Date: Tue, 18 Aug 1998 14:18:27 +0300 (IDT)
From: Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
To: djgpp-workers AT delorie DOT com
Subject: Strange code generated by GCC
Message-ID: <Pine.SUN.3.91.980818141311.12510B-100000@is>
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Can anybody please explain why does GCC generates such a strange code, as 
described below?  It is not an idle question: a couple of functions in the 
new libm (from v2.02) fail because of this.

Here's the deal.  The program below attempts to generate a float NaN by 
using the old trick of a union with float and unsigned int members.  
0x7fc00000 is the bit pattern for a float NaN.  The problem is that, as 
the comment says, the program prints 0xffc00000 instead (which is a
negative NaN).

I can solve the problem by using memcpy instead of the last assignment in 
SET_FLOAT_WORD macro, but I'd like to understand why is GCC generate such 
code, and why optimizations change that.

Thanks in advance for any help.

/*  This program prints the bit pattern of the NaN (Not-a-Number)
    which is 0x7fc00000.  Compiled without optimizations it indeed
    does so, but even -O1 causes it to print 0xffc00000 instead.  */

#include <stdio.h>
#include <math.h>
#include <float.h>

typedef union
{
  float value;
  unsigned word;
} float_shape_type;

/* Get a 32 bit int from a float.  */

#define GET_FLOAT_WORD(i,d)					\
do {								\
  float_shape_type gf_u;					\
  gf_u.value = (d);						\
  (i) = gf_u.word;						\
} while (0)

/* Set a float from a 32 bit int.  */

#define SET_FLOAT_WORD(d,i)					\
do {								\
  float_shape_type sf_u;					\
  sf_u.word = (i);						\
  (d) = sf_u.value;						\
} while (0)

int main (void)
{
  unsigned iv;
  float fv;

  _control87(0x033f, 0xffff);	/* mask all numeric exceptions */
  SET_FLOAT_WORD(fv, 0x7fc00000U);
  GET_FLOAT_WORD(iv, fv);
  printf ("SET_FLOAT_WORD: %f, GET_FLOAT_WORD: %x\n", fv, iv);
  return 0;
}