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From: "yinyuefengyi at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c++/109821] New: vect: Different output with -O2
 -ftree-loop-vectorize compared to -O2
Date: Fri, 12 May 2023 02:30:20 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109821

            Bug ID: 109821
           Summary: vect: Different output with -O2 -ftree-loop-vectorize
                    compared to -O2
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: yinyuefengyi at gmail dot com
  Target Milestone: ---

For this test code, it aims to generate special patterns different with mem=
cpy
or memmove, it generates different results with -O2 -ftree-loop-vectorize
compared to -O2, is this a bug of vectorizer that lack of checking the gap =
of
op-src should be larger than vector mode size (here only do vectorize if op=
 -
src > 16)?

copy.cpp:

#include <stdio.h>
#include <cstdint>
#include <stdlib.h>

#define UNALIGNED_LOAD64(_p) (*reinterpret_cast<const uint64_t *>(_p))
#define UNALIGNED_STORE64(_p, _val) (*reinterpret_cast<uint64_t *>(_p) =3D
(_val))

__attribute__((__noinline__))
static void IncrementalCopyFastPath(const char* src, char* op, int len) {
    while (op - src < 8) {
        UNALIGNED_STORE64(op, UNALIGNED_LOAD64(src));
        len -=3D op - src;
        op +=3D op - src;
    }
    while (len > 0) {
        UNALIGNED_STORE64(op, UNALIGNED_LOAD64(src));
        src +=3D 8;
        op +=3D 8;
        len -=3D 8;
    }
}

int main ()
{
  char src[] =3D "123456789abcdefghijklmnopqrstu";
  char *op =3D src+12;
  char * dst =3D op;
  IncrementalCopyFastPath (src, op, 36);
  int i =3D 0;
  while (i < 36)
    {printf("%x ", *(dst+i)), i++;}
  printf("\n");
  return 0;
}


$ gcc copy.cpp -O2 -o a.out.good
$ ./a.out.good
30 31 32 33 34 35 36 37 38 39 61 62 30 31 32 33 34 35 36 37 38 39 61 62 30 =
31
32 33 34 35 36 37 38 39 61 62
$ gcc copy.cpp -O2 -ftree-loop-vectorize  -o a.out.bad
$ ./a.out.bad
30 31 32 33 34 35 36 37 38 39 61 62 63 64 65 66 34 35 36 37 38 39 61 62 63 =
64
65 66 73 74 75 76 38 39 61 62


gimple after t.vect:

IncrementalCopyFastPath.constprop (const char * src, char * op)
{
...
  <bb 2> [local count: 118111600]:
  _4 =3D src_8(D) + 8;
  if (_4 !=3D op_9(D))    // <=3D  the check should be op_9 > src_8 + 16 he=
re?
    goto <bb 16>; [80.00%]
  else
    goto <bb 10>; [20.00%]
...
}=