[Bug c++/12667] New: max(enum) returns reference to temporary

[Bug c++/12667] New: max(enum) returns reference to temporary

[mdorey at bluearc dot com]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12667

           Summary: max(enum) returns reference to temporary
           Product: gcc
           Version: 3.3.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: mdorey at bluearc dot com
                CC: gcc-bugs at gcc dot gnu dot org

I think that max1() and max2() are both legal and should generate equivalent, 
legal code:

enum E {
    E1, E2
};
const E& max1(const E& a, const E& b) {
  bool c = a < b;
  return c ? b : a;
}
const E& max2(const E& a, const E& b) {
  return a < b ? b : a;
}

max2(), however, provokes a warning:

max-enum.cpp: In function `const E& max2(const E&, const E&)':
max-enum.cpp:9: warning: returning reference to temporary

And, worse, the warning's accurate in that, I think, incorrect code is 
generated.  I'm a PowerPC weenie at the moment, so this is the clearest 
demonstration of the problem in my eyes (3.3.1 sadly):

trevithick:~/playpen$ powerpc-eabi-g++ -W -Wall -pedantic -v -O2 -c max-
enum.cpp 
Reading specs from /usr/local/lib/gcc-lib/powerpc-eabi/3.3.1/specs
Configured with: ../gcc-3.3.1/configure --target=powerpc-eabi --with-newlib --
with-local-prefix=/usr/local/powerpc-eabi --enable-languages=c,c++ --enable-cxx-
flags=-fno-exceptions
Thread model: single
gcc version 3.3.1
 /usr/local/lib/gcc-lib/powerpc-eabi/3.3.1/cc1plus -quiet -v -D__GNUC__=3 -
D__GNUC_MINOR__=3 -D__GNUC_PATCHLEVEL__=1 max-enum.cpp -D__GNUG__=3 -quiet -
dumpbase max-enum.cpp -auxbase max-enum -O2 -W -Wall -pedantic -version -
o /tmp/cchuQIuU.s
GNU C++ version 3.3.1 (powerpc-eabi)
        compiled by GNU C version 3.3.1 (Debian).
GGC heuristics: --param ggc-min-expand=98 --param ggc-min-heapsize=129171
ignoring nonexistent directory "NONE/include"
ignoring nonexistent directory "/usr/local/powerpc-eabi/sys-include"
ignoring nonexistent directory "/usr/local/powerpc-eabi/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include/c++/3.3.1
 /usr/local/include/c++/3.3.1/powerpc-eabi
 /usr/local/include/c++/3.3.1/backward
 /usr/local/lib/gcc-lib/powerpc-eabi/3.3.1/include
End of search list.
max-enum.cpp: In function `const E& max2(const E&, const E&)':
max-enum.cpp:9: warning: returning reference to temporary
 /usr/local/lib/gcc-lib/powerpc-eabi/3.3.1/../../../../powerpc-eabi/bin/as -
mppc -V -Qy -o max-enum.o /tmp/cchuQIuU.s
GNU assembler version 2.14 (powerpc-eabi) using BFD version 2.14 20030612
trevithick:~/playpen$ powerpc-eabi-objdump --disassemble max-enum.o

max-enum.o:     file format elf32-powerpc

Disassembly of section .text:

00000000 <_Z4max1RK1ES1_>:
   0:   81 23 00 00     lwz     r9,0(r3)
   4:   7c 6b 1b 78     mr      r11,r3
   8:   80 04 00 00     lwz     r0,0(r4)
   c:   7c 83 23 78     mr      r3,r4
  10:   7c 09 00 00     cmpw    r9,r0
  14:   4d 80 00 20     bltlr   
  18:   7d 63 5b 78     mr      r3,r11
  1c:   4e 80 00 20     blr

00000020 <_Z4max2RK1ES1_>:
  20:   94 21 ff f0     stwu    r1,-16(r1)
  24:   38 61 00 08     addi    r3,r1,8
  28:   38 21 00 10     addi    r1,r1,16
  2c:   4e 80 00 20     blr
trevithick:~/playpen$ 

