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* [Bug c++/51270] New: constness violation is accepted without any warning but leads to a required function call being eliminated during optimization
@ 2011-11-22 15:41 michiel_dewilde at agilent dot com
2011-11-22 15:45 ` [Bug c++/51270] " michiel_dewilde at agilent dot com
` (7 more replies)
0 siblings, 8 replies; 9+ messages in thread
From: michiel_dewilde at agilent dot com @ 2011-11-22 15:41 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51270
Bug #: 51270
Summary: constness violation is accepted without any warning
but leads to a required function call being eliminated
during optimization
Classification: Unclassified
Product: gcc
Version: 4.6.2
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: michiel_dewilde@agilent.com
Created attachment 25884
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=25884
Standalone source file
Bug summary: constness violation is accepted without any warning but leads to a
required function call being eliminated during optimization
The code itself has a problem in the function "faulty_compiled_function":
Feeding the return value of "pass_through" as argument to "recast_reference"
violates "const" regulations.
This error is never caught by at least g++ 4.4.2 and g++ 4.6.2 on x86_64.
Far worse, when compiling with -O1 or bigger, the optimizer eliminates the call
to "pass_through" and feeds invalid data into "recast_reference".
Proposed treatment: Either there must be an error keeping this code from
compiling, or the optimization must be able to handle it properly.
Bug triggered as follows:
Compile using "g++ -O faulty_optimization_of_invalid_const_usage.cpp"
Run "./a.out ; echo $0"
This should return exit code 0. It returns exit code 1.
Without optimization, there is no error.
The error occurs at any nonzero optimization level, even when using
-fno-strict-aliasing -fno-inline -fno-omit-stack-frame.
$ gcc -v
COLLECT_GCC=gcc_x86_64
COLLECT_LTO_WRAPPER=/gfs/belgium/gntnas01/d3/hped_build_gent/tools/gcc/4.6.2/bin/../libexec/gcc/x86_64-unknown-linux-gnu/4.6.2/lto-wrapper
Target: x86_64-unknown-linux-gnu
Configured with: ./configure --prefix=/hped/builds/tools/gcc/4.6.2
--enable-languages=c,c++ --with-gmp=/hfs/d1/local/dbjornba/btmp
--with-mpfr=/hfs/d1/local/dbjornba/btmp --with-mpc=/hfs/d1/local/dbjornba/btmp
Thread model: posix
gcc version 4.6.2 (GCC)
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug c++/51270] constness violation is accepted without any warning but leads to a required function call being eliminated during optimization
2011-11-22 15:41 [Bug c++/51270] New: constness violation is accepted without any warning but leads to a required function call being eliminated during optimization michiel_dewilde at agilent dot com
@ 2011-11-22 15:45 ` michiel_dewilde at agilent dot com
2011-11-22 15:50 ` redi at gcc dot gnu.org
` (6 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: michiel_dewilde at agilent dot com @ 2011-11-22 15:45 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51270
--- Comment #1 from Michiel De Wilde <michiel_dewilde at agilent dot com> 2011-11-22 15:26:42 UTC ---
Created attachment 25885
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=25885
preprocessed source code (*.ii)
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug c++/51270] constness violation is accepted without any warning but leads to a required function call being eliminated during optimization
2011-11-22 15:41 [Bug c++/51270] New: constness violation is accepted without any warning but leads to a required function call being eliminated during optimization michiel_dewilde at agilent dot com
2011-11-22 15:45 ` [Bug c++/51270] " michiel_dewilde at agilent dot com
@ 2011-11-22 15:50 ` redi at gcc dot gnu.org
2011-11-22 16:10 ` redi at gcc dot gnu.org
` (5 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: redi at gcc dot gnu.org @ 2011-11-22 15:50 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51270
--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-11-22 15:47:45 UTC ---
If you want warnings then you should request them using -Wall etc.
