[Bug middle-end/61196] New: Optimizer does not handle memory accesses with two pointers to same location correctly

[bjoern.m.haase at web dot de]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61196

            Bug ID: 61196
           Summary: Optimizer does not handle memory accesses with two
                    pointers to same location correctly
           Product: gcc
           Version: 4.5.3
            Status: UNCONFIRMED
          Severity: major
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bjoern.m.haase at web dot de

Created attachment 32800
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=32800&action=edit
testcase program

The following test case program boils down an issue with gcc.4.5.3 on the
cygwin platform. The bug is not present in the mingw version 4.7.2 that I also
have on my system. It, thus, might be fixed, but adding the test case might be
worth the effort to check for regressions.

When compiling with -O0 -O1 it outputs the expected result, when compiling with 
-O2 or -O3, it produces an error. 

The code is somewhat strange but I stumbled over this segment in production
code of the libsodium crypto library. Since it's a wrong code bug on an
important platform (X86), I did classify it as "major" bug.


Output of "gcc testcase.c -O0 -o test.exe && test.exe" :
bbccddee 
66778899 
22334455 
ff001122 

Output of "gcc testcase.c -O3 -o test.exe && test.exe" :

0 
28d41c 
28ac88 
49435341 
We had an error.

Output of "gcc testcase.c -O2 -o test.exe && test.exe" :

bbccddee 
28d41c 
28ac68 
2d465455 
We had an error.


Declaring the tp pointer in the testcase as volatile fixes the problem. For
this reason, I doubt that it's rather the middle end than the X86 backend.

////////////////////////////////////////////////////////////////////
///
/// File testcase.c
///
////////////////////////////////////////////////////////////////////
#include <stdio.h>

typedef unsigned long int uint32;

typedef struct {
  unsigned long long a;
  unsigned long long b;
} int128;

int128 x = {0x1234567,0x1234567};

void copy2(int128 *r, const int128 *x)
{
  r->a = x->a;
  r->b = x->b;
}

void shufd(int128 *r, const int128 *x, const unsigned int c)
{
  int128 t;
  uint32 *tp = (uint32 *)&t;
  const uint32 *xp = (const uint32 *)x;
  tp[0] = xp[c&3];
  tp[1] = xp[(c>>2)&3];
  tp[2] = xp[(c>>4)&3];
  tp[3] = xp[(c>>6)&3];
  copy2(r,&t);
}

int main (void)
{
   int didErrorOccur = 0;

   x.a = 0x112233445566778899ll;
   x.b = 0xaabbccddeeff001122ll;
   shufd (&x,&x,0x93);
   printf ("%x \n", x.a);
   printf ("%x \n", (x.a >> 32));
   printf ("%x \n", x.b);
   printf ("%x \n", (x.b >> 32));
   if (x.a != 0x66778899bbccddeell)
   {
      didErrorOccur = 1;
   }
   if (x.b != 0xff00112222334455ll)
   {
      didErrorOccur = 1;
   }
   if (didErrorOccur)
   {
      printf ("We had an error.\n");
   }
}