[Bug middle-end/94815] Abusive -Wrestrict warning with sprintf

["msebor at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94815

Martin Sebor <msebor at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
           Keywords|                            |missed-optimization
             Blocks|                            |83819
             Status|REOPENED                    |RESOLVED
          Component|c                           |middle-end

--- Comment #5 from Martin Sebor <msebor at gcc dot gnu.org> ---
The test case in comment #4 isn't handled yet.  Even though the sprintf code
has access to the results of the strlen pass computed for prior string built-in
function calls and so knows the length of str3, it doesn't expose the lengths
of the strings it itself computes to the strlen pass (or track them across
statements) yet so it doesn't "know" the length of str2 computed at the prior
sprintf statement.

When the sprintf pass makes use of attribute alloc_size it will be able to use
the array sizes to improve its analysis but as I mentioned in comment #2 this
isn't implemented yet either.  The effect that we will get once it is can be
emulated by replacing buf with a fixed size array (char buf[30]).  It doesn't
eliminate the warning due to the missing bits above but the output of the
-fdump-tree-strlen option confirms that the warning is due to the strlen
limitation:

pr94815.c:15: sprintf: objsize = 20, fmtstr = "ABC%s"
  Directive 1 at offset 0: "ABC", length = 3
    Result: 3, 3, 3, 3 (3, 3, 3, 3)
  Directive 2 at offset 3: "%s"
    Result: 3, 3, 3, 3 (6, 6, 6, 6)
  Directive 3 at offset 5: "", length = 1
  Substituting 6 for return value.

pr94815.c:18: sprintf: objsize = 30, fmtstr = "DEF%s"
  Directive 1 at offset 0: "DEF", length = 3
    Result: 3, 3, 3, 3 (3, 3, 3, 3)
  Directive 2 at offset 3: "%s"
    Result: 0, 19, 19, 19 (3, 22, 22, 22)   <<< str2 length assumed to be up to
19
  Directive 3 at offset 5: "", length = 1

PR 92813 tracks the enhancement to make the string length information computed
by sprintf across statements (via the strlen pass).

*** This bug has been marked as a duplicate of bug 92813 ***


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83819
[Bug 83819] [meta-bug] missing strlen optimizations