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

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

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))
+		{
+		  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);
+}
-- 
2.36.1