[patch][4.7] Enhance XOR handling in simplify-rtx.c

[Chung-Lin Tang <cltang at codesourcery dot com>]

Hi,
this patch adds a bit more sophistication to the handled xor RTX cases
in foo().

This may look a bit ad hoc, but I am seeing it useful for some cases
where we combine zero_extend with (not (shift ...)).  The supplied ARM
testcase demonstrates when 3-insn combining comes up with:
(set (reg:SI 145)
    (xor:SI (and:SI (not:SI (reg/v:SI 135 [ crc ]))
            (const_int 32767 [0x7fff]))
        (const_int 65535 [0xffff])))

when it is actually equivalent to:
(set (reg:SI 145)
    (ior:SI (reg/v:SI 135 [ x ])
        (const_int 32768 [0x8000])))

This happens on ARM architecture levels v6 and above, due to its
possession of real zero_extend instructions.  On ARMv5 and earlier, the
use of two shifts for zero extending actually helped to work around
this, due to staged combining effects of optimizing the shifts away one
by one...

Cross-tested using QEMU for ARM-Linux, currently undergoing x86
bootstrapping and testing. If results are clear, is this okay for trunk
when stage1 opens again?

Thanks,
Chung-Lin

	* simplify-rtx.c (simplify_binary_operation_1): Handle
	(xor (and A B) C) case when B and C are both constants.

Attachment: xor.diff
Description: Text document