[Bug c/82286] New: Wrong array subscript is above array bounds

[hermantenbrugge at home dot nl]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82286

            Bug ID: 82286
           Summary: Wrong array subscript is above array bounds
           Product: gcc
           Version: 7.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hermantenbrugge at home dot nl
  Target Milestone: ---

Created attachment 42223
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=42223&action=edit
testcase

The attached testcase reports array bound error if compiled with:

gcc -O3 -Wall -c tst.c -DERROR
tst.c: In function 'mtrx_decompose_matrix':
tst.c:36:17: warning: array subscript is above array bounds [-Warray-bounds]
  sum += tmp.data[row][sub] * tmp.data[col][sub];
         ~~~~~~~~^~~~~

If -DERROR is not given the testcase compiles with no warning.