[Bug c++/96057] New: -Wreturn-type warning message disappears with -O1 to -Os in unnamed namespace definition

[Bug c++/96057] New: -Wreturn-type warning message disappears with -O1 to -Os in unnamed namespace definition

[haoxintu at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96057

            Bug ID: 96057
           Summary: -Wreturn-type warning message disappears with -O1 to
                    -Os in unnamed namespace definition
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: haoxintu at gmail dot com
  Target Milestone: ---

Hi, all.

This small code, bug.cc, the -Wreturn-type warning disappears when add -O1 or
higher optimization option.

$cat bug.cc
namespace {
    int foo() { 1 ? 0 : throw 0 ; }
}

$g++ -c -Wreturn-type -O0 bug.cc
bug.cc: In function 'int {anonymous}::foo()':
bug.cc:2:35: warning: control reaches end of non-void function [-Wreturn-type]
    2 |     int foo() { 1 ? 0 : throw 0 ; }
      |   

$g++ -c -Wreturn-type -O1 bug.cc
//emit nothing with -O1 to -Os option

While in Clang, the output of -O0 to -Os is the same. For example,

$clang++ -c -Wreturn-type -O1 bug.cc
bug.cc:2:35: warning: non-void function does not return a value [-Wreturn-type]
    int foo() { 1 ? 0 : throw 0 ; }
                                  ^
1 warning generated.


$g++ -v
Using built-in specs.
COLLECT_GCC=g++
COLLECT_LTO_WRAPPER=/home/haoxin/corpus-compilers/gcc-trunk/gcc-master/build/libexec/gcc/x86_64-pc-linux-gnu/11.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../configure
--prefix=/home/haoxin/corpus-compilers/gcc-trunk/gcc-master/build/
--enable-languages=c,c++ CC=gcc CXX=g++
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 11.0.0 20200630 (experimental) (GCC)

[Bug c++/96057] -Wreturn-type warning message disappears with -O1 to -Os in unnamed namespace definition

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96057

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
There was another report about static inline where this happens too. The same
reasoning applies here. The function is unused and there for there is no
undefined runtime behavior can happen.

[Bug c++/96057] [9/10/11/12 Regression] -Wreturn-type warning message disappears with -O1+ in anonymous namespace definition

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96057

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|-Wreturn-type warning       |[9/10/11/12 Regression]
                   |message disappears with -O1 |-Wreturn-type warning
                   |to -Os in unnamed namespace |message disappears with
                   |definition                  |-O1+ in anonymous namespace
                   |                            |definition
      Known to fail|                            |12.0, 4.4.7
     Ever confirmed|0                           |1
   Target Milestone|---                         |9.5
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2022-01-05
      Known to work|                            |4.1.2

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Confirmed.

[Bug c++/96057] [9/10/11/12 Regression] -Wreturn-type warning message disappears with -O1+ in anonymous namespace definition

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96057

Richard Biener <rguenth at gcc dot gnu.org> changed:

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

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
That's because the -Wreturn-type diagnostic happens after we remove unused
functions which we only do at -O1+:

t.c: At global scope:
t.c:2:9: warning: 'int {anonymous}::foo()' defined but not used
[-Wunused-function]
    2 |     int foo() { 1 ? 0 : throw 0 ; }
      |         ^~~

which in turn is because the function is local (in an anonymous namespace).
Likely GCC 4.1 removed the function only later.

IMHO a WONTFIX, sorry.