But the problem is also exhibited in the unoptimised build (I can follow-up 
with the disassembly if you like) and in 3.3.2 Intel builds:

trevithick:~/playpen$ g++ -W -Wall -pedantic -v -O2 -c max-enum.cpp 
Reading specs from /usr/lib/gcc-lib/i486-linux/3.3.2/specs
Configured with: ../src/configure -v --enable-
languages=c,c++,java,f77,pascal,objc,ada,treelang --prefix=/usr --
mandir=/usr/share/man --infodir=/usr/share/info --with-gxx-include-
dir=/usr/include/c++/3.3 --enable-shared --with-system-zlib --enable-nls --
without-included-gettext --enable-__cxa_atexit --enable-clocale=gnu --enable-
debug --enable-java-gc=boehm --enable-java-awt=xlib --enable-objc-gc i486-linux
Thread model: posix
gcc version 3.3.2 20031005 (Debian prerelease)
 /usr/lib/gcc-lib/i486-linux/3.3.2/cc1plus -quiet -v -D__GNUC__=3 -
D__GNUC_MINOR__=3 -D__GNUC_PATCHLEVEL__=2 -D_GNU_SOURCE max-enum.cpp -
D__GNUG__=3 -quiet -dumpbase max-enum.cpp -auxbase max-enum -O2 -W -Wall -
pedantic -version -o /tmp/ccyKxhqp.s
GNU C++ version 3.3.2 20031005 (Debian prerelease) (i486-linux)
        compiled by GNU C version 3.3.2 20031005 (Debian prerelease).
GGC heuristics: --param ggc-min-expand=98 --param ggc-min-heapsize=129171
ignoring nonexistent directory "/usr/i486-linux/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/c++/3.3
 /usr/include/c++/3.3/i486-linux
 /usr/include/c++/3.3/backward
 /usr/local/include
 /usr/lib/gcc-lib/i486-linux/3.3.2/include
 /usr/include
End of search list.
max-enum.cpp: In function `const E& max2(const E&, const E&)':
max-enum.cpp:9: warning: returning reference to temporary
 as -V -Qy -o max-enum.o /tmp/ccyKxhqp.s
GNU assembler version 2.14.90.0.6 (i386-linux) using BFD version 2.14.90.0.6 
20030820 Debian GNU/Linux
trevithick:~/playpen$ objdump --disassemble max-enum.o

max-enum.o:     file format elf32-i386

Disassembly of section .text:

00000000 <_Z4max1RK1ES1_>:
   0:   55                      push   %ebp
   1:   89 e5                   mov    %esp,%ebp
   3:   8b 55 0c                mov    0xc(%ebp),%edx
   6:   8b 4d 08                mov    0x8(%ebp),%ecx
   9:   8b 02                   mov    (%edx),%eax
   b:   39 01                   cmp    %eax,(%ecx)
   d:   7d 04                   jge    13 <_Z4max1RK1ES1_+0x13>
   f:   89 d0                   mov    %edx,%eax
  11:   5d                      pop    %ebp
  12:   c3                      ret    
  13:   89 c8                   mov    %ecx,%eax
  15:   eb fa                   jmp    11 <_Z4max1RK1ES1_+0x11>
  17:   90                      nop    
  18:   90                      nop    
  19:   8d b4 26 00 00 00 00    lea    0x0(%esi,1),%esi

00000020 <_Z4max2RK1ES1_>:
  20:   55                      push   %ebp
  21:   89 e5                   mov    %esp,%ebp
  23:   83 ec 04                sub    $0x4,%esp
  26:   89 ec                   mov    %ebp,%esp
  28:   8d 45 fc                lea    0xfffffffc(%ebp),%eax
  2b:   5d                      pop    %ebp
  2c:   c3                      ret    
  2d:   90                      nop    
  2e:   89 f6                   mov    %esi,%esi
trevithick:~/playpen$ 

The problem doesn't happen if E is typedef()d to be int.

