Prevent tree-ssa-dce.c from deleting stores at -Og

DCE tries to delete dead stores to local data and also tries to insert
debug binds for simple cases:

  /* If this is a store into a variable that is being optimized away,
     add a debug bind stmt if possible.  */
  if (MAY_HAVE_DEBUG_BIND_STMTS
      && gimple_assign_single_p (stmt)
      && is_gimple_val (gimple_assign_rhs1 (stmt)))
    {
      tree lhs = gimple_assign_lhs (stmt);
      if ((VAR_P (lhs) || TREE_CODE (lhs) == PARM_DECL)
	  && !DECL_IGNORED_P (lhs)
	  && is_gimple_reg_type (TREE_TYPE (lhs))
	  && !is_global_var (lhs)
	  && !DECL_HAS_VALUE_EXPR_P (lhs))
	{
	  tree rhs = gimple_assign_rhs1 (stmt);
	  gdebug *note
	    = gimple_build_debug_bind (lhs, unshare_expr (rhs), stmt);
	  gsi_insert_after (i, note, GSI_SAME_STMT);
	}
    }

But this doesn't help for things like "print *ptr" when ptr points
to the local variable (tests Og-dce-1.c and Og-dce-2.c).  It can
also introduce wrong debug info for earlier references (second test
in Og-dce-3.c) or make earlier references unavailable (first test
in Og-dce-3.c).

So for -Og I think it'd be better not to delete any stmts with
vdefs for now.  This also means that we can avoid the potentially
expensive vop walks (which already have a cut-off, but still).

The patch also fixes the Og failures in gcc.dg/guality/pr54970.c
(PR 86638).

2019-07-29  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	PR debug/86638
	* tree-ssa-dce.c (keep_all_vdefs_p): New function.
	(mark_stmt_if_obviously_necessary): Mark all stmts with vdefs as
	necessary if keep_all_vdefs_p is true.
	(mark_aliased_reaching_defs_necessary): Add a gcc_checking_assert
	that keep_all_vdefs_p is false.
	(mark_all_reaching_defs_necessary): Likewise.
	(propagate_necessity): Skip the vuse scan if keep_all_vdefs_p is true.

gcc/testsuite/
	* c-c++-common/guality/Og-dce-1.c: New test.
	* c-c++-common/guality/Og-dce-2.c: Likewise.
	* c-c++-common/guality/Og-dce-3.c: Likewise.

From-SVN: r273872

1 files changed, 23 insertions, 0 deletions

diff --git a/gcc/tree-ssa-dce.c b/gcc/tree-ssa-dce.c
index 90b3f4d..17a8d5e 100644
--- a/gcc/tree-ssa-dce.c
+++ b/gcc/tree-ssa-dce.c
@@ -115,6 +115,14 @@ static bool cfg_altered;
 static int *bb_postorder;
 
 
+/* True if we should treat any stmt with a vdef as necessary.  */
+
+static inline bool
+keep_all_vdefs_p ()
+{
+  return optimize_debug;
+}
+
 /* If STMT is not already marked necessary, mark it, and add it to the
    worklist if ADD_TO_WORKLIST is true.  */
 
@@ -317,6 +325,12 @@ mark_stmt_if_obviously_necessary (gimple *stmt, bool aggressive)
       return;
     }
 
+  if (gimple_vdef (stmt) && keep_all_vdefs_p ())
+    {
+      mark_stmt_necessary (stmt, true);
+      return;
+    }
+
   return;
 }
 
@@ -532,6 +546,9 @@ mark_aliased_reaching_defs_necessary_1 (ao_ref *ref, tree vdef, void *data)
 static void
 mark_aliased_reaching_defs_necessary (gimple *stmt, tree ref)
 {
+  /* Should have been caught before calling this function.  */
+  gcc_checking_assert (!keep_all_vdefs_p ());
+
   unsigned int chain;
   ao_ref refd;
   gcc_assert (!chain_ovfl);
@@ -610,6 +627,8 @@ mark_all_reaching_defs_necessary_1 (ao_ref *ref ATTRIBUTE_UNUSED,
 static void
 mark_all_reaching_defs_necessary (gimple *stmt)
 {
+  /* Should have been caught before calling this function.  */
+  gcc_checking_assert (!keep_all_vdefs_p ());
   walk_aliased_vdefs (NULL, gimple_vuse (stmt),
 		      mark_all_reaching_defs_necessary_1, NULL, &visited);
 }
@@ -813,6 +832,10 @@ propagate_necessity (bool aggressive)
 	  if (!use)
 	    continue;
 
+	  /* No need to search for vdefs if we intrinsicly keep them all.  */
+	  if (keep_all_vdefs_p ())
+	    continue;
+
 	  /* If we dropped to simple mode make all immediately
 	     reachable definitions necessary.  */
 	  if (chain_ovfl)