Re: [PATCH v2] DSE: Use the constant store source if possible

["H.J. Lu" <hjl.tools@gmail.com>]

On Wed, May 25, 2022 at 12:30 AM Richard Sandiford
<richard.sandiford@arm.com> wrote:
>
> "H.J. Lu via Gcc-patches" <gcc-patches@gcc.gnu.org> writes:
> > On Mon, May 23, 2022 at 12:38:06PM +0200, Richard Biener wrote:
> >> 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
> >
> > It tracks stored values only within the basic block.  When RTL loop
> > invariant motion hoists a constant initialization out of the loop into
> > a separate basic block, the constant store value becomes unknown
> > within the original basic block.
> >
> >> improve _that_ if it is not enough?  I also wonder if you need to
> >
> > My patch extends DSE stored value tracking to include the constant which
> > is set only once in another basic block.
> >
> >> verify the SET isn't partial?
> >>
> >
> > Here is the v2 patch to check that the constant is set by a non-partial
> > unconditional load.
> >
> > OK for master?
> >
> > Thanks.
> >
> > H.J.
> > ---
> > RTL DSE tracks redundant constant stores within a basic block.  When RTL
> > loop invariant motion hoists a constant initialization out of the loop
> > into a separate basic block, the constant store value becomes unknown
> > within the original basic block.  When recording store for RTL DSE, check
> > if the source register is set only once to a constant by a non-partial
> > unconditional load.  If yes, record the constant as the constant 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                               | 22 ++++++++++++
> >  gcc/testsuite/g++.target/i386/pr105638.C | 44 ++++++++++++++++++++++++
> >  2 files changed, 66 insertions(+)
> >  create mode 100644 gcc/testsuite/g++.target/i386/pr105638.C
> >
> > diff --git a/gcc/dse.cc b/gcc/dse.cc
> > index 30c11cee034..af8e88dac32 100644
> > --- a/gcc/dse.cc
> > +++ b/gcc/dse.cc
> > @@ -1508,6 +1508,28 @@ 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_FLAGS (def)
> > +                    & (DF_REF_PARTIAL | DF_REF_CONDITIONAL))
> > +               && !DF_REF_NEXT_REG (def))
>
> Can we introduce a helper for this?  There are already similar tests
> in ira and loop-iv, and it seems a bit too complex to have to open-code
> each time.

I can use find_single_def_src in loop-iv.cc:

/* If REGNO has a single definition, return its known value, otherwise return
   null.  */

rtx
find_single_def_src (unsigned int regno)

and do

             /* If RHS is set only once to a constant, set CONST_RHS
                 to the constant.  */
              rtx def_src = find_single_def_src (REGNO (rhs));
              if (def_src != nullptr && CONSTANT_P (def_src))
                {
                  df_ref def = DF_REG_DEF_CHAIN (REGNO (rhs));
                  if (!(DF_REF_FLAGS (def)
                        & (DF_REF_PARTIAL | DF_REF_CONDITIONAL)))
                  {
                      rtx_insn *def_insn = DF_REF_INSN (def);
                      rtx def_body = single_set (def_insn);
                      if (rhs == SET_DEST (def_body))
                        const_rhs = def_src;
                    }
                }
            }

Will it work?

Thanks.

> Thanks,
> Richard
>
> > +             {
> > +               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)
> > +                       && GET_MODE (def_src) == GET_MODE (rhs))
> > +                     const_rhs = def_src;
> > +                 }
> > +             }
> > +         }
> >       }
> >      }
> >
> > 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);
> > +}



-- 
H.J.