[Bug c++/12667] max(enum) returns reference to temporary

[pinskia at gcc dot gnu dot org]

PLEASE REPLY TO gcc-bugzilla@gcc.gnu.org ONLY, *NOT* gcc-bugs@gcc.gnu.org.

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12667



------- Additional Comments From pinskia at gcc dot gnu dot org  2003-10-18 00:15 -------
Fixed on the mainline, 3.4: Search converges between 2003-09-16-trunk (#403) and 2003-09
-17-trunk (#404).

[Bug c++/12667] [3.3 regression] max(enum) returns reference to temporary

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2003-11-15 11:31 -------
In 3.0.4 and before the code was right and included the compare so that is a regression 
but since I do not know if the code invokes undefined behavior, I am not confirming this 
one.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |pessimizes-code, wrong-code
            Summary|max(enum) returns reference |[3.3 regression] max(enum)
                   |to temporary                |returns reference to
                   |                            |temporary
   Target Milestone|---                         |3.3.3


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12667

[Bug c++/12667] [3.3 regression] max(enum) returns reference to temporary

[mdorey at bluearc dot com]

------- Additional Comments From mdorey at bluearc dot com  2003-11-15 14:06 -------
(In reply to comment #2)
> but since I do not know if the code invokes undefined behavior, I am not
confirming this 
> one.

Thanks for your interest.  Perhaps this would be a convincing demonstration (if
you can reproduce it):

martind@kenny:~/playpen$ cat max-enum2.cpp
#include <iostream>
enum E {
    E1, E2
};
const E& max2(const E& a, const E& b) {
  return a < b ? b : a;
}
int main() {
  std::cout << max2(E1, E2) << std::endl;
}
martind@kenny:~/playpen$ g++ -W -Wall -pedantic -v -O2 max-enum2.cpp
Reading specs from /usr/lib/gcc-lib/i486-linux/3.3.2/specs
Configured with: ../src/configure -v --enable-languages=c,c++,java,f77,pascal,ob
          jc,ada,treelang --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info            --with-gxx-include-dir=/usr/include/c++/3.3
--enable-shared --with-system-zlib -           -enable-nls
--without-included-gettext --enable-__cxa_atexit --enable-clocale=gn           u
--enable-debug --enable-java-gc=boehm --enable-java-awt=xlib --enable-objc-gc  
         i486-linux
Thread model: posix
gcc version 3.3.2 (Debian)
 /usr/lib/gcc-lib/i486-linux/3.3.2/cc1plus -quiet -v -D__GNUC__=3 -D__GNUC_MINOR
          __=3 -D__GNUC_PATCHLEVEL__=2 -D_GNU_SOURCE max-enum2.cpp -D__GNUG__=3
-quiet -du           mpbase max-enum2.cpp -auxbase max-enum2 -O2 -W -Wall
-pedantic -version -o /tmp/           ccNFFKYd.s
GNU C++ version 3.3.2 (Debian) (i486-linux)
        compiled by GNU C version 3.3.2 (Debian).
GGC heuristics: --param ggc-min-expand=34 --param ggc-min-heapsize=7673
ignoring nonexistent directory "/usr/i486-linux/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/c++/3.3
 /usr/include/c++/3.3/i486-linux
 /usr/include/c++/3.3/backward
 /usr/local/include
 /usr/lib/gcc-lib/i486-linux/3.3.2/include
 /usr/include
End of search list.
max-enum2.cpp: In function `const E& max2(const E&, const E&)':
max-enum2.cpp:6: warning: returning reference to temporary
 as -V -Qy -o /tmp/ccFrrEWo.o /tmp/ccNFFKYd.s
GNU assembler version 2.14.90.0.7 (i386-linux) using BFD version 2.14.90.0.7 200
          31029 Debian GNU/Linux
 /usr/lib/gcc-lib/i486-linux/3.3.2/collect2 --eh-frame-hdr -m elf_i386 -dynamic-
          linker /lib/ld-linux.so.2
/usr/lib/gcc-lib/i486-linux/3.3.2/../../../crt1.o /usr          
/lib/gcc-lib/i486-linux/3.3.2/../../../crti.o /usr/lib/gcc-lib/i486-linux/3.3.2/
          crtbegin.o -L/usr/lib/gcc-lib/i486-linux/3.3.2
-L/usr/lib/gcc-lib/i486-linux/3.3           .2/../../.. /tmp/ccFrrEWo.o -lstdc++
-lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /usr/li          
b/gcc-lib/i486-linux/3.3.2/crtend.o /usr/lib/gcc-lib/i486-linux/3.3.2/../../../c
          rtn.o
martind@kenny:~/playpen$ ./a.out
-1073742500
martind@kenny:~/playpen$ 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12667

