Re: [V2/PATCH] Fix tree-optimization/102216: missed optimization causing Warray-bounds

[Bernhard Reutner-Fischer]

On 27 October 2021 11:59:58 CEST, apinski--- via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
>From: Andrew Pinski <apinski@marvell.com>
>
>The problem here is tree-ssa-forwprop.c likes to produce
>&MEM <const char *> [(void *)_4 + 152B] which is the same as
>_4 p+ 152 which the rest of GCC likes better.
>This implements this transformation back to pointer plus to
>improve better code generation later on.
>
>OK? Bootstrapped and tested on aarch64-linux-gnu.
>
>Changes from v1:
>* v2: Add comments.
>
>gcc/ChangeLog:
>
>	PR tree-optimization/102216
>	* tree-ssa-forwprop.c (rewrite_assign_addr): New function.
>	(forward_propagate_addr_expr_1): Use rewrite_assign_addr
>	when rewriting into the addr_expr into an assignment.
>
>gcc/testsuite/ChangeLog:
>
>	PR tree-optimization/102216
>	* g++.dg/tree-ssa/pr102216.C: New test.
>---
> gcc/testsuite/g++.dg/tree-ssa/pr102216.C | 22 +++++++++
> gcc/tree-ssa-forwprop.c                  | 58 ++++++++++++++++++------
> 2 files changed, 67 insertions(+), 13 deletions(-)
> create mode 100644 gcc/testsuite/g++.dg/tree-ssa/pr102216.C
>
>diff --git a/gcc/testsuite/g++.dg/tree-ssa/pr102216.C b/gcc/testsuite/g++.dg/tree-ssa/pr102216.C
>new file mode 100644
>index 00000000000..b903e4eb57d
>--- /dev/null
>+++ b/gcc/testsuite/g++.dg/tree-ssa/pr102216.C
>@@ -0,0 +1,22 @@
>+/* { dg-options "-O2 -fdump-tree-optimized" } */
>+void link_error ();
>+void g ()
>+{
>+  const char **language_names;
>+
>+  language_names = new const char *[6];
>+
>+  const char **language_names_p = language_names;
>+
>+  language_names_p++;
>+  language_names_p++;
>+  language_names_p++;
>+
>+  if ( (language_names_p) - (language_names+3) != 0)
>+    link_error();
>+  delete[] language_names;
>+}
>+/* We should have removed the link_error on the gimple level as GCC should
>+   be able to tell that language_names_p is the same as language_names+3.  */
>+/* { dg-final { scan-tree-dump-times "link_error" 0 "optimized" } } */
>+
>diff --git a/gcc/tree-ssa-forwprop.c b/gcc/tree-ssa-forwprop.c
>index a830bab78ba..e4331c60525 100644
>--- a/gcc/tree-ssa-forwprop.c
>+++ b/gcc/tree-ssa-forwprop.c
>@@ -637,6 +637,47 @@ forward_propagate_into_cond (gimple_stmt_iterator *gsi_p)
>   return 0;
> }
> 
>+/* Rewrite the DEF_RHS as needed into the (plain) use statement.  */
>+
>+static void
>+rewrite_assign_addr (gimple_stmt_iterator *use_stmt_gsi, tree def_rhs)
>+{
>+  tree def_rhs_base;
>+  poly_int64 def_rhs_offset;
>+
>+  /* Get the base and offset.  */
>+  if ((def_rhs_base = get_addr_base_and_unit_offset (TREE_OPERAND (def_rhs, 0),
>+						     &def_rhs_offset)))
>+    {
>+      tree new_ptr;
>+      poly_offset_int off = 0;
>+
>+      /* If the base was a MEM, then add the offset to the other
>+         offset and adjust the base. */
>+      if (TREE_CODE (def_rhs_base) == MEM_REF)
>+	{
>+	  off += mem_ref_offset (def_rhs_base);
>+	  new_ptr = TREE_OPERAND (def_rhs_base, 0);
>+	}
>+      else
>+	new_ptr = build_fold_addr_expr (def_rhs_base);
>+
>+      /* If we have the new base is not an address express, then use a p+ expression
>+         as the new expression instead of &MEM[x, offset]. */
>+      if (TREE_CODE (new_ptr) != ADDR_EXPR)
>+	{
>+	  tree offset = wide_int_to_tree (sizetype, off);
>+	  def_rhs = build2 (POINTER_PLUS_EXPR, TREE_TYPE (def_rhs), new_ptr, offset);
>+	}
>+    }
>+
>+  /* Replace the rhs with the new expression.  */
>+  def_rhs = unshare_expr (def_rhs);
>+  gimple_assign_set_rhs_from_tree (use_stmt_gsi, def_rhs);
>+  gimple *use_stmt = gsi_stmt (*use_stmt_gsi);
>+  update_stmt (use_stmt);
>+}
>+
> /* We've just substituted an ADDR_EXPR into stmt.  Update all the
>    relevant data structures to match.  */
> 
>@@ -696,8 +737,8 @@ forward_propagate_addr_expr_1 (tree name, tree def_rhs,
>       if (single_use_p
> 	  && useless_type_conversion_p (TREE_TYPE (lhs), TREE_TYPE (def_rhs)))
> 	{
>-	  gimple_assign_set_rhs1 (use_stmt, unshare_expr (def_rhs));
>-	  gimple_assign_set_rhs_code (use_stmt, TREE_CODE (def_rhs));
>+	  rewrite_assign_addr (use_stmt_gsi, def_rhs);
>+	  gcc_assert (gsi_stmt (*use_stmt_gsi) == use_stmt);
> 	  return true;
> 	}
> 
>@@ -741,14 +782,7 @@ forward_propagate_addr_expr_1 (tree name, tree def_rhs,
>       if (forward_propagate_addr_expr (lhs, new_def_rhs, single_use_p))
> 	return true;
> 
>-      if (useless_type_conversion_p (TREE_TYPE (lhs),
>-				     TREE_TYPE (new_def_rhs)))
>-	gimple_assign_set_rhs_with_ops (use_stmt_gsi, TREE_CODE (new_def_rhs),
>-					new_def_rhs);
>-      else if (is_gimple_min_invariant (new_def_rhs))
>-	gimple_assign_set_rhs_with_ops (use_stmt_gsi, NOP_EXPR, new_def_rhs);
>-      else
>-	return false;
>+      rewrite_assign_addr (use_stmt_gsi, new_def_rhs);
>       gcc_assert (gsi_stmt (*use_stmt_gsi) == use_stmt);
>       update_stmt (use_stmt);

ISTM the above update_stmt is redundant now?
thanks,

>       return true;
>@@ -951,9 +985,7 @@ forward_propagate_addr_expr_1 (tree name, tree def_rhs,
> 					      unshare_expr (def_rhs),
> 					      fold_convert (ptr_type_node,
> 							    rhs2)));
>-      gimple_assign_set_rhs_from_tree (use_stmt_gsi, new_rhs);
>-      use_stmt = gsi_stmt (*use_stmt_gsi);
>-      update_stmt (use_stmt);
>+      rewrite_assign_addr (use_stmt_gsi, new_rhs);
>       tidy_after_forward_propagate_addr (use_stmt);
>       return true;
>     }