No warning of violating strict aliasing rule and produce wrong code

[Bingfeng Mei]

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.
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? 

Cheers,
Bingfeng Mei 
Broadcom UK


tst.c

#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");
}