Re: [PATCH] Fix missed DSE opportunity with operator delete.

[Richard Biener <richard dot guenther at gmail dot com>]

On Sat, Apr 16, 2016 at 11:32 PM, Mikhail Maltsev <maltsevm@gmail.com> wrote:
> Hi, all!
>
> Currently GCC can optimize away the following dead store:
>
> void test(char *x)
> {
>   *x = 1;
>   free(x);
> }
>
> but not this one (Clang handles both cases):
>
> void test(char *x)
> {
>   *x = 1;
>   delete x;
> }
>
> The attached patch fixes this by introducing a new __attribute__((free)). I
> first tried to add new built-ins for each version of operator delete (there are
> four of them), but it looked a little clumsy, and would require some special
> handling for warning about taking address of built-in function.
>
> Is such approach (i.e. adding a new attribute) OK? Bootstrapped and regtested on
> x86_64-pc-linux-gnu.

Enlarging tree_function_decl is bad.  I think as you made this a decl attribute
it won't work for virtual delete methods which are called indirectly.

Passes should get at the info via flags_from_decl_or_type () and a new
ECF_FREE.

Richard.

> --
> Regards,
>     Mikhail Maltsev
>
> gcc/c/ChangeLog:
>
> 2016-04-16  Mikhail Maltsev  <maltsevm@gmail.com>
>
>         * c-decl.c (merge_decls): Handle free_flag.
>
> gcc/ChangeLog:
>
> 2016-04-16  Mikhail Maltsev  <maltsevm@gmail.com>
>
>         * builtin-attrs.def: Add attribute free.
>         * builtins.def (free): Add attribute free.
>         * doc/extend.texi: Document attribute free.
>         * gtm-builtins.def (_ITM_free): Add attribute free.
>         * tree-core.h (struct tree_function_decl): Add free_flag.
>         * tree-ssa-alias.c (ref_maybe_used_by_call_p_1): Handle free_flag.
>         (call_may_clobber_ref_p_1): Likewise.
>         (stmt_kills_ref_p): Likewise.
>         * tree-streamer-in.c (unpack_ts_function_decl_value_fields): Likewise.
>         * tree-streamer-out.c (pack_ts_function_decl_value_fields): Likewise.
>         * tree.h (DECL_IS_FREE): New accessor macro.
>
> gcc/testsuite/ChangeLog:
>
> 2016-04-16  Mikhail Maltsev  <maltsevm@gmail.com>
>
>         * g++.dg/opt/op-delete-dse.C: New test.
>         * gcc.dg/attr-free.c: New test.
>
> gcc/c-family/ChangeLog:
>
> 2016-04-16  Mikhail Maltsev  <maltsevm@gmail.com>
>
>
>
>         * c-common.c (handle_free_attribute): New function.
>
>
>
>
>
> gcc/cp/ChangeLog:
>
>
>
>
>
> 2016-04-16  Mikhail Maltsev  <maltsevm@gmail.com>
>
>
>
>
>
>         * decl.c (cxx_init_decl_processing): Set flag_free for operator delete.