[pushed] recog: Fix insn_change_watermark destructor

[Richard Sandiford via Gcc-patches]

Noticed while working on something else that the insn_change_watermark
destructor could call cancel_changes for changes that no longer exist.
The loop in cancel_changes is a nop in that case, but:

  num_changes = num;

can mess things up.

I think this would only affect nested uses of insn_change_watermark.

Tested on aarch64-linux-gnu, aarch64_be-elf and x86_64-linux-gnu,
pushed as obvious.

Richard


gcc/
        * recog.h (insn_change_watermark::~insn_change_watermark): Avoid
        calling cancel_changes for changes that no longer exist.
---
 gcc/recog.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/gcc/recog.h b/gcc/recog.h
index 269094a30f1..e96e66e99f2 100644
--- a/gcc/recog.h
+++ b/gcc/recog.h
@@ -547,13 +547,19 @@ class insn_change_watermark
 {
 public:
   insn_change_watermark () : m_old_num_changes (num_validated_changes ()) {}
-  ~insn_change_watermark () { cancel_changes (m_old_num_changes); }
+  ~insn_change_watermark ();
   void keep () { m_old_num_changes = num_validated_changes (); }
 
 private:
   int m_old_num_changes;
 };
 
+inline insn_change_watermark::~insn_change_watermark ()
+{
+  if (m_old_num_changes < num_validated_changes ())
+    cancel_changes (m_old_num_changes);
+}
+
 #endif
 
 #endif /* GCC_RECOG_H */