optimization/997: Incorrect behavior in presence of glibc memset optimization

Tue Dec 5 23:56:00 GMT 2000

>Number:         997
>Category:       optimization
>Synopsis:       Incorrect behavior in presence of glibc memset optimization
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Tue Dec 05 23:56:00 PST 2000
>Closed-Date:
>Last-Modified:
>Originator:     Dana Jacobsen
>Release:        gcc version 2.97 20001127 (experimental)
>Organization:
>Environment:
i686-pc-linux-gnu, Pentium III, RedHat 6.2 Linux
>Description:
This program uses memset with a small compile-time fixed size.  If the glibc string optimizations are allowed (the default when optimization is used), then the memset is replaced with a large chunk of code using __builtin_memset among other things.

The test will compile properly with all snapshots I have.  When compiled with "-mcpu=i686 -O" I see these results with the following compilers:

2.95.2     OK
20000313   OK
20000807   Abort
20000904   Abort
20001002   OK
20001106   OK
20001113   Abort
20001127   Abort
20001204   Abort

The problem is seen in the check2() function where on ia32 we should be returning 1 and we do not.  check1() is identical except for an early constant return.  We abort if they are not equal, which should let us just test for this error.

To add more information, with -mcpu=i386, I get failure with -O but success with -Os, -O2, or -O3.  -mcpu=i686 gives failure on all -O levels >= 1.  Compiling without optimization avoids the problem.
>How-To-Repeat:
Use included .i file to ensure you see the same memset optimization.

#include <string.h>

typedef union {
    unsigned short  a_ushort;
    unsigned long   a_ulong;
} align_t;

static int check1(void)
{
   /* Returns 1 on ia32 */
   int r;
   align_t a;
   memset(&a,0xff,sizeof(a));
   if  (a.a_ushort == 0xFFFF)
      return 1;
   r = (a.a_ushort == 0xFFFF)  ?  1  :  0;
   return r;
}

static int check2(void)
{
   /* Should return 1 on ia32 */
   int r;
   align_t a;
   memset(&a,0xff,sizeof(a));
   r = (a.a_ushort == 0xFFFF)  ?  1  :  0;
   return r;
}

int main(void)
{
   if (check1() != check2())
      abort();
   return 0;
}
>Fix:
1. Compile without optimization (-O0).

2. Compile the source with -D__NO_STRING_INLINES to prevent glibc from touching memset.

3. Use "-O2 -mcpu=i386"
>Release-Note:
>Audit-Trail:
>Unformatted:
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