[Bug c/66397] New: sanitize=undefined triggers extra -Warray-bounds warning

[Bug c/66397] New: sanitize=undefined triggers extra -Warray-bounds warning

[sirl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66397

            Bug ID: 66397
           Summary: sanitize=undefined triggers extra -Warray-bounds
                    warning
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sirl at gcc dot gnu.org
  Target Milestone: ---

Created attachment 35691
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35691&action=edit
testcase, warns with gcc-6 -c -O2 -fsanitize=undefined

The attached testcase derived from a C++ iterator implementation relies on the
fact that the undefined pCurrent-1 is immediately nullified again by the
following pCurrent++ by the optimizer. But in current trunk r224064 the
following warning is issued:

test.c: In function 'test':
test.c:19:53: warning: array subscript is below array bounds [-Warray-bounds]
     ths->pCurrent = (ths->pStart) ? ths->pStart - 1 : (stru *) 0;
                                                     ^

Though the warning is not completely wrong, -Warray-bounds usually triggers
only when the value is really accessed, or? gcc-5.1 compiles the testcase
without warning.

[Bug c/66397] sanitize=undefined triggers extra -Warray-bounds warning

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66397

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Comment on attachment 35691
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=35691
testcase, warns with gcc-6 -c -O2 -fsanitize=undefined

Except it is undefined behavior to have one element before the start of an
array. And that is why it is warning. Does not matter if there is a ++ later
on.

[Bug c/66397] sanitize=undefined triggers extra -Warray-bounds warning

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66397

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |mpolacek at gcc dot gnu.org

--- Comment #2 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Yeah, I think this isn't a bug.

[Bug c/66397] sanitize=undefined triggers extra -Warray-bounds warning

[sirl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66397

--- Comment #3 from Franz Sirl <sirl at gcc dot gnu.org> ---
Yeah, I feared so :-(. This is a bit unfortunate though, as for our code base
we compile with -Werror=array-bounds, now when I add -fsanitize=undefined I
need to downgrade the error to a warning again. Another special casing in the
build system... From this POV I would prefer to get only the
-fsanitize=undefined runtime error.

[Bug c/66397] sanitize=undefined triggers extra -Warray-bounds warning

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66397

Marek Polacek <mpolacek at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #4 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Closing.