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.
--
Marc Glisse
