[Bug c/28073] Type-punned pointer passed as function parameter generates bad assembly sequence

["dberlin at dberlin dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #4 from dberlin at gcc dot gnu dot org  2006-06-19 18:55 -------
Subject: Re:  Type-punned pointer passed as function parameter
 generates bad assembly sequence

sorenj at us dot ibm dot com wrote:
> ------- Comment #2 from sorenj at us dot ibm dot com  2006-06-19 16:44 -------
> Changing just one line of the test program to the (AFAIK) legal C code.  By
> casting through void *, we are addressing Andrew's concerns about violating the
> C rules. 

No you aren't.
The only thing that matters is what the type of the dereferenced pointer
is, not the intermediate casts.

For example,

int *foo
float b;
float *c;

b = 5.0
foo = (int*)&b
c = (float *)foo
printf("%f\n", *c);


is legal.


> 
>   Foo *pFoo = *(Foo **) ((void *)&longPtr); /* // BAD! */

Still not legal.

> 
> eliminates the type-punned warning, even at the highest possible warning level,
> and continues to generate code the results in a bad return value.  This test
> case illustrates that this problem is actually worse than we originally
> thought, as now incorrect code is generated without any warning.

We can't issue warnings in every case because it is impossible to detect
every case.

We could probably issue a warning in this case.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=28073