[Bug middle-end/98373] New: [11 Repression] Bogus error: ‘symbuf’ may be used uninitialized at -O0

[Bug middle-end/98373] New: [11 Repression] Bogus error: ‘symbuf’ may be used uninitialized at -O0

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98373

            Bug ID: 98373
           Summary: [11 Repression] Bogus error: ‘symbuf’ may be used
                    uninitialized at -O0
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hjl.tools at gmail dot com
  Target Milestone: ---

Created attachment 49795
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=49795&action=edit
A testcase

[hjl@gnu-cfl-2 gold]$ /usr/gcc-11.0.0-x32/bin/g++ -S /tmp/plugin.ii -W -Wall
/export/linux/src/binutils/binutils/gold/plugin.cc: In member function ‘void
gold::Sized_pluginobj<size, big_endian>::do_add_symbols(gold::Symbol_table*,
gold::Read_symbols_data*, gold::Layout*) [with int size = 32; bool big_endian =
false]’:
/export/linux/src/binutils/binutils/gold/plugin.cc:1400:33: warning: ‘symbuf’
may be used uninitialized [-Wmaybe-uninitialized]
 1400 |   elfcpp::Sym<size, big_endian> sym(symbuf);
      |                                 ^~~
In file included from /export/linux/src/binutils/binutils/gold/script.h:37,
                 from /export/linux/src/binutils/binutils/gold/layout.h:33,
                 from /export/linux/src/binutils/binutils/gold/plugin.cc:73:
/export/linux/src/binutils/binutils/gold/../elfcpp/elfcpp.h:1532:3: note: by
argument 2 of type ‘const unsigned char*’ to ‘elfcpp::Sym<size,
big_endian>::Sym(const unsigned char*) [with int size = 32; bool big_endian =
false]’ declared here
 1532 |   Sym(const unsigned char* p)
      |   ^~~
/export/linux/src/binutils/binutils/gold/plugin.cc:1399:17: note: ‘symbuf’
declared here
 1399 |   unsigned char symbuf[sym_size];
      |                 ^~~~~~
/export/linux/src/binutils/binutils/gold/plugin.cc: In member function ‘void
gold::Sized_pluginobj<size, big_endian>::do_add_symbols(gold::Symbol_table*,
gold::Read_symbols_data*, gold::Layout*) [with int size = 64; bool big_endian =
false]’:
/export/linux/src/binutils/binutils/gold/plugin.cc:1400:33: warning: ‘symbuf’
may be used uninitialized [-Wmaybe-uninitialized]
 1400 |   elfcpp::Sym<size, big_endian> sym(symbuf);
      |                                 ^~~
In file included from /export/linux/src/binutils/binutils/gold/script.h:37,
                 from /export/linux/src/binutils/binutils/gold/layout.h:33,
                 from /export/linux/src/binutils/binutils/gold/plugin.cc:73:
/export/linux/src/binutils/binutils/gold/../elfcpp/elfcpp.h:1532:3: note: by
argument 2 of type ‘const unsigned char*’ to ‘elfcpp::Sym<size,
big_endian>::Sym(const unsigned char*) [with int size = 64; bool big_endian =
false]’ declared here
 1532 |   Sym(const unsigned char* p)
      |   ^~~
/export/linux/src/binutils/binutils/gold/plugin.cc:1399:17: note: ‘symbuf’
declared here
 1399 |   unsigned char symbuf[sym_size];
      |                 ^~~~~~
[hjl@gnu-cfl-2 gold]$ /usr/gcc-11.0.0-x32/bin/g++ -S /tmp/plugin.ii -W -Wall
-O2
[hjl@gnu-cfl-2 gold]$

[Bug middle-end/98373] [11 Repression] Bogus error: ‘symbuf’ may be used uninitialized at -O0

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98373

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |WAITING
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2020-12-18
                 CC|                            |msebor at gcc dot gnu.org

--- Comment #1 from Martin Sebor <msebor at gcc dot gnu.org> ---
The warning looks justified to me.  Would you mind explaining why you think
it's invalid?  (It seems like a basic expectation of any report.)

