[Bug tree-optimization/85175] [8 regression] false-positive -Wformat-overflow= warning with gcc-8 -Os

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85175

--- Comment #2 from Martin Sebor <msebor at gcc dot gnu.org> ---
The warning appeared with r257857:

r257857 | law | 2018-02-20 13:59:22 -0500 (Tue, 20 Feb 2018) | 8 lines

        PR middle-end/82123
        PR tree-optimization/81592
        PR middle-end/79257
        * gimple-ssa-sprintf.c (format_integer): Query EVRP range analyzer
        for range data rather than using global data.

The difference in the dump between the revision prior to it and r257857 is:

  Directive 2 at offset 2: "%d"
    Result: 1, 1, 11, 11 (3, 3, 13, 13)

and

  Directive 2 at offset 2: "%d"
    Result: 1, 11, 11, 11 (3, 13, 13, 13)

I.e., GCC now thinks the likely output of the %d directive is 11 bytes (the
second number on the Result: line).  Prior to r257857 the range type of the
argument was VR_VARYING for which the warning doesn't trigger.  With r257857
the range is  [-2147483647, 3] of which the warning uses the larger bound to
trigger.

So with the change above GCC is doing a better but imperfect job of determining
the range.  Changing the variable to unsigned constrains the lower bound to
zero and eliminates the warning even at -Os.