middle-end: delay checking for alignment to load [PR118464]

middle-end: delay checking for alignment to load [PR118464]

This fixes two PRs on Early break vectorization by delaying the safety checks to
vectorizable_load when the VF, VMAT and vectype are all known.

This patch does add two new restrictions:

1. On LOAD_LANES targets, where the buffer size is known, we reject non-power
   of two group sizes, as they are unaligned every other iteration and so may
   cross a page unwittingly.  For those cases require partial masking support.

2. On LOAD_LANES targets when the buffer is unknown, we reject vectorization if
   we cannot peel for alignment, as the alignment requirement is quite large at
   GROUP_SIZE * vectype_size.  This is unlikely to ever be beneficial so we
   don't support it for now.

There are other steps documented inside the code itself so that the reasoning
is next to the code.

As a fall-back, when the alignment fails we require partial vector support.

For VLA targets like SVE return element alignment as the desired vector
alignment.  This means that the loads are never misaligned and so annoying it
won't ever need to peel.

So what I think needs to happen in GCC 16 is that.

1. during vect_compute_data_ref_alignment we need to take the max of
   POLY_VALUE_MIN and vector_alignment.

2. vect_do_peeling define skip_vector when PFA for VLA, and in the guard add a
   check that ncopies * vectype does not exceed POLY_VALUE_MAX which we use as a
   proxy for pagesize.

3. Force LOOP_VINFO_USING_PARTIAL_VECTORS_P to be true in
   vect_determine_partial_vectors_and_peeling since the first iteration has to
   be partial. Require LOOP_VINFO_MUST_USE_PARTIAL_VECTORS_P otherwise we have
   to fail to vectorize.

4. Create a default mask to be used, so that vect_use_loop_mask_for_alignment_p
   becomes true and we generate the peeled check through loop control for
   partial loops.  From what I can tell this won't work for
   LOOP_VINFO_FULLY_WITH_LENGTH_P since they don't have any peeling support at
   all in the compiler.  That would need to be done independently from the
   above.

In any case, not GCC 15 material so I've kept the WIP patches I have downstream.

Bootstrapped Regtested on aarch64-none-linux-gnu,
arm-none-linux-gnueabihf, x86_64-pc-linux-gnu
-m32, -m64 and no issues.

gcc/ChangeLog:

	PR tree-optimization/118464
	PR tree-optimization/116855
	* doc/invoke.texi (min-pagesize): Update docs with vectorizer use.
	* tree-vect-data-refs.cc (vect_analyze_early_break_dependences): Delay
	checks.
	(vect_compute_data_ref_alignment): Remove alignment checks and move to
	get_load_store_type, increase group access alignment.
	(vect_enhance_data_refs_alignment): Add note to comment needing
	investigating.
	(vect_analyze_data_refs_alignment): Likewise.
	(vect_supportable_dr_alignment): For group loads look at first DR.
	* tree-vect-stmts.cc (get_load_store_type):
	Perform safety checks for early break pfa.
	* tree-vectorizer.h (dr_set_safe_speculative_read_required,
	dr_safe_speculative_read_required, DR_SCALAR_KNOWN_BOUNDS): New.
	(need_peeling_for_alignment): Renamed to...
	(safe_speculative_read_required): .. This
	(class dr_vec_info): Add scalar_access_known_in_bounds.

gcc/testsuite/ChangeLog:

	PR tree-optimization/118464
	PR tree-optimization/116855
	* gcc.dg/vect/bb-slp-pr65935.c: Update, it now vectorizes because the
	load type is relaxed later.
	* gcc.dg/vect/vect-early-break_121-pr114081.c: Update.
	* gcc.dg/vect/vect-early-break_22.c: Require partial vectors.
	* gcc.dg/vect/vect-early-break_128.c: Likewise.
	* gcc.dg/vect/vect-early-break_26.c: Likewise.
	* gcc.dg/vect/vect-early-break_43.c: Likewise.
	* gcc.dg/vect/vect-early-break_44.c: Likewise.
	* gcc.dg/vect/vect-early-break_2.c: Require load_lanes.
	* gcc.dg/vect/vect-early-break_7.c: Likewise.
	* gcc.dg/vect/vect-early-break_132-pr118464.c: New test.
	* gcc.dg/vect/vect-early-break_133_pfa1.c: New test.
	* gcc.dg/vect/vect-early-break_133_pfa11.c: New test.
	* gcc.dg/vect/vect-early-break_133_pfa10.c: New test.
	* gcc.dg/vect/vect-early-break_133_pfa2.c: New test.
	* gcc.dg/vect/vect-early-break_133_pfa3.c: New test.
	* gcc.dg/vect/vect-early-break_133_pfa4.c: New test.
	* gcc.dg/vect/vect-early-break_133_pfa5.c: New test.
	* gcc.dg/vect/vect-early-break_133_pfa6.c: New test.
	* gcc.dg/vect/vect-early-break_133_pfa7.c: New test.
	* gcc.dg/vect/vect-early-break_133_pfa8.c: New test.
	* gcc.dg/vect/vect-early-break_133_pfa9.c: New test.
	* gcc.dg/vect/vect-early-break_39.c: Update testcase for misalignment.
	* gcc.dg/vect/vect-early-break_18.c: Likewise.
	* gcc.dg/vect/vect-early-break_20.c: Likewise.
	* gcc.dg/vect/vect-early-break_21.c: Likewise.
	* gcc.dg/vect/vect-early-break_38.c: Likewise.
	* gcc.dg/vect/vect-early-break_6.c: Likewise.
	* gcc.dg/vect/vect-early-break_53.c: Likewise.
	* gcc.dg/vect/vect-early-break_56.c: Likewise.
	* gcc.dg/vect/vect-early-break_57.c: Likewise.
	* gcc.dg/vect/vect-early-break_81.c: Likewise.