[Bug c/44848] New: Bogus "array subscript is below array bounds" with loops

[eli dot friedman at gmail dot com]

Testcase (derived from ffmpeg):

void av_solve_lls(double (*factor)[33], int count, int min_order){
    int i,j,k;
    for(i=0; i<count; i++){
        for(j=i; j<count; j++){
            double sum = factor[i][j];

            for(k=i-1; k>=0; k--)
                sum -= factor[i][k]*factor[j][k];

            factor[i][i]= sum;
        }
    }

    for(j=count-1; j>=min_order; j--){
        for(i=j; i>=0; i--){
            factor[0][j]= 0;
        }
        factor[0][j] = 0;
        for(i=0; i<=j; i++){
            factor[0][j] += 0;
        }
    }
}

With the following command-line:
gcc l.c -S -Wall -O2

Gives the following warning:
/tmp/l.c: In function ‘av_solve_lls’:
/tmp/l.c:18:18: warning: array subscript is below array bounds

Using gcc 4.5.0 on x86-64-pc-linux-gnu, built from source, configured with
"configure --enable-languages=c --disable-bootstrap".

The testcase is a little large, but I can't figure out how to reduce it more;
the issue seems very sensitive to the precise structure of the code.


-- 
           Summary: Bogus "array subscript is below array bounds" with loops
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: eli dot friedman at gmail dot com
  GCC host triplet: x86-64-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=44848