simplify-rtx.c (mode_signbit_p): New function to check whether an RTX is an immediate constant that represents...


	* simplify-rtx.c (mode_signbit_p): New function to check whether
	an RTX is an immediate constant that represents the most significant
	bit of a given machine mode.
	(simplify_unary_operation) <NOT>: Optimize ~(X+C) as X ^ ~C, where
	C is the sign bit.
	(simplify_binary_operation) <PLUS>: Optimize (X^C1) + C2 as X^(C1^C2)
	when C2 is the sign bit.
	(simplify_binary_operation) <XOR>: Canonicalize X^C as X+C when C
	is the sign bit.  Optimize (X+C1) ^ C2 as X^(C1^C2) when C1 is the
	sign bit.

	* gcc.c-torture/execute/20040409-1.c: New test case.
	* gcc.c-torture/execute/20040409-2.c: New test case.
	* gcc.c-torture/execute/20040409-3.c: New test case.

From-SVN: r80568