improving auto increment expressions detection across basic blocks.

["Ramana Radhakrishnan" <ramana dot r at gmail dot com>]

Hi ,

I've been looking to sort out a case where one sees the possibility of
doing some kind of redundancy elimination with auto-increment
expressions across basic blocks. I understand based on earlier
conversations as well as looking at auto-inc-dec.c that all
infrastructure for auto-inc-dec works only in one basic block. Looking
at a number of loops in a codec and other internal benchmarks I notice
that looking at auto-increments outside of a basic block loop would
help cases where you have a possibility of merging the auto-increment
for the

The basic case is as explained below.

for (i = 0; i < 100; i ++)
{
 if (....)
{
  a[i] = something;
}
else
 a[i] = something else..

}

The specific testcase I have is the following where you can see that


void Test (int n, int out[], int in[])
{
  int i;
  for (i = 0; i < n; i++) {
    if (in[i]) {
      out[i] = 0x12;

    } else {
      out[i] = 0x98;
    }
  }
}

Test:
        cmpslt  $c6,$zero,$c1
        brz     $c6,$link
        incsi   $c1,-1
        i2cs    $c4,152
        i2cs    $c5,18
.L5:
        ldw     $c6,($c3[0])
        briz    $c6,.L3
        stw     ($c2[0]),$c5
.L4:
        addzi   $c3,$c3,4.... This can be merged with the load above
to remove this extra addition and can be used with auto increment.
        addzi   $c2,$c2,4 ... This has the potential of being merged
with the above store instruction.
        brinzdec       $c1,.L5
        brz     $zero,$link
.L3:
        stw     ($c2[0]),$c4
        briz    $zero,.L4


The code generated does not use a post increment operation for the
address calculation for out where we could very well have done so. I
am wondering if there could be a pass after the current local auto
increment spotter which does this detection and improvement across
basic blocks. I am not clear where one could do this and how to
improve this particular case of code generation. Any suggestions would
be most helpful.



cheers
Ramana


-- 
Ramana Radhakrishnan