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Subject: [Bug middle-end/105638] New: Redundant stores aren't removed by DSE
Date: Tue, 17 May 2022 23:29:05 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105638

            Bug ID: 105638
           Summary: Redundant stores aren't removed by DSE
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hjl.tools at gmail dot com
  Target Milestone: ---

For

$ cat foo.cpp
#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;=20=20=20=20
};

class FastState {
public:=20=20=20=20=20=20=20=20
    FastBoard board;

    int movenum;=20=20=20=20=20=20=20=20=20=20=20=20=20=20
protected:
    FastBoard::scoredlist_t scoredmoves;
};

class KoState : public FastState {
private:=20=20=20=20=20=20=20=20=20
    std::vector<uint64_t> ko_hash_history;=20=20=20
    std::vector<uint64_t> hash_history;=20=20=20=20=20
};

class GameState : public KoState {
public:=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20=20
    void foo ();=20=20=20=20=20=20
private:
    std::vector<KoState> game_history;=20=20=20=20=20=20=20=20=20=20=20=20=
=20=20=20=20=20=20=20=20=20=20=20=20=20=20
};

void GameState::foo() {
    game_history.resize(movenum);
}
$ g++ -O2 -march=3Dskylake foo.cpp -S

generates:

...
        movl    $280, %edx
        xorl    %esi, %esi
        call    memset
        movq    %rax, %rcx
        vpxor   %xmm0, %xmm0, %xmm0
        addq    $280, %rcx
        vmovdqu %xmm0, 36(%rax)
        vmovdqu %xmm0, 52(%rax)
        vmovdqu %xmm0, 68(%rax)
        vmovdqu %xmm0, 84(%rax)
        vmovdqu %xmm0, 100(%rax)
        vmovdqu %xmm0, 116(%rax)
        vmovdqu %xmm0, 132(%rax)
        vmovdqu %xmm0, 148(%rax)
        vmovdqu %xmm0, 164(%rax)
        vmovdqu %xmm0, 180(%rax)
        vmovdqu %xmm0, 196(%rax)
        vmovdqu %xmm0, 212(%rax)
...

Here memset has cleared 280 bytes starting from RAX.  There is no need to
clear these bytes again.  The optimized tree dump shows:

  <bb 14> [local count: 444773291]:
  # __cur_154 =3D PHI <__cur_42(14), _6(13)>
  # __n_155 =3D PHI <__n_41(14), __n_20(D)(13)>
  *__cur_154 =3D {};
  MEM[(int * *)__cur_154] =3D 0B;=20
  MEM[(int * *)__cur_154 + 8B] =3D 0B;=20
  MEM[(int * *)__cur_154 + 16B] =3D 0B;=20
  MEM[(struct array *)__cur_154 + 36B]._M_instance =3D {};=20
  MEM <vector(4) long unsigned int> [(long unsigned int * *)__cur_154 + 232=
B] =3D
{ 0, 0, 0, 0 };=20
  MEM[(long unsigned int * *)__cur_154 + 264B] =3D 0B;=20
  MEM[(long unsigned int * *)__cur_154 + 272B] =3D 0B;

Some of them are removed by RTL DSE.  But vector stores aren't.  Should
SSA DSE remove them?=