Q263606: BUG: Optimization Bug When Using Array with memset Initializer

Article: Q263606
Product(s): Microsoft C Compiler
Version(s): winnt:6.0
Operating System(s): 
Keyword(s): kbCodeGen kbCompiler kbVC600bug
Last Modified: 03-MAY-2001

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The information in this article applies to:

- The C/C++ Compiler (CL.EXE), included with:
   - Microsoft Visual C++, 32-bit Enterprise Edition, version 6.0 
   - Microsoft Visual C++, 32-bit Professional Edition, version 6.0 
   - Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
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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 this to be a bug in the Microsoft products 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;
  }

Additional query words:

======================================================================
Keywords          : kbCodeGen kbCompiler kbVC600bug 
Technology        : kbVCsearch kbAudDeveloper kbCVCComp
Version           : winnt:6.0
Issue type        : kbbug

=============================================================================

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