[Bug c/46045] New: incorrect code generated if redecalring local variable in do-while(0)

[Bug c/46045] New: incorrect code generated if redecalring local variable in do-while(0)

[mbuilov at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46045

           Summary: incorrect code generated if redecalring local variable
                    in do-while(0)
           Product: gcc
           Version: 4.5.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: mbuilov@gmail.com


following program gives incorrect results if compiled with -O0 and crashes if
compiled with -O2

$cat test.c

struct A
{
        int k;
};

static int *foo(struct A *a)
{
        do {
                struct A *a = a;
                return &a->k;
        } while (0);
}

int main(int argc, char *argv[])
{
        struct A a = {1};
        return *foo(&a);
}

$gcc -O0 test.c && ./a.out ; echo $?
72

$gcc -O2 test.c && ./a.out ; echo $?
Segmentation fault
139

Renaming local variable in do-while fixes the problem:
<                struct A *a = a;
<                return &a->k;
---
>                struct A *b = a;
>                return &b->k;

$gcc -O0 test.c && ./a.out ; echo $?
1

$gcc -O2 test.c && ./a.out ; echo $?
1

[Bug c/46045] incorrect code generated if redecalring local variable in do-while(0)

[zsojka at seznam dot cz]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46045

Zdenek Sojka <zsojka at seznam dot cz> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |zsojka at seznam dot cz

--- Comment #1 from Zdenek Sojka <zsojka at seznam dot cz> 2010-10-16 13:31:58 UTC ---
> struct A *a = a;

Refers to the just declared variable, thus the variable is initialised by
itself.

$ gcc-4.6.0-pre9999 test.c -Wall -W -Wshadow -Winit-self
test.c: In function 'foo':
test.c:9:27: warning: declaration of 'a' shadows a parameter [-Wshadow]
test.c:6:27: warning: shadowed declaration is here [-Wshadow]
test.c:6:27: warning: unused parameter 'a' [-Wunused-parameter]
test.c: In function 'main':
test.c:14:14: warning: unused parameter 'argc' [-Wunused-parameter]
test.c:14:26: warning: unused parameter 'argv' [-Wunused-parameter]
test.c: In function 'foo':
test.c:9:27: warning: 'a' is used uninitialized in this function
[-Wuninitialized]

You get the warning with -Winit-self -Wuninitialized

[Bug c/46045] incorrect code generated if redecalring local variable in do-while(0)

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46045

Richard Guenther <rguenth at gcc dot gnu.org> changed:

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

--- Comment #2 from Richard Guenther <rguenth at gcc dot gnu.org> 2010-10-16 18:43:42 UTC ---
.

[Bug c/46045] incorrect code generated if redecalring local variable in do-while(0)

[mbuilov at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46045

--- Comment #3 from Michael Builov <mbuilov at gmail dot com> 2010-10-17 15:51:16 UTC ---
Just a warning with -Wextra, but no warnings with -Wall -pedantic.

I found a bug with this code:

struct A {


#definer dump(_a_) \
do { \
  Struct A *a = _a_; \
  printf("%d, %d\n", a->k, a->m); \
} while (0)

Struct *b, a;
dump(b); /* fine */
dump(a); /* crash */

[Bug c/46045] incorrect code generated if redecalring local variable in do-while(0)

[mbuilov at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46045

--- Comment #4 from Michael Builov <mbuilov at gmail dot com> 2010-10-17 16:19:37 UTC ---
Just a warning with -Wextra, but no warnings with -Wall -pedantic.

I found a bug with this code:

struct A {
        int k;
        int m;
};

#define dump(_a_) \
        do { \
                struct A *a = _a_; \
                printk("%d, %d\n", a->k, a->m); \
        } while (0)

void foo(struct A *b, struct A *a)
{
        dump(b); /* fine */
        dump(a); /* crash */
}

This code compiles and runs fine with Sun cc, but crashes with gcc.

I think it is definitely a bug in gcc, this code compiled only with extra
warnings (not enabled by default in linux kernel) should not crash.

The best way to fix this issue - generate an error message, like when
redeclaring variable in function:

void bar(struct A *a)
{
        struct A *a = a; /* error: 'a' redeclared as different kind of symbol
*/
}

strange, but no errors/non-extra warnings with:

void bar(struct A *a)
{
        {
                struct A *a = a;
        }
}