[V2/PATCH] Fix tree-optimization/102216: missed optimization causing Warray-bounds
Richard Biener
richard.guenther@gmail.com
Wed Oct 27 10:41:19 GMT 2021
On Wed, Oct 27, 2021 at 12:00 PM 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.
Why do you think so? Can you pin-point the transform that now
fixes the new testcase?
Comments below
> 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)))
So this will cause us to rewrite &MEM[p_1].a.b.c; to a pointer-plus,
right? Don't
we want to preserve that for object-size stuff? So maybe directly pattern
match ADDR_EXPR <MEM_REF <SSA_NAME, ..>> only?
> + {
> + 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);
Ick. You should be able to use gimple_assign_set_rhs_with_ops.
> + }
> + }
> +
> + /* Replace the rhs with the new expression. */
> + def_rhs = unshare_expr (def_rhs);
and definitely no need to unshare anything here?
> + 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);
so you only do this after addr_expr forwarding but not on stmts in
general? You could
do it that way in the 2nd loop over the BB.
> tidy_after_forward_propagate_addr (use_stmt);
> return true;
> }
> --
> 2.17.1
>
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