[Bug c/68329] [4.8 4.9]gcc using array index to accelerate loop running , why turn off at gcc 5.X

[zuogang at huawei dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=68329

zuogang <zuogang at huawei dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |amd64 ppwerpc32 (tested)
                   |                            |and others(no testing)
      Known to work|                            |4.4.1, 5.2.0
               Host|                            |amd64
      Known to fail|                            |4.8.5, 4.9.4
              Build|                            |amd64

--- Comment #1 from zuogang <zuogang at huawei dot com> ---
the src code file is:
/*
 * compiling options:
 * -g -w -c -Wall -fno-common -fno-omit-frame-pointer -Wextra
-Wformat-nonliteral -Wformat-security -Wswitch-default -O2 -fstrength-reduce
-fno-strict-aliasing
 *
 *  */
#define NULL 0
typedef unsigned int uint4;
typedef unsigned short ushort;
typedef unsigned long ulong;

typedef struct team {
    ulong size;
    ulong count;
    ulong flag;
    ulong peer;
 ulong event;
 ulong index[1];
 ulong addr;
}Team;

typedef struct entry {
    uint4 *p1;
 uint4 *p2;
 char simple[64];
 char full[64];
 uint4 index;
}Entry;

extern uint4 get1(uint4 p1, uint4 *p2);
extern uint4 get2(uint4 p1, uint4 p2, Entry **p3);
extern void hand(uint4 p1, uint4 p2, Entry *p3);

void recvt(ushort id, Team *pTeam)
{
    uint4         num = 0;
    uint4         index   = 0;
    Entry         *pEntry       = NULL;
    uint4         count     = 0;
    uint4         i       = 0;

    if (0x01 & pTeam->flag)
    {
        count = pTeam->count;

        for (i = 0; i < count; i++)
        {
            index = *(pTeam->index + i);
   if(0 != get1(index, &num))
            {
                continue;
            }
            if (0 == index)
            {
                continue;
            }

            if ( 0 != get2(num ,index, &pEntry))
            {
                continue;
            }

            hand(num, pTeam->event, pEntry);
        }
    }

    return;
}

compile this file using :
gcc -g -w -c -Wall -fno-common -fno-omit-frame-pointer -Wextra
-Wformat-nonliteral -Wformat-security -Wswitch-default -O2 -fstrength-reduce
-fno-strict-aliasing test-loop.c -o testloop

see the disassemble file , wile find that the loop "for (i = 0; i < count;
i++)" run only once.

I think this "bug" is because gcc find the loop index "i", is a array's index,
and the array 's len is 1, so gcc tell the loop can only run once, so gcc don't
generate loop-specific asm codes.

my question is why this op are not support in gcc version 5. any option to
control this "bug"?