One more limits question

[Mark Mitchell <mark@codesourcery.com>]

I'll try not to open such a huge can of worms this time. :-)

I'm trying to figure out how this code works.

      static const bool has_quiet_NaN
	= __builtin_nanf ("") != __builtin_nanf ("");

The constant-folding machinery seems to always consider this
expression to evaluate to true, due to this code in fold:

	  /* If either operand is a NaN, the result is false with two
	     exceptions: First, an NE_EXPR is true on NaNs, but that case
	     is already handled correctly since we will be inverting the
	     result for NE_EXPR.  Second, if we had inverted a LE_EXPR
	     or a GE_EXPR into a LT_EXPR, we must return true so that it
	     will be inverted into false.  */

	  if (REAL_VALUE_ISNAN (TREE_REAL_CST (arg0))
	      || REAL_VALUE_ISNAN (TREE_REAL_CST (arg1)))
	    t1 = build_int_2 (invert && code == LT_EXPR, 0);

So I'm trying to figure out why this isn't just:

  // We don't yet have a way of computing this correctly.
  static const bool has_quiet_NaN = true;

I'm sure I'm being dense; lay it on me.

Thanks,

--
Mark Mitchell                   mark@codesourcery.com
CodeSourcery, LLC               http://www.codesourcery.com