Amend match.pd syntax with force-simplified results

This adds a ! marker to result expressions that should simplify
(and if not fail the simplification).  This can for example be
used like

(simplify
  (plus (vec_cond:s @0 @1 @2) @3)
  (vec_cond @0 (plus! @1 @3) (plus! @2 @3)))

to make the simplification only apply in case both plus operations
in the result end up simplified to a simple operand.

2020-07-31  Richard Biener  <rguenther@suse.de>

	* genmatch.c (expr::force_leaf): Add and initialize.
	(expr::gen_transform): Honor force_leaf by passing
	NULL as sequence argument to maybe_push_res_to_seq.
	(parser::parse_expr): Allow ! marker on result expression
	operations.
	* doc/match-and-simplify.texi: Amend.

1 files changed, 15 insertions, 0 deletions

diff --git a/gcc/doc/match-and-simplify.texi b/gcc/doc/match-and-simplify.texi
index 1df8b90..41980ac 100644
--- a/gcc/doc/match-and-simplify.texi
+++ b/gcc/doc/match-and-simplify.texi
@@ -361,6 +361,21 @@ Usually the types of the generated result expressions are
 determined from the context, but sometimes like in the above case
 it is required that you specify them explicitely.
 
+Another modifier for generated expressions is @code{!} which
+tells the machinery to only consider the simplification in case
+the marked expression simplified to a simple operand.  Consider
+for example
+
+@smallexample
+(simplify
+  (plus (vec_cond:s @@0 @@1 @@2) @@3)
+  (vec_cond @@0 (plus! @@1 @@3) (plus! @@2 @@3)))
+@end smallexample
+
+which moves the outer @code{plus} operation to the inner arms
+of the @code{vec_cond} expression but only if the actual plus
+operations both simplify.
+
 As intermediate conversions are often optional there is a way to
 avoid the need to repeat patterns both with and without such
 conversions.  Namely you can mark a conversion as being optional