BUG: Optimization Bug When Using Array with memset Initializer (263606)


The information in this article applies to:

  • Microsoft Visual C++, 32-bit Enterprise Edition 6.0
  • Microsoft Visual C++, 32-bit Professional Edition 6.0
  • Microsoft Visual C++, 32-bit Learning Edition 6.0
This article was previously published under Q263606

SYMPTOMS

The C/C++ compiler may generate incorrect code when compiled with the /O2 option if an array pointer is initialized with a call to the memset initializer.

RESOLUTION

Split the array initialization and pointer initialization into two steps as shown in the sample code below.

STATUS

Microsoft has confirmed that this is a bug in the Microsoft products that are listed at the beginning of this article.

MORE INFORMATION

Steps to Reproduce Behavior

The following sample code demonstrates the bug and the workaround: 
//test.cpp
// compiler option needed: /O2  OR  /Og /Oi /Ot /Oy

//#define WORKAROUND /* Uncomment this line for workaround.*/ 
#include <stdio.h>
#include <string.h>

typedef struct {signed char s[118];} A;
typedef struct {signed char s[12];} B;

void *memset_surrogate(void *p, int i, size_t s)
{
  return memset(p, i, s);
}

int main()
{
  A a;
#ifndef WORKAROUND
  A *pa= (A *)memset(&a, 0, sizeof(A));
#else
  memset(&a, 0, sizeof(A));
  A *pa= &a;
#endif

  B bs[5];
  memset_surrogate(bs, -1, sizeof(B));
  printf("%d\n", pa->s[0]); // should print 0, prints -1
  return 0;
}
Modification Type:MinorLast Reviewed:7/5/2005
Keywords:kbBug kbCodeGen kbCompiler KB263606
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