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