Although doing so at -O2 gives this, which isn't actually very helpful:
e.cpp:67:60: warning: '<anonymous>' is used uninitialized in this function
[-Wuninitialized]
There is no constness error, the argument to recast_reference creates a
temporary of type 'char const*' which binds to a reference-to-const to that
type:
You can rewrite it as:
int*& faulty_compiled_function(char*& val)
{
char const* tmp = pass_through(val);
return recast_reference(tmp);
}
pass_through and recast_reference are not needed, they simply serve to hide the
error in your code, which is that you return a reference to the temporary, and
use it in main() after the temporary has gone out of scope. Therefore the code
is invalid.
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug c++/51270] constness violation is accepted without any warning but leads to a required function call being eliminated during optimization
2011-11-22 15:41 [Bug c++/51270] New: constness violation is accepted without any warning but leads to a required function call being eliminated during optimization michiel_dewilde at agilent dot com
2011-11-22 15:45 ` [Bug c++/51270] " michiel_dewilde at agilent dot com
2011-11-22 15:50 ` redi at gcc dot gnu.org
@ 2011-11-22 16:10 ` redi at gcc dot gnu.org
2011-11-22 16:11 ` redi at gcc dot gnu.org
` (4 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: redi at gcc dot gnu.org @ 2011-11-22 16:10 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51270
--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-11-22 15:50:33 UTC ---
If you eliminate the obfuscation you get this:
int*& faulty_compiled_function(char*& val)
{
char const* tmp = val;
return (int*&)tmp;
}
char* input = (char*)0x1234;
int* output = (int*)0x1234;
int main(int argc, char* argv[])
{
return (faulty_compiled_function(input) == output ? 0 : 1);
}
Which correctly warns you about the problem even without -Wall
If there's a bug in GCC it's that it doesn't see through two layers of
functions to determine that you're returning a reference to a temporary.
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug c++/51270] constness violation is accepted without any warning but leads to a required function call being eliminated during optimization
2011-11-22 15:41 [Bug c++/51270] New: constness violation is accepted without any warning but leads to a required function call being eliminated during optimization michiel_dewilde at agilent dot com
` (2 preceding siblings ...)
2011-11-22 16:10 ` redi at gcc dot gnu.org
@ 2011-11-22 16:11 ` redi at gcc dot gnu.org
2011-11-22 16:55 ` [Bug c++/51270] missed warning about returning reference to temporary redi at gcc dot gnu.org
` (3 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: redi at gcc dot gnu.org @ 2011-11-22 16:11 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51270
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Keywords| |diagnostic
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-11-22 15:55:41 UTC ---
This variation is a bit closer to your original and doesn't get a warning:
int*& faulty_compiled_function(char*& val)
{
char const* const& tmp = (const char*)val;
return (int*&)tmp;
}
So maybe there is a missed warning bug. Other compilers miss it too.
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug c++/51270] missed warning about returning reference to temporary
2011-11-22 15:41 [Bug c++/51270] New: constness violation is accepted without any warning but leads to a required function call being eliminated during optimization michiel_dewilde at agilent dot com
` (3 preceding siblings ...)
2011-11-22 16:11 ` redi at gcc dot gnu.org
@ 2011-11-22 16:55 ` redi at gcc dot gnu.org
2011-11-22 18:50 ` michiel_dewilde at agilent dot com
` (2 subsequent siblings)
7 siblings, 0 replies; 9+ messages in thread
From: redi at gcc dot gnu.org @ 2011-11-22 16:55 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51270
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Summary|constness violation is |missed warning about
|accepted without any |returning reference to
|warning but leads to a |temporary
|required function call |
|being eliminated during |
|optimization |
--- Comment #5 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-11-22 16:41:16 UTC ---
Very similar to my PR 49974.
Still no warning even without the (highly questionable) cast:
char const* const& faulty_compiled_function(char*& val)
{
char const* const& tmp = (const char*)val;
return tmp;
}
N.B. Clang warns for that variation (but not the others):
faulty_optimization_of_invalid_const_usage.cpp:4:10: warning: returning
reference to local temporary object [-Wreturn-stack-address]
return tmp;
^~~
faulty_optimization_of_invalid_const_usage.cpp:3:22: note: binding reference
variable 'tmp' here
char const* const& tmp = (const char*)val;
^ ~~~~~~~~~~~~~~~~
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug c++/51270] missed warning about returning reference to temporary
2011-11-22 15:41 [Bug c++/51270] New: constness violation is accepted without any warning but leads to a required function call being eliminated during optimization michiel_dewilde at agilent dot com
` (4 preceding siblings ...)
