* builtins.c (dconstpi, dconste): New mathematical constants.
	(init_builtin_dconsts): New function to initialize dconstpi
	and dconste.
	(fold_builtin): Optimize exp(1.0) = e.  Evaluate exp(x) at
	compile time with -ffast-math when x is an integer constant.
	Optimize tan(0.0) = 0.0.  Optimize atan(0.0) = 0.0,
	atan(1.0) = pi/4 and tan(atan(x)) = x with -ffast-math.

	* gcc.dg/builtins-2.c: Add tests for tan(atan(x)).
	* gcc.dg/builtins-3.c: Add tests for tan(0.0) and atan(0.0).
	* gcc.dg/builtins-7.c: Add tests for tan(atan(x)) == x.
	* gcc.dg/builtins-17.c: New test case.
	* gcc.dg/i386-387-4.c: New test case.
	* gcc.c-torture/execute/ieee/mzero4.c: New test case.

From-SVN: r67438