Let the target choose a vectorisation alignment

Programming / Compilers / GCC - rsandifo [138bc75d-0d04-0410-961f-82ee72b054a4] - 22 September 2017 12:44 EDT

The vectoriser aligned vectors to TYPE_ALIGN unconditionally, although there was also a hard-coded assumption that this was equal to the type size. This was inconvenient for SVE for two reasons:

- When compiling for a specific power-of-2 SVE vector length, we might want to align to a full vector. However, the TYPE_ALIGN is governed by the ABI alignment, which is 128 bits regardless of size.

- For vector-length-agnostic code it doesn't usually make sense to align, since the runtime vector length might not be a power of two. Even for power of two sizes, there's no guarantee that aligning to the previous 16 bytes will be an improveent.

This patch therefore adds a target hook to control the preferred
vectoriser (as opposed to ABI) alignment.

2017-09-22 Richard Sandiford Alan Hayward David Sherwood

gcc/
- target.def (preferred_vector_alignment): New hook.
- doc/tm.texi.in (TARGET_VECTORIZE_PREFERRED_VECTOR_ALIGNMENT): New hook.
- doc/tm.texi: Regenerate.
- targhooks.h (default_preferred_vector_alignment): Declare.
- targhooks.c (default_preferred_vector_alignment): New function.
- tree-vectorizer.h (dataref_aux): Add a target_alignment field. Expand commentary. (DR_TARGET_ALIGNMENT): New macro. (aligned_access_p): Update commentary. (vect_known_alignment_in_bytes): New function.
- tree-vect-data-refs.c (vect_calculate_required_alignment): New function. (vect_compute_data_ref_alignment): Set DR_TARGET_ALIGNMENT. Calculate the misalignment based on the target alignment rather than the vector size. (vect_update_misalignment_for_peel): Use DR_TARGET_ALIGMENT rather than TYPE_ALIGN / BITS_PER_UNIT to update the misalignment. (vect_enhance_data_refs_alignment): Mask the byte misalignment with the target alignment, rather than masking the element misalignment with the number of elements in a vector. Also use the target alignment when calculating the maximum number of peels. (vect_find_same_alignment_drs): Use vect_calculate_required_alignment instead of TYPE_ALIGN_UNIT. (vect_duplicate_ssa_name_ptr_info): Remove stmt_info parameter. Measure DR_MISALIGNMENT relative to DR_TARGET_ALIGNMENT. (vect_create_addr_base_for_vector_ref): Update call accordingly. (vect_create_data_ref_ptr): Likewise. (vect_setup_realignment): Realign by ANDing with
-DR_TARGET_MISALIGNMENT.
- tree-vect-loop-manip.c (vect_gen_prolog_loop_niters): Calculate the number of peels based on DR_TARGET_ALIGNMENT.
- tree-vect-stmts.c (get_group_load_store_type): Compare the gap with the guaranteed alignment boundary when deciding whether overrun is OK. (vectorizable_mask_load_store): Interpret DR_MISALIGNMENT relative to DR_TARGET_ALIGNMENT instead of TYPE_ALIGN_UNIT. (ensure_base_align): Remove stmt_info parameter. Get the target base alignment from DR_TARGET_ALIGNMENT. (vectorizable_store): Update call accordingly. Interpret DR_MISALIGNMENT relative to DR_TARGET_ALIGNMENT instead of TYPE_ALIGN_UNIT. (vectorizable_load): Likewise.

gcc/testsuite/
- gcc.dg/vect/vect-outer-3a.c: Adjust dump scan for new wording of alignment message.
- gcc.dg/vect/vect-outer-3a-big-array.c: Likewise.

aec313e Let the target choose a vectorisation alignment
gcc/ChangeLog | 48 +++++++++++
gcc/doc/tm.texi | 12 +++
gcc/doc/tm.texi.in | 2 +
gcc/target.def | 14 ++++
gcc/targhooks.c | 9 +++
gcc/targhooks.h | 1 +
gcc/testsuite/ChangeLog | 8 ++
.../gcc.dg/vect/vect-outer-3a-big-array.c | 2 +-
gcc/testsuite/gcc.dg/vect/vect-outer-3a.c | 2 +-
gcc/tree-vect-data-refs.c | 92 ++++++++++++----------
gcc/tree-vect-loop-manip.c | 51 ++++++------
gcc/tree-vect-stmts.c | 83 ++++++++++---------
gcc/tree-vectorizer.h | 23 +++++-
13 files changed, 242 insertions(+), 105 deletions(-)

Upstream: gcc.gnu.org


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