[patch] Reduce over-promotion of vector operations

[Ira Rosen <ira dot rosen at linaro dot org>]

Hi,

This patch tries to reduce over-promotion of vector operations that
could be done with narrower elements, e.g., for

char a;
int b, c;
short d;

b = (int) a;
c = b << 2;
d = (short) c;

we currently produce six vec_unpack_lo/hi_expr statements for
char->int conversion and then two vec_pack_trunc_expr for short->int.
While the shift can be performed on short, using only two
vec_unpack_lo/hi_expr operations for char->short conversion in this
example.

With this patch we detect such over-promoted sequences that start with
a type promotion operation and end with a type demotion operation. The
statements in between are checked if they can be performed using
smaller type (this patch only adds a support for shifts and bit
operations with a constant). If a sequence is detected we create a
sequence of scalar pattern statements to be vectorized instead the
original one.  Since there may be two pattern statements created for
the same original statement - the operation itself (on an intermediate
type) and a type promotion (from a smaller type to the intermediate
type) for the non-constant operand - this patch adds a new field to
struct _stmt_vec_info to keep that pattern def statement.

Bootstrapped and tested on powerpc64-suse-linux.
Comments are welcome.

Thanks,
Ira

ChangeLog:

   * tree-vectorizer.h (struct _stmt_vec_info): Add new field for
   pattern def statement, and its access macro.
   (NUM_PATTERNS): Set to 5.
   * tree-vect-loop.c (vect_determine_vectorization_factor): Handle
   pattern def statement.
   (vect_transform_loop): Likewise.
   * tree-vect-patterns.c (vect_vect_recog_func_ptrs): Add new
   function vect_recog_over_widening_pattern ().
   (vect_operation_fits_smaller_type): New function.
   (vect_recog_over_widening_pattern, vect_mark_pattern_stmts):
   Likewise.
   (vect_pattern_recog_1): Move the code that marks pattern
   statements to vect_mark_pattern_stmts (), and call it.  Update
   documentation.
   * tree-vect-stmts.c (vect_supportable_shift): New function.
   (vect_analyze_stmt): Handle pattern def statement.
   (new_stmt_vec_info): Initialize pattern def statement.

testsuite/ChangeLog:

   * gcc.dg/vect/vect-over-widen-1.c: New test.
   * gcc.dg/vect/vect-over-widen-2.c: New test.
   * gcc.dg/vect/vect-over-widen-3.c: New test.
   * gcc.dg/vect/vect-over-widen-4.c: New test.

Attachment: over-widen.txt
Description: Text document