[Bug tree-optimization/108821] New: Extra volatile access with -O2 -ftree-loop-im since GCC-11

["sirl at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108821

            Bug ID: 108821
           Summary: Extra volatile access with -O2 -ftree-loop-im since
                    GCC-11
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sirl at gcc dot gnu.org
  Target Milestone: ---

Hi,

this small example

extern volatile int *x;
static int gCrc;

static int crc16Add(int crc, int b) __attribute__((noinline));
static int crc16Add(int crc, int b)
{
  return crc + b;
}

void f(int data, int dataSz)
{
  int i;

  for(i=0;i<dataSz;i++)
  {
    gCrc = crc16Add(gCrc, data);
    *x = data;
  }
}

adds an extra volatile access after the loop (ARM assembler, but x64 shows the
same problem):

f(int, int):
        mov     r2, r1
        cmp     r2, #0
        ble     .L8
        push    {r3, r4, r5, lr}
        movw    r5, #:lower16:.LANCHOR0
        movt    r5, #:upper16:.LANCHOR0
        movw    r4, #:lower16:x
        movt    r4, #:upper16:x
        mov     r1, r0
        movs    r3, #0
        ldr     r0, [r5]
        ldr     r4, [r4]
.L5:
        adds    r3, r3, #1
        bl      crc16Add(int, int)
        cmp     r2, r3
        str     r1, [r4]  @ <-- the last store here
        bne     .L5
        str     r0, [r5]
        str     r1, [r4]  @ <-- is duplicated here
        pop     {r3, r4, r5, pc}
.L8:
        bx      lr


The tree dumps shows the extra access is added during the lim2 pass.
Compiling with -fno-tree-loop-im avoids the invalid extra access to volatile
memory.

I'm not enough of a language lawyer to be sure that the extra volatile access
is invalid in C/C++, but at least it's a bad optimization.