Re: [PATCH] ifcvt/vect: Emit COND_ADD for conditional scalar reduction.

[Robin Dapp <rdapp.gcc@gmail.com>]

> Hmm, the function is called at transform time so this shouldn't help
> avoiding the ICE.  I expected we refuse to vectorize _any_ reduction
> when sign dependent rounding is in effect?  OTOH maybe sign-dependent
> rounding is OK but only when we use a unconditional fold-left
> (so a loop mask from fully masking is OK but not an original COND_ADD?).

So we currently only disable the use of partial vectors

      else if (reduction_type == FOLD_LEFT_REDUCTION
	       && reduc_fn == IFN_LAST
	       && FLOAT_TYPE_P (vectype_in)
	       && HONOR_SIGNED_ZEROS (vectype_in)
	       && HONOR_SIGN_DEPENDENT_ROUNDING (vectype_in))
	{
	  if (dump_enabled_p ())
	    dump_printf_loc (MSG_MISSED_OPTIMIZATION, vect_location,
			     "can't operate on partial vectors because"
			     " signed zeros cannot be preserved.\n");
	  LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P (loop_vinfo) = false;

which is inside a LOOP_VINFO_CAN_USE_PARTIAL_VECTORS_P block.

For the fully masked case we continue (and then fail the assertion
on aarch64 at transform time).

I didn't get why that case is ok, though?  We still merge the initial
definition with the identity/neutral op (i.e. possibly -0.0) based on
the loop mask.  Is that different to partial masking?

Regards
 Robin