Re: Optimizer bug in 2.95.1

[Jeffrey A Law <law at cygnus dot com>]

  In message <199909160256.WAA18668@mojo.dhcp>you write:
  >  * The following is a simplified example of the problem found when
  >  * compiling dtoa.c using gcc 2.95.1 on my Linux box.  I'm using H.J.
  >  * Lu's build, though I found the same problem on a build of my own
  >  * using straight GNU sources.  The bug doesn't occur in egcs 1.1.x.
  >  *
  >  * dtoa.c can be found at http://www.cs.bell-labs.com/netlib/fp/dtoa.c.gz
  >  * H.J. Lu's gcc from     ftp://ftp.varesearch.com/pub/support/hjl/gcc
  >  *
  >  * I suspect the word0(x) macro is confusing the optimizer, and it doesn't
  >  * expect me to be doing integer manipulations on a double.  When compiled
  >  * with -O2, the value of d2 is stored *after* the bit fiddling, so the
  >  * subtraction takes place before the data is valid, and the output is
  >  * then the unsubtracted version.
  >  *
  >  * Try running compiled with -O and it works fine.  With -O2 it's wrong.
  >  * Here you can see that the high bit subtraction happens before the 
  >  * floating point value is actually stored:
  >  *
  >  *    .stabn 68,0,43,.LM3-foo
  >  *    .LM3:
  >  *    	      xorl %edx,%edx
  >  *    	      movl %eax,-8(%ebp)                 <--- load up x
  >  *    	      movl %edx,-4(%ebp)
  >  *    	      fildll -8(%ebp)
  >  *    .stabn 68,0,46,.LM4-foo
  >  *    .LM4:
  >  *    	      addl $-32505856,-12(%ebp)          <--- subtraction fiddl
  > ing
  >  *    .stabn 68,0,43,.LM5-foo
  >  *    .LM5:
  >  *    	      fstl -16(%ebp)                     <--- store d2
  >  */
  > 
  > #define word0(x) ((unsigned long *)&x)[1]
  > #define word1(x) ((unsigned long *)&x)[0]
  > 
  > double foo(double d)
  > {
  >     unsigned long x = word1(d)<<20;
  >     double d2;
  > 
  >     /* Assign a (double) value to d2 */
  >     d2 = x;
  >  
  >     /* Fiddle with the bits */
  >     word0(d2) -= 31*0x100000;
  > 
  >     return d2;
  > }    
Your code violates ANSI/ISO aliasing rules.

You access the memory for "d" using two different types.

Please see the FAQ.
jeff