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

gcc/ChangeLog

@@ -1,3 +1,14 @@
+2019-07-29  Richard Sandiford  <richard.sandiford@arm.com>
+
+	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.
+
 2019-07-29  Richard Sandiford  <richard.sandiford@arm.com>
 
 	* common.opt (Og): Change the initial value of flag_dse to 0.

gcc/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2019-07-29  Richard Sandiford  <richard.sandiford@arm.com>
+
+	* 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.
+
 2019-07-29  Richard Sandiford  <richard.sandiford@arm.com>
 
 	* c-c++-common/guality/Og-global-dse-1.c: New test.

gcc/testsuite/c-c++-common/guality/Og-dce-1.c

@@ -0,0 +1,14 @@
+/* { dg-do run } */
+/* { dg-options "-g" } */
+
+int *__attribute__((noipa)) consume (int *ptr) { return ptr; }
+
+int
+main (void)
+{
+  int x;
+  int *volatile ptr = consume (&x);
+  x = 0;
+  x = 1;	/* { dg-final { gdb-test . "*ptr" "0" } } */
+  return 0;	/* { dg-final { gdb-test . "*ptr" "1" } } */
+}

gcc/testsuite/c-c++-common/guality/Og-dce-2.c

@@ -0,0 +1,19 @@
+/* { dg-do run } */
+/* { dg-options "-g" } */
+
+struct s { int a, b, c, d; };
+
+struct s gs1 = { 1, 2, 3, 4 };
+struct s gs2 = { 5, 6, 7, 8 };
+
+struct s *__attribute__((noipa)) consume (struct s *ptr) { return ptr; }
+
+int
+main (void)
+{
+  struct s x;
+  struct s *volatile ptr = consume (&x);
+  x = gs1;
+  x = gs2;	/* { dg-final { gdb-test . "ptr->a" "1" } } */
+  return 0;	/* { dg-final { gdb-test . "ptr->a" "5" } } */
+}

gcc/testsuite/c-c++-common/guality/Og-dce-3.c

@@ -0,0 +1,29 @@
+/* { dg-do run } */
+/* { dg-options "-g" } */
+
+volatile int amount = 10;
+
+void __attribute__((noipa))
+do_something (int *ptr)
+{
+  *ptr += 10;
+}
+
+int __attribute__((noipa))
+foo (int count)
+{
+  int x = 1;
+  for (int i = 0; i < count; ++i)
+    do_something (&x); /* { dg-final { gdb-test . "x" "1" } } */
+  int res = x; /* { dg-final { gdb-test . "x" "101" } } */
+  x = res + 1;
+  return res; /* { dg-final { gdb-test . "x" "102" } } */
+  
+}
+
+int
+main (void)
+{
+  foo (10);
+  return 0;
+}

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)