Re: [PATCH] DSE: Use the constant source if possible

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

On Sat, May 21, 2022 at 5:02 AM H.J. Lu via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> When recording store for RTL dead store elimination, check if the source
> register is set only once to a constant.  If yes, record the constant
> as the store source.  It eliminates unrolled zero stores after memset 0
> in a loop where a vector register is used as the zero store source.
>
> gcc/
>
>         PR rtl-optimization/105638
>         * dse.cc (record_store): Use the constant source if the source
>         register is set only once.
>
> gcc/testsuite/
>
>         PR rtl-optimization/105638
>         * g++.target/i386/pr105638.C: New test.
> ---
>  gcc/dse.cc                               | 19 ++++++++++
>  gcc/testsuite/g++.target/i386/pr105638.C | 44 ++++++++++++++++++++++++
>  2 files changed, 63 insertions(+)
>  create mode 100644 gcc/testsuite/g++.target/i386/pr105638.C
>
> diff --git a/gcc/dse.cc b/gcc/dse.cc
> index 30c11cee034..0433dd3d846 100644
> --- a/gcc/dse.cc
> +++ b/gcc/dse.cc
> @@ -1508,6 +1508,25 @@ record_store (rtx body, bb_info_t bb_info)
>
>           if (tem && CONSTANT_P (tem))
>             const_rhs = tem;
> +         else
> +           {
> +             /* If RHS is set only once to a constant, set CONST_RHS
> +                to the constant.  */
> +             df_ref def = DF_REG_DEF_CHAIN (REGNO (rhs));
> +             if (def != nullptr
> +                 && !DF_REF_IS_ARTIFICIAL (def)
> +                 && !DF_REF_NEXT_REG (def))
> +               {
> +                 rtx_insn *def_insn = DF_REF_INSN (def);
> +                 rtx def_body = PATTERN (def_insn);
> +                 if (GET_CODE (def_body) == SET)
> +                   {
> +                     rtx def_src = SET_SRC (def_body);
> +                     if (CONSTANT_P (def_src))
> +                       const_rhs = def_src;

doesn't DSE have its own tracking of stored values?  Shouldn't we
improve _that_ if it is not enough?  I also wonder if you need to
verify the SET isn't partial?

Richard.

> +                   }
> +               }
> +           }
>         }
>      }
>
> diff --git a/gcc/testsuite/g++.target/i386/pr105638.C b/gcc/testsuite/g++.target/i386/pr105638.C
> new file mode 100644
> index 00000000000..ff40a459de1
> --- /dev/null
> +++ b/gcc/testsuite/g++.target/i386/pr105638.C
> @@ -0,0 +1,44 @@
> +/* { dg-do compile { target { ! ia32 } } } */
> +/* { dg-options "-std=gnu++20 -O2 -march=skylake" } */
> +/* { dg-final { scan-assembler-not "vpxor" } } */
> +
> +#include <stdint.h>
> +#include <vector>
> +#include <tr1/array>
> +
> +class FastBoard {
> +public:
> +    typedef std::pair<int, int> movescore_t;
> +    typedef std::tr1::array<movescore_t, 24> scoredlist_t;
> +
> +protected:
> +    std::vector<int> m_critical;
> +
> +    int m_boardsize;
> +};
> +
> +class FastState {
> +public:
> +    FastBoard board;
> +
> +    int movenum;
> +protected:
> +    FastBoard::scoredlist_t scoredmoves;
> +};
> +
> +class KoState : public FastState {
> +private:
> +    std::vector<uint64_t> ko_hash_history;
> +    std::vector<uint64_t> hash_history;
> +};
> +
> +class GameState : public KoState {
> +public:
> +    void foo ();
> +private:
> +    std::vector<KoState> game_history;
> +};
> +
> +void GameState::foo() {
> +    game_history.resize(movenum);
> +}
> --
> 2.36.1
>