2011-11-22 16:55 ` [Bug c++/51270] missed warning about returning reference to temporary redi at gcc dot gnu.org
@ 2011-11-22 18:50 ` michiel_dewilde at agilent dot com
2011-11-22 19:23 ` redi at gcc dot gnu.org
2011-11-22 20:49 ` redi at gcc dot gnu.org
7 siblings, 0 replies; 9+ messages in thread
From: michiel_dewilde at agilent dot com @ 2011-11-22 18:50 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51270
--- Comment #6 from Michiel De Wilde <michiel_dewilde at agilent dot com> 2011-11-22 18:20:44 UTC ---
Hi Jonathan, thanks for the quick analysis.
I did not realize that "char * &" and "char const * &" are not
reference-compatible, leading to a temporary being introduced when initializing
a "char const * const &" reference from a "char * &".
Therefore this is not a true gcc bug.
It would be nice though to have a warning for this case, i.e. when only the
different constnesses after pointer dereferencing are responsible for
temporaries being created for const lvalue initializations from other lvalues.
I understand this may be difficult.
Improved tracking of the reference to the temporary would also be nice.
Thanks,
Michiel
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug c++/51270] missed warning about returning reference to temporary
2011-11-22 15:41 [Bug c++/51270] New: constness violation is accepted without any warning but leads to a required function call being eliminated during optimization michiel_dewilde at agilent dot com
` (5 preceding siblings ...)
2011-11-22 18:50 ` michiel_dewilde at agilent dot com
@ 2011-11-22 19:23 ` redi at gcc dot gnu.org
2011-11-22 20:49 ` redi at gcc dot gnu.org
7 siblings, 0 replies; 9+ messages in thread
From: redi at gcc dot gnu.org @ 2011-11-22 19:23 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51270
--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-11-22 18:56:37 UTC ---
I don't think the warning should be restricted to conversions that only involve
different const-ness.
Here's a simpler testcase for the missed warning:
const int& f(long l)
{
const int& i = l;
return i;
}
See also PR 51066
^ permalink raw reply [flat|nested] 9+ messages in thread
* [Bug c++/51270] missed warning about returning reference to temporary
2011-11-22 15:41 [Bug c++/51270] New: constness violation is accepted without any warning but leads to a required function call being eliminated during optimization michiel_dewilde at agilent dot com
` (6 preceding siblings ...)
2011-11-22 19:23 ` redi at gcc dot gnu.org
@ 2011-11-22 20:49 ` redi at gcc dot gnu.org
7 siblings, 0 replies; 9+ messages in thread
From: redi at gcc dot gnu.org @ 2011-11-22 20:49 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51270
Jonathan Wakely <redi at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2011-11-22
Ever Confirmed|0 |1
--- Comment #8 from Jonathan Wakely <redi at gcc dot gnu.org> 2011-11-22 19:39:27 UTC ---
since at least one other compiler (clang) warns about the testcase in comment 7
I'm going to confirm this
^ permalink raw reply [flat|nested] 9+ messages in thread
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2011-11-22 15:41 [Bug c++/51270] New: constness violation is accepted without any warning but leads to a required function call being eliminated during optimization michiel_dewilde at agilent dot com
2011-11-22 15:45 ` [Bug c++/51270] " michiel_dewilde at agilent dot com
2011-11-22 15:50 ` redi at gcc dot gnu.org
2011-11-22 16:10 ` redi at gcc dot gnu.org
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2011-11-22 16:55 ` [Bug c++/51270] missed warning about returning reference to temporary redi at gcc dot gnu.org
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2011-11-22 19:23 ` redi at gcc dot gnu.org
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