[Bug c/67433] ?: expression returns unexpected value when condition is a bool variable which memory is not true/false

[Bug c/67433] ?: expression returns unexpected value when condition is a bool variable which memory is not true/false

[xuejuncao at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67433

xuejuncao <xuejuncao at gmail dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
            Summary|?: expresion returns        |?: expression returns
                   |unexpected value when       |unexpected value when
                   |condition is a bool         |condition is a bool
                   |variable and it's value     |variable which memory is
                   |above 1                     |not true/false

--- Comment #1 from xuejuncao <xuejuncao at gmail dot com> ---
#include <stdio.h>
#include <stdbool.h>
#include <string.h>

int main()
{
    union u {
        bool b;
        char c;
    } u;
    memset((void *)&u, -1, sizeof(u));
    bool b = (u.b ? 1 : 0);
    int i = (u.b ? 1 : 0);
    int j = (u.b ? 1 : 2);
    printf("b = %d, i = %d, j = %d\n", b, i, j);
    return 0;
}

without optimisation, the output is:
b = 1, i = 255, j = 1

with -O2, is:
b = 255, i = 255, j = 1

[Bug c/67433] ?: expression returns unexpected value when condition is a bool variable which memory is not true/false

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67433

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

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

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Bool can only be 0 or 1, any other value causes undefined behavior.

[Bug c/67433] ?: expression returns unexpected value when condition is a bool variable which memory is not true/false

[xuejuncao at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67433

--- Comment #3 from xuejuncao <xuejuncao at gmail dot com> ---
(In reply to Andrew Pinski from comment #2)
> Bool can only be 0 or 1, any other value causes undefined behavior.

but it seems like a trap, the below code will not work fine, and will make
program abort.
(expect to write "1/0" that only takes 1 byte, but write 2 bytes when value is
negative)

https://github.com/google/protobuf/blob/master/src/google/protobuf/wire_format_lite_inl.h

inline void WireFormatLite::WriteBoolNoTag(bool value,
                                           io::CodedOutputStream* output) {
  output->WriteVarint32(value ? 1 : 0);
}

[Bug c/67433] ?: expression returns unexpected value when condition is a bool variable which memory is not true/false

[xuejuncao at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67433

--- Comment #4 from xuejuncao <xuejuncao at gmail dot com> ---
btw, the same code works fine on Mac:

$gcc --version
Configured with: --prefix=/Library/Developer/CommandLineTools/usr
--with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
Target: x86_64-apple-darwin14.5.0
Thread model: posix

[Bug c/67433] ?: expression returns unexpected value when condition is a bool variable which memory is not true/false

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67433

--- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
Try -fsantitized=undefined to catch this error.

[Bug c/67433] ?: expression returns unexpected value when condition is a bool variable which memory is not true/false

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67433

--- Comment #6 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to xuejuncao from comment #4)
> btw, the same code works fine on Mac:
> 
> $gcc --version
> Configured with: --prefix=/Library/Developer/CommandLineTools/usr
> --with-gxx-include-dir=/usr/include/c++/4.2.1
> Apple LLVM version 6.1.0 (clang-602.0.53) (based on LLVM 3.6.0svn)
> Target: x86_64-apple-darwin14.5.0
> Thread model: posix

So this is undefined behavior.

[Bug c/67433] ?: expression returns unexpected value when condition is a bool variable which memory is not true/false

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67433

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I should say the value that is stored in the bool variable can only be 0 or 1;
anything else is undefined bahavior.

[Bug c/67433] ?: expression returns unexpected value when condition is a bool variable which memory is not true/false

[trippels at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67433

Markus Trippelsdorf <trippels at gcc dot gnu.org> changed:

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

--- Comment #9 from Markus Trippelsdorf <trippels at gcc dot gnu.org> ---
(In reply to xuejuncao from comment #8)
> thanks, i get the below result with -fsanitize=undefined 
> 
> boolmagic.c:12:16: runtime error: load of value 255, which is not a valid
> value for type '_Bool'
> boolmagic.c:13:15: runtime error: load of value 255, which is not a valid
> value for type '_Bool'
> boolmagic.c:14:15: runtime error: load of value 255, which is not a valid
> value for type '_Bool'
> b = 1, i = 1, j = 1
> 
> but, can i say the "-fsanitize=undefined" will _corrects_ the undefined
> behaviour?
> 
> or replace the code by if/else, which works fine (lucky?)
> 
>     int i; // = (u.b ? 1 : 0);
>     if (u.b) i = 1; else i = 0;

No, you're still invoking undefined behavior. -fsanitize=undefined will not
fix this at all; it just points out the issues.

You need to get rid of the underlying problem, but this is not the place
to discuss this further.

[Bug c/67433] ?: expression returns unexpected value when condition is a bool variable which memory is not true/false

[xuejuncao at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67433

--- Comment #10 from xuejuncao <xuejuncao at gmail dot com> ---
when bool value read from unpacked stream or file, wen can not ensure it's 1 or
0;
so maybe the best solution is compile with "-D_Bool=char" for now :-/

[Bug c/67433] ?: expression returns unexpected value when condition is a bool variable which memory is not true/false

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67433

--- Comment #11 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to xuejuncao from comment #10)
> when bool value read from unpacked stream or file, wen can not ensure it's 1
> or 0;
> so maybe the best solution is compile with "-D_Bool=char" for now :-/

You can always do the read with 'char' and then convert to _Bool.

You can also "fix" your testcase (I think, didn't try) by using

union u {
  bool b : 1;
  char c;
} u;

(in case unions allow bitfield members)

[Bug c/67433] ?: expression returns unexpected value when condition is a bool variable which memory is not true/false

[xuejuncao at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=67433

--- Comment #12 from xuejuncao <xuejuncao at gmail dot com> ---
(In reply to Richard Biener from comment #11)
> (In reply to xuejuncao from comment #10)
> > when bool value read from unpacked stream or file, wen can not ensure it's 1
> > or 0;
> > so maybe the best solution is compile with "-D_Bool=char" for now :-/
> 
> You can always do the read with 'char' and then convert to _Bool.
> 
> You can also "fix" your testcase (I think, didn't try) by using
> 
> union u {
>   bool b : 1;
>   char c;
> } u;
> 
> (in case unions allow bitfield members)

yes, it works, but likes a trick:-)

bit operation to bool is also undefined
int i = (u.b & 1 ? 1 : 0); // works without -O2, useless with -O2