[Bug c++/12667] [3.3 regression] max(enum) returns reference to temporary

[bangerth at dealii dot org]

------- Additional Comments From bangerth at dealii dot org  2003-11-16 17:47 -------
The code in comment #3 is definitely wrong. You call 
  max(E1,E2) 
both E1 and E2 are names that when evaluated yield temporary variables. These 
temporaries are then bound to the constant references in the args to max, and  
you return a reference to a temporary from that function. The lifetime of the temporary 
ends at the end of the call expression, so you get garbage when using the result int 
the call to operator<< 
 
W. 

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12667

[Bug c++/12667] [3.3 regression] max(enum) returns reference to temporary

[falk at debian dot org]

------- Additional Comments From falk at debian dot org  2003-11-16 17:55 -------
(In reply to comment #4)
> The code in comment #3 is definitely wrong. You call 
>   max(E1,E2) 
> both E1 and E2 are names that when evaluated yield temporary variables.

I don't think so. The standard says:

"The identifiers in an enumerator-list are declared as constants, and can appear
wherever constants are required."


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12667

[Bug c++/12667] [3.3 regression] max(enum) returns reference to temporary

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2003-11-18 05:28 -------
Is a type of "const enum &" an lvalue, if it is then this is a bug in 3.3, otherwise there is a bug in 
3.4 because of 5.16/4 If the second and third operands are lvalues and have the same type, the 
result is of that type is an lvalue.

-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12667

[Bug c++/12667] [3.3 regression] max(enum) returns reference to temporary

[gdr at integrable-solutions dot net]

------- Additional Comments From gdr at integrable-solutions dot net  2003-11-19 04:23 -------
Subject: Re:  [3.3 regression] max(enum) returns reference to temporary

"bangerth at dealii dot org" <gcc-bugzilla@gcc.gnu.org> writes:

| The code in comment #3 is definitely wrong. You call 
|   max(E1,E2) 
| both E1 and E2 are names that when evaluated yield temporary variables. These 
| temporaries are then bound to the constant references in the args to max, and  
| you return a reference to a temporary from that function. The lifetime of the temporary 
| ends at the end of the call expression, 

No, the temporaries last until the end of the full expression.

-- Gaby


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12667

[Bug c++/12667] [3.3 regression] max(enum) returns reference to temporary

[mdorey at bluearc dot com]

------- Additional Comments From mdorey at bluearc dot com  2003-11-20 07:16 -------
(In reply to comment #7)
> No, the temporaries last until the end of the full expression.

