Hi,

This is the updated version of patch to deal with the bwaves_r
degradation due to vector construction fed by strided loads.

As Richi's comments [1], this follows the similar idea to over
price the vector construction fed by VMAT_ELEMENTWISE or
VMAT_STRIDED_SLP.  Instead of adding the extra cost on vector
construction costing immediately, it firstly records how many
loads and vectorized statements in the given loop, later in
rs6000_density_test (called by finish_cost) it computes the
load density ratio against all vectorized stmts, and check
with the corresponding thresholds DENSITY_LOAD_NUM_THRESHOLD
and DENSITY_LOAD_PCT_THRESHOLD, do the actual extra pricing
if both thresholds are exceeded.

Note that this new load density heuristic check is based on
some fields in target cost which are updated as needed when
scanning each add_stmt_cost entry, it's independent of the
current function rs6000_density_test which requires to scan
non_vect stmts.  Since it's checking the load stmts count
vs. all vectorized stmts, it's kind of density, so I put
it in function rs6000_density_test.  With the same reason to
keep it independent, I didn't put it as an else arm of the
current existing density threshold check hunk or before this
hunk.

In the investigation of -1.04% degradation from 526.blender_r
on Power8, I noticed that the extra penalized cost 320 on one
single vector construction with type V16QI is much exaggerated,
which makes the final body cost unreliable, so this patch adds
one maximum bound for the extra penalized cost for each vector
construction statement.

Bootstrapped/regtested on powerpc64le-linux-gnu P9.

Full SPEC2017 performance evaluation on Power8/Power9 with
option combinations:
  * -O2 -ftree-vectorize {,-fvect-cost-model=very-cheap} {,-ffast-math}
  * {-O3, -Ofast} {,-funroll-loops}

bwaves_r degradations on P8/P9 have been fixed, nothing else
remarkable was observed.

Is it ok for trunk?

[1] https://gcc.gnu.org/pipermail/gcc-patches/2021-May/570076.html

BR,
Kewen
-----
gcc/ChangeLog:

	* config/rs6000/rs6000.c (struct rs6000_cost_data): New members
	nstmts, nloads and extra_ctor_cost.
	(rs6000_density_test): Add load density related heuristics and the
	checks, do extra costing on vector construction statements if need.
	(rs6000_init_cost): Init new members.
	(rs6000_update_target_cost_per_stmt): New function.
	(rs6000_add_stmt_cost): Factor vect_nonmem hunk out to function
	rs6000_update_target_cost_per_stmt and call it.