gimple-isel: Recognize vec_extract pattern.

In gimple-isel we already deduce a vec_set pattern from an
ARRAY_REF(VIEW_CONVERT_EXPR).  This patch does the same for a
vec_extract.

The code is largely similar to the vec_set one
including the addition of a can_vec_extract_var_idx_p function
in optabs.cc to check if the backend can handle a register
operand as index.  We already have can_vec_extract in
optabs-query but that one checks whether we can extract
specific modes.

With the introduction of an internal function for vec_extract
the expander must not FAIL.  For vec_set this has already been
the case so adjust the documentation accordingly.

Additionally, clarify the wording of the vector-vector case for
vec_extract.

gcc/ChangeLog:

	* doc/md.texi: Document that vec_set and vec_extract must not
	fail.
	* gimple-isel.cc (gimple_expand_vec_set_expr): Rename this...
	(gimple_expand_vec_set_extract_expr): ...to this.
	(gimple_expand_vec_exprs): Call renamed function.
	* internal-fn.cc (vec_extract_direct): Add.
	(expand_vec_extract_optab_fn): New function to expand
	vec_extract optab.
	(direct_vec_extract_optab_supported_p): Add.
	* internal-fn.def (VEC_EXTRACT): Add.
	* optabs.cc (can_vec_extract_var_idx_p): New function.
	* optabs.h (can_vec_extract_var_idx_p): Declare.

1 files changed, 6 insertions, 1 deletions

diff --git a/gcc/doc/md.texi b/gcc/doc/md.texi
index f14dd32..b30a824 100644
--- a/gcc/doc/md.texi
+++ b/gcc/doc/md.texi
@@ -5091,6 +5091,8 @@ Mask elements @var{i} with @var{i} > (operand 5 + operand 6) are ignored.
 Set given field in the vector value.  Operand 0 is the vector to modify,
 operand 1 is new value of field and operand 2 specify the field index.
 
+This pattern is not allowed to @code{FAIL}.
+
 @cindex @code{vec_extract@var{m}@var{n}} instruction pattern
 @item @samp{vec_extract@var{m}@var{n}}
 Extract given field from the vector value.  Operand 1 is the vector, operand 2
@@ -5098,7 +5100,10 @@ specify field index and operand 0 place to store value into.  The
 @var{n} mode is the mode of the field or vector of fields that should be
 extracted, should be either element mode of the vector mode @var{m}, or
 a vector mode with the same element mode and smaller number of elements.
-If @var{n} is a vector mode, the index is counted in units of that mode.
+If @var{n} is a vector mode the index is counted in multiples of
+mode @var{n}.
+
+This pattern is not allowed to @code{FAIL}.
 
 @cindex @code{vec_init@var{m}@var{n}} instruction pattern
 @item @samp{vec_init@var{m}@var{n}}