[PATCH] Clear DECL_CHAIN of local vars

Richard Biener rguenther@suse.de
Thu Oct 6 13:29:00 GMT 2016


The following patch makes sure to clear DECL_CHAIN of gimple_bind_vars
once we dissolve that (but makes sure to preserve the BLOCK_VARS tail).
This should allow better GC in the face of remove-unused-locals eventually
removing the last reference to a decl (and not keep it live via DECL_CHAIN
of some others).

I ran into the /* Ugh */ comment during libsubc++ build;  rather than
trying to nail that down I papered over it ...

Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.

Richard.

2016-10-06  Richard Biener  <rguenther@suse.de>

	* gimple-low.c (lower_gimple_bind): Clear DECL_CHAIN of
	vars in gimple_bind_vars but not in BLOCK_VARS.

Index: gcc/gimple-low.c
===================================================================
--- gcc/gimple-low.c	(revision 240831)
+++ gcc/gimple-low.c	(working copy)
@@ -416,6 +416,23 @@ lower_gimple_bind (gimple_stmt_iterator
     }
 
   record_vars (gimple_bind_vars (stmt));
+
+  /* Scrap DECL_CHAIN up to BLOCK_VARS to ease GC after we no longer
+     need gimple_bind_vars.  */
+  tree next;
+  tree end = NULL_TREE;
+  if (gimple_bind_block (stmt))
+    end = BLOCK_VARS (gimple_bind_block (stmt));
+  for (tree var = gimple_bind_vars (stmt); var != end; var = next)
+    {
+      /* Ugh, something is violating the constraint that BLOCK_VARS
+         is a sub-chain of gimple_bind_vars.  */
+      if (! var)
+	break;
+      next = DECL_CHAIN (var);
+      DECL_CHAIN (var) = NULL_TREE;
+    }
+
   lower_sequence (gimple_bind_body_ptr (stmt), data);
 
   if (new_block)



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