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From: "juzhe.zhong at rivai dot ai" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/112331] New: middle-end: Fail vectorization
Date: Wed, 01 Nov 2023 09:20:33 +0000
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X-Bugzilla-Product: gcc
X-Bugzilla-Component: c
X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D112331

            Bug ID: 112331
           Summary: middle-end: Fail vectorization
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: juzhe.zhong at rivai dot ai
  Target Milestone: ---

https://gcc.godbolt.org/z/x7GGzezGh


#include <stdio.h>
#define LEN 32000
#define ntimes 200000
#define TYPE float
#define lll LEN
#define LEN2 256
#define ALIGNMENT 16
__attribute__ ((aligned(ALIGNMENT))) TYPE X[lll],Y[lll],Z[lll],U[lll],V[lll=
];

struct GlobalData {
  __attribute__((aligned(ALIGNMENT))) TYPE a[LEN];
  int pad1[3];
  __attribute__((aligned(ALIGNMENT))) TYPE b[LEN];
  int pad2[5];
  __attribute__((aligned(ALIGNMENT))) TYPE c[LEN];
  int pad3[7];
  __attribute__((aligned(ALIGNMENT))) TYPE d[LEN];
  int pad4[11];
  __attribute__((aligned(ALIGNMENT))) TYPE e[LEN];

  int pad5[13];
  __attribute__((aligned(ALIGNMENT))) TYPE aa[LEN2][LEN2];
  int pad6[17];
  __attribute__((aligned(ALIGNMENT))) TYPE bb[LEN2][LEN2];
  int pad7[19];
  __attribute__((aligned(ALIGNMENT))) TYPE cc[LEN2][LEN2];
  int pad8[23];
  __attribute__((aligned(ALIGNMENT))) TYPE tt[LEN2][LEN2];
} global_data;

__attribute__((aligned(ALIGNMENT))) TYPE * const a =3D global_data.a;
__attribute__((aligned(ALIGNMENT))) TYPE * const b =3D global_data.b;
__attribute__((aligned(ALIGNMENT))) TYPE * const c =3D global_data.c;
__attribute__((aligned(ALIGNMENT))) TYPE * const d =3D global_data.d;
__attribute__((aligned(ALIGNMENT))) TYPE * const e =3D global_data.e;
__attribute__((aligned(ALIGNMENT))) TYPE (* const aa)[LEN2] =3D global_data=
.aa;
__attribute__((aligned(ALIGNMENT))) TYPE (* const bb)[LEN2] =3D global_data=
.bb;
__attribute__((aligned(ALIGNMENT))) TYPE (* const cc)[LEN2] =3D global_data=
.cc;
__attribute__((aligned(ALIGNMENT))) TYPE (* const tt)[LEN2] =3D global_data=
.tt;

int foo()
{

//      linear dependence testing
//      no dependence - vectorizable


        for (int nl =3D 0; nl < 2*ntimes; nl++) {
//              #pragma vector always
                for (int i =3D 1; i < LEN; i +=3D 2) {
                        a[i] =3D a[i - 1] + b[i];
                }
        }

        return 0;
}

Both RVV and ARM SVE faild to vectorize it wheras Clang can vectorize it.=