Re: [PATCH] Improve maybe_remove_writeonly_store to do a simple DCE for defining statement

[Richard Biener <richard.guenther@gmail.com>]

On Thu, Oct 21, 2021 at 7:53 AM apinski--- via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> From: Andrew Pinski <apinski@marvell.com>
>
> Instead of putting a full blow DCE after execute_fixup_cfg, it makes sense
> to try to remove the defining statement for the store that is being removed.
> Using simple_dce_from_worklist makes this easier, just mark the ssa_name on
> the rhs side of the store (if it was one) in a bitmap and then call
> simple_dce_from_worklist at the end.
>
> gcc.dg/pr36902.c needed to be changed such that the static array was no
> longer a static array but a global array.  This is because this new dce
> will remove the load as it is dead. I also filed PR 102864 for the warning
> on dead loads.

OK.

Thanks,
Richard.

> gcc/ChangeLog:
>
>         * tree-cfg.c (maybe_remove_writeonly_store): Add dce_ssa_names argument.
>         Mark the ssa-name of the rhs as one to be removed.
>         (execute_fixup_cfg): Update call to maybe_remove_writeonly_store.
>         Call simple_dce_from_worklist at the end to a simple dce.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.dg/pr36902.c: Move buf to be a non-static variable.
> ---
>  gcc/testsuite/gcc.dg/pr36902.c |  5 +----
>  gcc/tree-cfg.c                 | 18 ++++++++++++++++--
>  2 files changed, 17 insertions(+), 6 deletions(-)
>
> diff --git a/gcc/testsuite/gcc.dg/pr36902.c b/gcc/testsuite/gcc.dg/pr36902.c
> index 7dafc9ac171..365a26e26b7 100644
> --- a/gcc/testsuite/gcc.dg/pr36902.c
> +++ b/gcc/testsuite/gcc.dg/pr36902.c
> @@ -24,10 +24,9 @@ struct {
>    unsigned char pcr_select[4];
>  } sel;
>
> +unsigned char buf[64];
>  int bar(void)
>  {
> -  static unsigned char buf[64];
> -
>    sel.size_of_select = 3;
>    foo(buf, sel.pcr_select, sel.size_of_select);
>
> @@ -52,8 +51,6 @@ foo2(unsigned char * to, const unsigned char * from, int n)
>
>  int baz(void)
>  {
> -  static unsigned char buf[64];
> -
>    sel.size_of_select = 5;
>    foo2(buf, sel.pcr_select, sel.size_of_select);
>
> diff --git a/gcc/tree-cfg.c b/gcc/tree-cfg.c
> index dbbf6beb6e4..b3a27bcd17c 100644
> --- a/gcc/tree-cfg.c
> +++ b/gcc/tree-cfg.c
> @@ -54,6 +54,7 @@ along with GCC; see the file COPYING3.  If not see
>  #include "value-prof.h"
>  #include "tree-inline.h"
>  #include "tree-ssa-live.h"
> +#include "tree-ssa-dce.h"
>  #include "omp-general.h"
>  #include "omp-expand.h"
>  #include "tree-cfgcleanup.h"
> @@ -9669,7 +9670,8 @@ make_pass_warn_unused_result (gcc::context *ctxt)
>  /* Maybe Remove stores to variables we marked write-only.
>     Return true if a store was removed. */
>  static bool
> -maybe_remove_writeonly_store (gimple_stmt_iterator &gsi, gimple *stmt)
> +maybe_remove_writeonly_store (gimple_stmt_iterator &gsi, gimple *stmt,
> +                             bitmap dce_ssa_names)
>  {
>    /* Keep access when store has side effect, i.e. in case when source
>       is volatile.  */
> @@ -9692,6 +9694,15 @@ maybe_remove_writeonly_store (gimple_stmt_iterator &gsi, gimple *stmt)
>        print_gimple_stmt (dump_file, stmt, 0,
>                          TDF_VOPS|TDF_MEMSYMS);
>      }
> +
> +  /* Mark ssa name defining to be checked for simple dce. */
> +  if (gimple_assign_single_p (stmt))
> +    {
> +      tree rhs = gimple_assign_rhs1 (stmt);
> +      if (TREE_CODE (rhs) == SSA_NAME
> +         && !SSA_NAME_IS_DEFAULT_DEF (rhs))
> +       bitmap_set_bit (dce_ssa_names, SSA_NAME_VERSION (rhs));
> +    }
>    unlink_stmt_vdef (stmt);
>    gsi_remove (&gsi, true);
>    release_defs (stmt);
> @@ -9714,6 +9725,7 @@ execute_fixup_cfg (void)
>    profile_count num = node->count;
>    profile_count den = ENTRY_BLOCK_PTR_FOR_FN (cfun)->count;
>    bool scale = num.initialized_p () && !(num == den);
> +  auto_bitmap dce_ssa_names;
>
>    if (scale)
>      {
> @@ -9754,7 +9766,7 @@ execute_fixup_cfg (void)
>              }
>
>           /* Remove stores to variables we marked write-only. */
> -         if (maybe_remove_writeonly_store (gsi, stmt))
> +         if (maybe_remove_writeonly_store (gsi, stmt, dce_ssa_names))
>             {
>               todo |= TODO_update_ssa | TODO_cleanup_cfg;
>               continue;
> @@ -9820,6 +9832,8 @@ execute_fixup_cfg (void)
>        && (todo & TODO_cleanup_cfg))
>      loops_state_set (LOOPS_NEED_FIXUP);
>
> +  simple_dce_from_worklist (dce_ssa_names);
> +
>    return todo;
>  }
>
> --
> 2.17.1
>