Suspicion of regression in uninitialized value detection

[David Brown]

On 05/12/2011 22:43, Jeff Law wrote:
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> On 12/02/11 06:03, Patrice B wrote:
>> Sorry for the noise, the problem is already tracked here:
>> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18501
>>
>> Le 2 décembre 2011 10:42, Patrice Bouchand<pbfwdlist@gmail.com>  a
>> écrit :
>>> Hello,
>>>
>>> I suspect a regression in uninitialized value detection, but
>>> before opening a bug I request your advices on the following
>>> problem:
>>>
>>> I compile the following code :
>>> ---------------------------------------------------- #include
>>> <stdio.h>  #include<stdlib.h>
>>>
>>> int main( int argc, char **argv ) { int j; int rtn; int k,t;
>>>
>>> j = atoi( argv[1] );
>>>
>>> if ( j>  5 ) { rtn = 10; }
>>>
>>> k=t;
>>>
>>> printf("rtn = %d\n", rtn);
>>>
>>> exit(0); } ----------------------------------------------------
>>>
>>> With gcc 4.0:
>>>
>>> bash-4.2$ gcc-4.0  -O2 -Wall ./test_gcc2.c -o test_gcc
>>> ./test_gcc2.c: In function 'main': ./test_gcc2.c:17: warning: 't'
>>> is used uninitialized in this function ./test_gcc2.c:7: warning:
>>> 'rtn' may be used uninitialized in this function
>>>
>>> With gcc 4.6.1, the warning on rtn disappears :
>>>
>>> bash-4.2$  gcc  -O2 -Wall ./test_gcc2.c -o test_gcc
>>> ./test_gcc2.c: In function ‘main’: ./test_gcc2.c:8:8: attention :
>>> variable ‘k’ set but not used [-Wunused-but-set-variable]
>>> ./test_gcc2.c:17:5: attention : ‘t’ is used uninitialized in this
>>> function [-Wuninitialized]
>>>
>>>
>>> Do I need to pass special options to gcc 4.6.1 to enable this
>>> detection or is it a gcc problem ?
> The uninitialized warning for rtn is not emitted because gcc-4.6.1
> optimizes the code better than gcc-4.0.  It effectively realizes the
> value is uninitialized on one path through the CFG while on the other
> path RTN has the value 10.
>
> When the uninitialized&  initialized to 10 paths meet, the compiler
> (correctly) pretends the value for the uninitialized path is 10 as
> well.  It then propagates the value 10 into the use of RTN in the
> printf statement.
>
> Thus by the time the uninitialized warning code runs all uses of the
> RTN variable have been eliminated leaving just a use of the constant
> 10.  Thus you do not get a warning.
>
> jeff

That sounds to me like a bug.

The point of a warning like "unintialised variable" is static error 
checking - it is to help spot mistakes in your code.  And if there is a 
path through the function that uses an uninitialised variable, that's 
almost certainly a bug in your code - one you would like the warning to 
tell you about.

This is a case of the compiler seeing that there is undefined behaviour, 
and picking it's own way to define that behaviour.  It is of course free 
to do that (you can implement undefined behaviour any way you want) - 
and picking the shortest, fastest way is a good idea.  But it should 
trigger the warning.

mvh.,

David