Just so but, for any sceptics in the audience, this produces similarly 
incorrect output:

trevithick:~/playpen$ cat max-enum3.cpp && g++ -W -Wall -pedantic -v -O2 max-
enum3.cpp && ./a.out
#include <iostream>
enum E {
    E1, E2
};
const E& max2(const E& a, const E& b) {
  return a < b ? b : a;
}
int main() {
  E e1 = E1;
  E e2 = E2;
  std::cout << max2(e1, e2) << std::endl;
}
Reading specs from /usr/lib/gcc-lib/i486-linux/3.3.2/specs
Configured with: ../src/configure -v --enable-
languages=c,c++,java,f77,pascal,objc,ada,treelang --prefix=/usr --
mandir=/usr/share/man --infodir=/usr/share/info --with-gxx-include-
dir=/usr/include/c++/3.3 --enable-shared --with-system-zlib --enable-nls --
without-included-gettext --enable-__cxa_atexit --enable-clocale=gnu --enable-
debug --enable-java-gc=boehm --enable-java-awt=xlib --enable-objc-gc i486-linux
Thread model: posix
gcc version 3.3.2 (Debian)
 /usr/lib/gcc-lib/i486-linux/3.3.2/cc1plus -quiet -v -D__GNUC__=3 -
D__GNUC_MINOR__=3 -D__GNUC_PATCHLEVEL__=2 -D_GNU_SOURCE max-enum3.cpp -
D__GNUG__=3 -quiet -dumpbase max-enum3.cpp -auxbase max-enum3 -O2 -W -Wall -
pedantic -version -o /tmp/ccHEBlEU.s
GNU C++ version 3.3.2 (Debian) (i486-linux)
        compiled by GNU C version 3.3.2 (Debian).
GGC heuristics: --param ggc-min-expand=98 --param ggc-min-heapsize=129171
ignoring nonexistent directory "/usr/i486-linux/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/include/c++/3.3
 /usr/include/c++/3.3/i486-linux
 /usr/include/c++/3.3/backward
 /usr/local/include
 /usr/lib/gcc-lib/i486-linux/3.3.2/include
 /usr/include
End of search list.
max-enum3.cpp: In function `const E& max2(const E&, const E&)':
max-enum3.cpp:6: warning: returning reference to temporary
 as -V -Qy -o /tmp/ccALcrp6.o /tmp/ccHEBlEU.s
GNU assembler version 2.14.90.0.7 (i386-linux) using BFD version 2.14.90.0.7 
20031029 Debian GNU/Linux
 /usr/lib/gcc-lib/i486-linux/3.3.2/collect2 --eh-frame-hdr -m elf_i386 -dynamic-
linker /lib/ld-linux.so.2 /usr/lib/gcc-lib/i486-
linux/3.3.2/../../../crt1.o /usr/lib/gcc-lib/i486-
linux/3.3.2/../../../crti.o /usr/lib/gcc-lib/i486-linux/3.3.2/crtbegin.o -
L/usr/lib/gcc-lib/i486-linux/3.3.2 -L/usr/lib/gcc-lib/i486-
linux/3.3.2/../../.. /tmp/ccALcrp6.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -
lgcc /usr/lib/gcc-lib/i486-linux/3.3.2/crtend.o /usr/lib/gcc-lib/i486-
linux/3.3.2/../../../crtn.o
-1073744148

Cheers,


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12667

[Bug c++/12667] [3.3 regression] max(enum) returns reference to temporary

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2003-11-20 07:18 -------
Remember this is fixed for 3.4 already.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2003-11-20 07:18:20
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12667

[Bug c++/12667] [3.3 regression] max(enum) returns reference to temporary

[mdorey at bluearc dot com]

------- Additional Comments From mdorey at bluearc dot com  2003-11-20 07:23 -------
(In reply to comment #9)
> Remember this is fixed for 3.4 already.

And 3.3 (at least the version and configuration I just submitted the -v for) is 
producing a warning even with no flags - so it's not a *silent* code-gen bug.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12667

[Bug c++/12667] [3.3 regression] max(enum) returns reference to temporary

[pinskia at gcc dot gnu dot org]

------- Additional Comments From pinskia at gcc dot gnu dot org  2003-12-22 22:57 -------
Too much for the branch so closing as will not fix for 3.3.3.

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


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=12667