RFC: missed loop optimizations from loop induction variable copies

["Rahul Kharche" <rahul@IceraSemi.com>]

The following causes missed loop optimizations in O2 from creating
unnecessary loop induction variables. Or, is a case of IV opts not
able to coalesce copies of induction variables. A previous post related
to this was made in PR41026 which had a type promoted loop index
variable
copied. I believe this example makes the problem more obvious.

struct struct_t {
  int* data;
};

void testAutoIncStruct (struct struct_t* sp, int start, int end)
{
    int i;
    for (i = 0; i+start < end; i++)
      {
        sp->data[i+start] = 0;
      }
}

With GCC v4.4.1 release) and gcc -O2 -fdump-tree-all on the above case
we get the following dump from IVOpts

testAutoIncStruct (struct struct_t * sp, int start, int end)
{
  unsigned int D.1283;
  unsigned int D.1284;
  int D.1282;
  unsigned int ivtmp.32;
  int * pretmp.17;
  int i;
  int * D.1245;
  unsigned int D.1244;
  unsigned int D.1243;

<bb 2>:
  if (start_3(D) < end_5(D))
    goto <bb 3>;
  else
    goto <bb 6>;

<bb 3>:
  pretmp.17_22 = sp_6(D)->data;
  D.1282_23 = start_3(D) + 1;
  ivtmp.32_25 = (unsigned int) D.1282_23;
  D.1283_27 = (unsigned int) end_5(D);
  D.1284_28 = D.1283_27 + 1;

<bb 4>:
  # start_20 = PHI <start_4(5), start_3(D)(3)>
  # ivtmp.32_7 = PHI <ivtmp.32_24(5), ivtmp.32_25(3)>
  D.1243_9 = (unsigned int) start_20;
  D.1244_10 = D.1243_9 * 4;
  D.1245_11 = pretmp.17_22 + D.1244_10;
  *D.1245_11 = 0;
  start_26 = (int) ivtmp.32_7;
  start_4 = start_26;
  ivtmp.32_24 = ivtmp.32_7 + 1;
  if (ivtmp.32_24 != D.1284_28)
    goto <bb 5>;
  else
    goto <bb 6>;

<bb 5>:
  goto <bb 4>;

<bb 6>:
  return;

}

IVOpts cannot identify start_26, start_4 and ivtmp_32_7 to be copies.
The root cause is that expression 'i + start' is identified as a common
expression between the test in the header and the index operation in the
latch. This is unified by copy propagation or FRE prior to loop
optimizations
and creates a new induction variable.

If we disable tree copy propagation and FRE with
gcc -O2 -fno-tree-copy-prop -fno-tree-fre -fdump-tree-all we get

testAutoIncStruct (struct struct_t * sp, int start, int end)
{
  unsigned int D.1287;
  unsigned int D.1288;
  unsigned int D.1289;
  int D.1290;
  unsigned int D.1284;
  unsigned int D.1285;
  unsigned int D.1286;
  int * pretmp.17;
  int i;
  int * D.1245;
  unsigned int D.1244;
  unsigned int D.1243;
  int D.1242;
  int * D.1241;

<bb 2>:
  if (start_3(D) < end_5(D))
    goto <bb 3>;
  else
    goto <bb 6>;

<bb 3>:
  pretmp.17_23 = sp_6(D)->data;
  D.1287_27 = (unsigned int) end_5(D);
  D.1288_28 = (unsigned int) start_3(D);
  D.1289_29 = D.1287_27 - D.1288_28;
  D.1290_30 = (int) D.1289_29;

<bb 4>:
  # i_20 = PHI <i_12(5), 0(3)>
  D.1241_7 = pretmp.17_23;
  D.1284_26 = (unsigned int) start_3(D);
  D.1285_25 = (unsigned int) i_20;
  D.1286_24 = D.1284_26 + D.1285_25;
  MEM[base: pretmp.17_23, index: D.1286_24, step: 4] = 0;
  i_12 = i_20 + 1;
  if (i_12 != D.1290_30)
    goto <bb 5>;
  else
    goto <bb 6>;

<bb 5>:
  goto <bb 4>;

<bb 6>:
  return;

}

The correct single induction variable as been identified here. This is
not
a loop header copying problem either. If we disable loop header copying,
we
still get multiple induction variables created. In fact in the above
case
loop header copying correctly enables post-increment mode on our port.

testAutoIncStruct (struct struct_t * sp, int start, int end)
{
  unsigned int D.1282;
  unsigned int ivtmp.31;
  unsigned int ivtmp.29;
  int i;
  int * D.1245;
  unsigned int D.1244;
  unsigned int D.1243;
  int D.1242;
  int * D.1241;

<bb 2>:
  ivtmp.29_18 = (unsigned int) start_3(D);
  D.1282_21 = (unsigned int) start_3(D);
  ivtmp.31_22 = D.1282_21 * 4;
  goto <bb 4>;

<bb 3>:
  D.1241_7 = sp_6(D)->data;
  D.1244_10 = ivtmp.31_19;
  D.1245_11 = D.1241_7 + D.1244_10;
  *D.1245_11 = 0;
  ivtmp.29_17 = ivtmp.29_8 + 1;
  ivtmp.31_20 = ivtmp.31_19 + 4;

<bb 4>:
  # ivtmp.29_8 = PHI <ivtmp.29_18(2), ivtmp.29_17(3)>
  # ivtmp.31_19 = PHI <ivtmp.31_22(2), ivtmp.31_20(3)>
  D.1242_23 = (int) ivtmp.29_8;
  if (D.1242_23 < end_5(D))
    goto <bb 3>;
  else
    goto <bb 5>;

<bb 5>:
  return;

}

Does this imply we try and not copy propagate or FRE potential induction
variables? Or is this simply a missed case in IVOpts?

Rahul