode) == GET_MODE_INNER (reg_mode));
> > + gcc_assert (GET_CODE (op0_subop1) == PARALLEL);
> > +
> > + if (XVECLEN (trueop1, 0) == XVECLEN (op0_subop1, 0))
> > + {
> > + /* Apply the second ordering vector to the first.
> > + If the result is { 0, 1, ..., n-1 } then the
> > + two VEC_SELECTs cancel. */
> > + for (int i = 0; i < XVECLEN (trueop1, 0); ++i)
> > + {
> > + rtx x = XVECEXP (trueop1, 0, i);
> > + gcc_assert (CONST_INT_P (x));
> > + rtx y = XVECEXP (op0_subop1, 0, INTVAL (x));
> > + gcc_assert (CONST_INT_P (y));
> > + if (i != INTVAL (y))
> > + return 0;
> > + }
> > + return XEXP (trueop0, 0);
> > + }
> > + }
>
> I may have missed it, but don't you want to check that what you are
> returning has the right mode/length (or generate the obvious vec_select
> otherwise)? I don't know if any platform has such constructions (probably
> not), but in principle you could start from a vector of size 4, extract
> {1,0} from it, extract {1,0} from that, and you don't want to return the
> initial vector as is. On the other hand, I don't think you really care
> whether trueop1 is smaller than op0_subop1. Starting from a vector of size
> 2, extracting {1,0,1,0} then {3,0} gives the initial vector just fine.
>
