[16/23] recog: Add a way of temporarily undoing changes

[Richard Sandiford <richard.sandiford@arm.com>]

In some cases, it can be convenient to roll back the changes that
have been made by validate_change to see how things looked before,
then reroll the changes.  For example, this makes it possible
to defer calculating the cost of an instruction until we know that
the result is actually needed.  It can also make dumps easier to read.

This patch adds a couple of helper functions for doing that.

gcc/
	* recog.h (temporarily_undo_changes, redo_changes): Declare.
	* recog.c (swap_change, temporarily_undo_changes): New functions.
	(redo_changes): Likewise.
---
 gcc/recog.c | 40 ++++++++++++++++++++++++++++++++++++++++
 gcc/recog.h |  2 ++
 2 files changed, 42 insertions(+)

diff --git a/gcc/recog.c b/gcc/recog.c
index 65125b8f0d1..309a578a151 100644
--- a/gcc/recog.c
+++ b/gcc/recog.c
@@ -577,6 +577,46 @@ cancel_changes (int num)
   num_changes = num;
 }
 
+/* Swap the status of change NUM from being applied to not being applied,
+   or vice versa.  */
+
+static void
+swap_change (int num)
+{
+  if (changes[num].old_len >= 0)
+    std::swap (XVECLEN (*changes[num].loc, 0), changes[num].old_len);
+  else
+    std::swap (*changes[num].loc, changes[num].old);
+  if (changes[num].object && !MEM_P (changes[num].object))
+    std::swap (INSN_CODE (changes[num].object), changes[num].old_code);
+}
+
+/* Temporarily undo all the changes numbered NUM and up, with a view
+   to reapplying them later.  The next call to the changes machinery
+   must be:
+
+      redo_changes (NUM)
+
+   otherwise things will end up in an invalid state.  */
+
+void
+temporarily_undo_changes (int num)
+{
+  for (int i = num_changes - 1; i >= num; i--)
+    swap_change (i);
+}
+
+/* Redo the changes that were temporarily undone by:
+
+      temporarily_undo_changes (NUM).  */
+
+void
+redo_changes (int num)
+{
+  for (int i = num; i < num_changes; ++i)
+    swap_change (i);
+}
+
 /* Reduce conditional compilation elsewhere.  */
 /* A subroutine of validate_replace_rtx_1 that tries to simplify the resulting
    rtx.  */
diff --git a/gcc/recog.h b/gcc/recog.h
index e152e2bb591..facf36e7c08 100644
--- a/gcc/recog.h
+++ b/gcc/recog.h
@@ -96,6 +96,8 @@ extern void confirm_change_group (void);
 extern int apply_change_group (void);
 extern int num_validated_changes (void);
 extern void cancel_changes (int);
+extern void temporarily_undo_changes (int);
+extern void redo_changes (int);
 extern int constrain_operands (int, alternative_mask);
 extern int constrain_operands_cached (rtx_insn *, int);
 extern int memory_address_addr_space_p (machine_mode, rtx, addr_space_t);
-- 
2.17.1