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

[Martin Sebor]

On 10/27/21 3:59 AM, apinski--- via Gcc-patches 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.

Since the purpose of this transformation is to avoid a bogus
-Warray-bounds can you please include a test case showing
the difference it makes? (I.e., one that warns without
the patch and doesn't with it.  The test in the patch doesn't
trigger a warning for me.)

Thanks
Martin

> 
> 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);
>         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;
>       }
>