[Bug middle-end/98373] [11 Repression] Bogus error: ‘symbuf’ may be used uninitialized at -O0

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98373

H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|WAITING                     |NEW

--- Comment #2 from H.J. Lu <hjl.tools at gmail dot com> ---
[hjl@gnu-cfl-2 gold]$ cat /tmp/foo.cc 
template<int size>
struct Sym_data;

template<>
struct Sym_data<32>
{
  unsigned char st_info;
  unsigned char st_other;
};

template<>
struct Sym_data<64>
{
  unsigned char st_info;
  unsigned char st_other;
};

template<int size, bool big_endian>
class Sym
{
 public:
  Sym(const unsigned char* p)
    : p_(reinterpret_cast<const Sym_data<size>*>(p))
  { }

 private:
  const Sym_data<size>* p_;
};

template<int size, bool big_endian>
class Sized_relobj_incr
{
 private:
  void do_add_symbols();
  Sym<size, big_endian> *symbols_;
};

template<int size, bool big_endian>
Sym<size, big_endian> *
foo(Sym<size, big_endian> *);

template<int size, bool big_endian>
void
Sized_relobj_incr<size, big_endian>::do_add_symbols()
{
  unsigned char symbuf[100];
  Sym<size, big_endian> sym(symbuf);
  Sym<size, big_endian>* res = foo(&sym);
  this->symbols_ = res;
}

template
class Sized_relobj_incr<32, false>;
[hjl@gnu-cfl-2 gold]$ /usr/gcc-11.0.0-x32/bin/g++ -Wall -S /tmp/foo.cc
/tmp/foo.cc: In member function ‘void Sized_relobj_incr<size,
big_endian>::do_add_symbols() [with int size = 32; bool big_endian = false]’:
/tmp/foo.cc:47:25: warning: ‘symbuf’ may be used uninitialized
[-Wmaybe-uninitialized]
   47 |   Sym<size, big_endian> sym(symbuf);
      |                         ^~~
/tmp/foo.cc:22:3: note: by argument 2 of type ‘const unsigned char*’ to
‘Sym<size, big_endian>::Sym(const unsigned char*) [with int size = 32; bool
big_endian = false]’ declared here
   22 |   Sym(const unsigned char* p)
      |   ^~~
/tmp/foo.cc:46:17: note: ‘symbuf’ declared here
   46 |   unsigned char symbuf[100];
      |                 ^~~~~~
[hjl@gnu-cfl-2 gold]$

[Bug middle-end/98373] [11 Repression] Bogus error: ‘symbuf’ may be used uninitialized at -O0 by r11-959

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98373

H.J. Lu <hjl.tools at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|[11 Repression] Bogus       |[11 Repression] Bogus
                   |error: ‘symbuf’ may be used |error: ‘symbuf’ may be used
                   |uninitialized at -O0        |uninitialized at -O0 by
                   |                            |r11-959

--- Comment #3 from H.J. Lu <hjl.tools at gmail dot com> ---
It is caused by r11-959.

[Bug middle-end/98373] [11 Repression] Bogus error: ‘symbuf’ may be used uninitialized at -O0 by r11-959

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98373

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

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

--- Comment #4 from Martin Sebor <msebor at gcc dot gnu.org> ---
Yes, but you still haven't explained why you think it's invalid.

The warning is by design as documented in the manual

Wmaybe-uninitialized
    ...
    In addition, passing a pointer (or in C++, a reference) to an uninitialized
object to a const-qualified function argument is also diagnosed by this
warning. (-Wuninitialized is issued for built-in functions known to read the
object.) Annotating the function with attribute access (none) indicates that
the argument isn’t used to access the object and avoids the warning (see Common
Function Attributes).