[Bug tree-optimization/41488] New: IVOpts cannot coalesce multiple induction variables

[rahul at icerasemi dot com]

Using GCC 4.4.1 release version and compiling the following test with

gcc -O2 -fdump-tree-all

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;
      }
}

IVOpts dump shows induction variables (start and ivtmp.32) cannot be coalesced

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;

}

The problem arises from expression "i + start" being identified as a common
expression between the header and the latch. This seems to creates an extra
induction variable and a PHI in the latch. If we disable tree FRE and tree copy
propagation with

gcc -O2 -fno-tree-fre -fno-tree-copy-prop

We get

<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>;

The induction variable and the memory reference is now correctly identified.


-- 
           Summary: IVOpts cannot coalesce multiple induction variables
           Product: gcc
           Version: 4.4.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rahul at icerasemi dot com
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=41488