Re: No warning of violating strict aliasing rule and produce wrong code

[Andrew Haley]

Bingfeng Mei wrote:
> Hello, 
> 
> In following code, gcc (mainline version as well as previous versions)
> produces wrong code without giving any warning regarding strict aliasing
> violation.
> 
> ~/work/trunk-x86/bin/gcc tst.c -O3 -o tst -Wstrict-aliasing=2
> ./tst
> barrier1
> Miscompilation
> 
> 
> If I compile with 
> ~/work/trunk-x86/bin/gcc tst.c -O3 -o tst -fno-strict-aliasing
> ./tst
> Barrier1
> 
> Then it executes correctly. The problem is with LSocketId, which is cast
> to a different pointer type. GCC thinks the dereferenced pointer does
> not alias with the original variable and goes on to produce wrong code.

Actually, that's not true: the program is undefined, and therefore gcc
is not generating wrong code.

> This kind of error is really difficult to detect. It would be nice to at
> least give a warning. Should I report it as a bug? 

It warns me:

zorro:~ $ gcc --version
gcc (GCC) 4.3.0 20080428 (Red Hat 4.3.0-8)
Copyright (C) 2008 Free Software Foundation, Inc.

zorro:~ $ gcc t.c -O3 -o tst -Wstrict-aliasing=2
t.c: In function 'foo':
t.c:20: warning: dereferencing type-punned pointer will break strict-aliasing 
rules

zorro:~ $ ~/gcc/trunk/install/bin/gcc --version
gcc (GCC) 4.4.0 20080618 (experimental)
Copyright (C) 2008 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

zorro:~ $ ~/gcc/trunk/install/bin/gcc t.c -O3 -o tst -Wstrict-aliasing=2
t.c: In function 'foo':
t.c:20: warning: dereferencing type-punned pointer will break strict-aliasing 
rules

Andrew.


#include <stdio.h>
unsigned long long core_id;

unsigned long bar(unsigned long extchan, unsigned long* intchan)
{
  *intchan = extchan + 6;

  return extchan-5;
}


 __attribute__((noinline)) int foo (unsigned long *channelId)
{
  long long LSocketId = *channelId;
  printf("barrier1\n");

  if ((core_id) == 0)
  {
     unsigned long destcore = bar(LSocketId, (unsigned
long*)&LSocketId);

   }

   return LSocketId;
}



int main(){
  unsigned long a = 5;
  core_id = 0;
  if(foo(&a) != 11)
    printf("Miscompilation\n");
}