Re: [PATCH] vect: Add a popcount fallback.

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

> We seem to be looking at promotions of the call argument, lhs_type
> is the same as the type of the call LHS.  But the comment mentions .POPCOUNT
> and the following code also handles others, so maybe handling should be
> moved.  Also when we look to vectorize popcount (x) instead of popcount((T)x)
> we can simply promote the result accordingly.

IMHO lhs_type is the type of the conversion

  lhs_oprnd = gimple_assign_lhs (last_stmt);
  lhs_type = TREE_TYPE (lhs_oprnd);

and rhs/unprom_diff has the type of the call's input argument

  rhs_oprnd = gimple_call_arg (call_stmt, 0);
  vect_look_through_possible_promotion (vinfo, rhs_oprnd, &unprom_diff);

So we can potentially have
  T0 arg
  T1 in = (T1)arg
  T2 ret = __builtin_popcount (in)
  T3 lhs = (T3)ret

and we're checking if precision (T0) == precision (T3).

This will never be true for a proper __builtin_popcountll except if
the return value is cast to uint64_t (which I just happened to do
in my test...).  Therefore it still doesn't really make sense to me.

Interestingly though, it helps for an aarch64 __builtin_popcountll
testcase where we abort here and then manage to vectorize via
vectorizable_call.  When we skip this check, recognition succeeds
and replaces the call with the pattern.  Then scalar costs are lower
than in the vectorizable_call case because __builtin_popcountll is
not STMT_VINFO_RELEVANT_P anymore (not live or so?).
Then, vectorization costs are too high compared to the wrong scalar
costs and we don't vectorize... Odd, might require fixing separately.
We might need to calculate the scalar costs in advance?

> It looks like vect_recog_popcount_clz_ctz_ffs_pattern is specifcally for
> the conversions, so your fallback should possibly apply even when not
> matching them.

Mhm, yes it appears to only match when casting the return value to
something else than an int.  So we'd need a fallback in vectorizable_call?
And it would potentially look a bit out of place there only handling
popcount and not ctz, clz, ...  Not sure if it is worth it then?

Regards
 Robin