frontend: Add novector C++ pragma

FORTRAN currently has a pragma NOVECTOR for indicating that vectorization should
not be applied to a particular loop.

ICC/ICX also has such a pragma for C and C++ called #pragma novector.

As part of this patch series I need a way to easily turn off vectorization of
particular loops, particularly for testsuite reasons.

This patch proposes a #pragma GCC novector that does the same for C++
as gfortan does for FORTRAN and what ICX/ICX does for C++.

I added only some basic tests here, but the next patch in the series uses this
in the testsuite in about ~800 tests.

gcc/cp/ChangeLog:

	* cp-tree.h (RANGE_FOR_NOVECTOR): New.
	(cp_convert_range_for, finish_while_stmt_cond, finish_do_stmt,
	finish_for_cond): Add novector param.
	* init.cc (build_vec_init): Default novector to false.
	* method.cc (build_comparison_op): Likewise.
	* parser.cc (cp_parser_statement): Likewise.
	(cp_parser_for, cp_parser_c_for, cp_parser_range_for,
	cp_convert_range_for, cp_parser_iteration_statement,
	cp_parser_omp_for_loop, cp_parser_pragma): Support novector.
	(cp_parser_pragma_novector): New.
	* pt.cc (tsubst_expr): Likewise.
	* semantics.cc (finish_while_stmt_cond, finish_do_stmt,
	finish_for_cond): Likewise.

gcc/ChangeLog:

	* doc/extend.texi: Document it.

gcc/testsuite/ChangeLog:

	* g++.dg/vect/vect.exp (support vect- prefix).
	* g++.dg/vect/vect-novector-pragma.cc: New test.

1 files changed, 19 insertions, 0 deletions

diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
index e06caf3..89c5b4e 100644
--- a/gcc/doc/extend.texi
+++ b/gcc/doc/extend.texi
@@ -24581,6 +24581,25 @@ void ignore_vec_dep (int *a, int k, int c, int m)
 @}
 @end smallexample
 
+@cindex pragma GCC novector
+@item #pragma GCC novector
+
+With this pragma, the programmer asserts that the following loop should be
+prevented from executing concurrently with SIMD (single instruction multiple
+data) instructions.
+
+For example, the compiler cannot vectorize the following loop with the pragma:
+
+@smallexample
+void foo (int n, int *a, int *b, int *c)
+@{
+  int i, j;
+#pragma GCC novector
+  for (i = 0; i < n; ++i)
+    a[i] = b[i] + c[i];
+@}
+@end smallexample
+
 @cindex pragma GCC unroll @var{n}
 @item #pragma GCC unroll @var{n}