[Bug c/95044] New: -Wreturn-local-addr false alarm in GCC 10.1.0 (regression)

[Bug c/95044] New: -Wreturn-local-addr false alarm in GCC 10.1.0 (regression)

[eggert at cs dot ucla.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95044

            Bug ID: 95044
           Summary: -Wreturn-local-addr false alarm in GCC 10.1.0
                    (regression)
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eggert at cs dot ucla.edu
  Target Milestone: ---

Created attachment 48501
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=48501&action=edit
Illustrate the -Wreturn-local-addr bug.

I ran into this problem when compiling GNU Emacs master with GCC 10.1.0 x86-64,
which I built from source on RHEL 7.7.

Compile the attached program u.c with:

gcc -S -Wreturn-local-addr -O2 u.c

The output is:

u.c: In function 'careadlinkat':
cc1: warning: function may return address of local variable
[-Wreturn-local-add\
r]
u.c:7:8: note: declared here
    7 |   char stack_buf[1024];
      |        ^~~~~~~~~


It's obviously impossible for the function to return that address. This false
alarm is not generated by GCC 9.3.0, so this is a regression.

There are really two bugs here:

1. GCC is ignoring the '#pragma GCC diagnostic ignored "-Wreturn-local-addr"'
and is outputting the diagnostic anyway.

2. GCC should not issue a diagnostic even if the pragma is absent, as it's
obviously impossible for the function to return the address of stack_buf.

Both bugs are new to GCC 10.1.0; they do not occur with GCC 9.3.0.

[Bug c/95044] [10/11 Regression] -Wreturn-local-addr false alarm since r10-1741-gaac9480da1ffd037

[marxin at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95044

Martin Liška <marxin at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |marxin at gcc dot gnu.org,
                   |                            |msebor at gcc dot gnu.org
            Summary|-Wreturn-local-addr false   |[10/11 Regression]
                   |alarm in GCC 10.1.0         |-Wreturn-local-addr false
                   |(regression)                |alarm since
                   |                            |r10-1741-gaac9480da1ffd037
   Last reconfirmed|                            |2020-05-11
      Known to work|                            |9.3.0
     Ever confirmed|0                           |1
      Known to fail|                            |10.1.0, 11.0
             Status|UNCONFIRMED                 |NEW

[Bug c/95044] [10/11 Regression] -Wreturn-local-addr false alarm since r10-1741-gaac9480da1ffd037

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95044

--- Comment #1 from Marc Glisse <glisse at gcc dot gnu.org> ---
I think there is another very similar bug report.

  # buf_1 = PHI <&stack_buf(2), buf_15(6)>
[...]
  if (&stack_buf != buf_1)

in each branch, we thus know what buf_1 is, so we could replace it with buf_15
in

  # _3 = PHI <_17(5), buf_1(4)>
  return _3;

(or is that bad for register pressure?)

Or use it as a hint to thread that path, or add some logic to
Wreturn_local_addr, but that's getting more complicated.

[Bug c/95044] [10/11 Regression] -Wreturn-local-addr false alarm since r10-1741-gaac9480da1ffd037

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95044

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Peeling off the first iteration might be the answer.  Thread jumping could do
that but normally is not used for that, especially through other condtionals.

[Bug c/95044] [10/11 Regression] -Wreturn-local-addr false alarm since r10-1741-gaac9480da1ffd037

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95044

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |DUPLICATE
             Status|NEW                         |RESOLVED

--- Comment #3 from Martin Sebor <msebor at gcc dot gnu.org> ---
This looks like a duplicate of pr93644, including the test case involving the
careadlinkat function.

*** This bug has been marked as a duplicate of bug 93644 ***