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

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

On Mon, May 23, 2022 at 11:42 PM Richard Biener
<richard.guenther@gmail.com> wrote:
>
> On Mon, May 23, 2022 at 8:34 PM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > 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 really use df-chain here and rely that a single definition is
> the only one?  If rhs is a hardreg does df-chain include implicit
> sets of function argument registers for example?  Don't we need RD
> here or at least verify the single df-chain definition dominates the
> use here (if we can rely on the reg otherwise be uninitialized and thus
> the use invoking undefined behavior we could use the constant even
> in non-dominating context, but WRT the function args & hardregs I'm not
> sure we can tell).

Does the hard register have a regular DEF?  Won't the function args &
hardregs have DF_REF_ARTIFICIAL?

>
> > +               {
> > +                 rtx_insn *def_insn = DF_REF_INSN (def);
> > +                 rtx def_body = PATTERN (def_insn);
> > +                 if (GET_CODE (def_body) == SET)
>
> I think you should be able to use
>
>            rtx def_body = single_set (def_insn);
>            if (def_body)
>
> here to get at some more cases.

Will change it.

> > +                   {
> > +                     rtx def_src = SET_SRC (def_body);
> > +                     if (CONSTANT_P (def_src)
> > +                         && GET_MODE (def_src) == GET_MODE (rhs))
>
> possibly verify SET_DEST (def_body) == rhs to rule out subregs and mode punning
> instead?  Then def_src should be automatically OK.  I wonder whether it's worth

Will change it.

> tracking (or if DSE even can) loads from the constant pool with constant.> REG_EQUAL/EQUIV notes?

CONSTANT_P returns true for a constant pool.   I am not
sure if DSE can handle a constant pool.

Thanks.

>
> > +                       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);
> > +}
> > --
> > 2.36.1
> >



-- 
H.J.