This patch makes CONST_VECTOR use the same encoding as VECTOR_CST.

One problem that occurs in RTL but not at the tree level is that a fair
amount of code uses XVEC and XVECEXP directly on CONST_VECTORs (which is
valid, just with looser checking).  This is complicated by the fact that
vectors are also represented as PARALLELs in some target interfaces,
so using XVECEXP is a good polymorphic way of handling both forms.

Rather than try to untangle all that, the best approach seemed to be to
continue to encode every element in a fixed-length vector.  That way only
target-independent and AArch64 code need to be precise about using
CONST_VECTOR_ELT over XVECEXP.

After this change is no longer valid to modify CONST_VECTORs in-place.
This needed some fix-up in the powerpc backends.

2018-01-02  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* doc/rtl.texi (const_vector): Describe new encoding scheme.
	* Makefile.in (OBJS): Add rtx-vector-builder.o.
	* rtx-vector-builder.h: New file.
	* rtx-vector-builder.c: Likewise.
	* rtl.h (rtx_def::u2): Add a const_vector field.
	(CONST_VECTOR_NPATTERNS): New macro.
	(CONST_VECTOR_NELTS_PER_PATTERN): Likewise.
	(CONST_VECTOR_DUPLICATE_P): Likewise.
	(CONST_VECTOR_STEPPED_P): Likewise.
	(CONST_VECTOR_ENCODED_ELT): Likewise.
	(const_vec_duplicate_p): Check for a duplicated vector encoding.
	(unwrap_const_vec_duplicate): Likewise.
	(const_vec_series_p): Check for a non-duplicated vector encoding.
	Say that the function only returns true for integer vectors.
	* emit-rtl.c: Include rtx-vector-builder.h.
	(gen_const_vec_duplicate_1): Delete.
	(gen_const_vector): Call gen_const_vec_duplicate instead of
	gen_const_vec_duplicate_1.
	(const_vec_series_p_1): Operate directly on the CONST_VECTOR encoding.
	(gen_const_vec_duplicate): Use rtx_vector_builder.
	(gen_const_vec_series): Likewise.
	(gen_rtx_CONST_VECTOR): Likewise.
	* config/powerpcspe/powerpcspe.c: Include rtx-vector-builder.h.
	(swap_const_vector_halves): Take an rtx pointer rather than rtx.
	Build a new vector rather than modifying a CONST_VECTOR in-place.
	(handle_special_swappables): Update call accordingly.
	* config/rs6000/rs6000-p8swap.c: Include rtx-vector-builder.h.
	(swap_const_vector_halves): Take an rtx pointer rather than rtx.
	Build a new vector rather than modifying a CONST_VECTOR in-place.
	(handle_special_swappables): Update call accordingly.

From-SVN: r256102