[Bug tree-optimization/21278] New: Move maximum out of a loop

[tkoenig at gcc dot gnu dot org]

This could help in reducing the amount of work for bounds
checking.

$ cat mymax.c
#include <stdlib.h>
#include <stdio.h>

int main()
{
  char *p;
  int i,j,n;

  scanf("%d",&n);
  j = 0;
  for (i=0; i<n; i++)
    {
        if (j < i)
            j = i;
    }
    printf("%d\n",j);
    return 0;
}
$ gcc -g -funroll-all-loops -S -O3 -fdump-tree-optimized  mymax.c

still does the do loop:

<L13>:;
  j = 0;
  i = 0;

<L0>:;
  j = MAX_EXPR <j, i>;
  i = i + 1;
  if (i != n.10) goto <L0>; else goto <L4>;

when the loop could be optimized to

    if (n <= 0)
        j = 0;
    else
        j = n-1;

-- 
           Summary: Move maximum out of a loop
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P2
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tkoenig at gcc dot gnu dot org
                CC: gcc-bugs at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21278