Re: Add __builtin_clrsb, similar to clz/ctz

[Jakub Jelinek <jakub at redhat dot com>]

On Mon, Jun 20, 2011 at 09:38:22PM +0200, Bernd Schmidt wrote:
> D'oh. Blackfin has a (clrsb:HI (operand:SI)) instruction, so adding this
> showed a problem with some of the existing simplify_const_unop cases:
> for ffs/clz/ctz/clrsb/parity/popcount, we should look at the mode of the
> operand, rather than the mode of the operation. This limits what we can
> do in that function, since op_mode is sometimes VOIDmode - we really
> should add builtin folders for these at some point.

> 	* simplify-rtx.c (simplify_const_unary_operation): Likewise.
> 	Use op_mode rather than mode when optimizing ffs, clz, ctz, parity
> 	and popcount.

This change is IMHO wrong, see e.g.
PR50161 where we have (subreg:SI (popcount:DI (const_int -1))).  This
is supposed to yield 64, but with your changes
it yields 128 - the op_mode here is VOIDmode, so the first if that used
to handle it is no longer used, but as width is <= 2 *
HOST_BITS_PER_WIDE_INT, it is treated as TImode constant.

IMHO best would be just to mandate that for these unary ops like
FFS, CLZ, CLRSB, CTZ, POPCOUNT, PARITY, BSWAP the operand has the same mode
(or VOIDmode) as the unary rtx and that the operation is being carried in
the unop's mode, it shouldn't be hard to adjust the few
*.md patterns (mainly in avr.md, bfin.md).
I think it is bad enough that ZERO_EXTEND must not have CONST_INT argument,
making CONST_INT undefined also for all these unary ops is unnecessary.
I think for NEG/NOT we already have such a guarantee (and thus your change
pessimizes it anyway).
avr.md/bfin.md etc. can use (subreg:HI (popcount:SI (match_operand:SI ...)))
(or (zero_extend:HI (popcount:QI (match_operand:QI ...))) and similar.

Or the
  /* We can do some operations on integer CONST_DOUBLEs.  Also allow
     for a DImode operation on a CONST_INT.  */
  else if (GET_MODE (op) == VOIDmode
           && width <= HOST_BITS_PER_WIDE_INT * 2
           && (GET_CODE (op) == CONST_DOUBLE
               || CONST_INT_P (op)))
case would need to change too to test that op_width ==
HOST_BITS_PER_WIDE_INT * 2 (but then, it would again pessimize at least
NEG/NOT/ABS that are defined sanely).  But we'd also need to change many
other places, e.g. cse_process_notes_1, that currently special case
ZERO_EXTEND/SUBREG (and sometimes SIGN_EXTEND) and pessimize them because
those rtxes aren't allowed to have VOIDmode arguments.  cse_process_notes_1
perhaps could be changed for VOIDmode new_rtx to try to
simplify_replace_rtx it...

	Jakub