x86/process: Optimize TIF checks in __switch_to_xtra()

x86/process: Optimize TIF checks in __switch_to_xtra()

commit af8b3cd3934ec60f4c2a420d19a9d416554f140b upstream

Help the compiler to avoid reevaluating the thread flags for each checked
bit by reordering the bit checks and providing an explicit xor for
evaluation.

With default defconfigs for each arch,

x86_64: arch/x86/kernel/process.o
text       data     bss     dec     hex
3056       8577      16   11649    2d81	Before
3024	   8577      16	  11617	   2d61	After

i386: arch/x86/kernel/process.o
text       data     bss     dec     hex
2957	   8673	      8	  11638	   2d76	Before
2925	   8673       8	  11606	   2d56	After

Originally-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Kyle Huey <khuey@kylehuey.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Link: http://lkml.kernel.org/r/20170214081104.9244-2-khuey@kylehuey.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
[dwmw2: backported to make TIF_RDS handling simpler.
        No deferred TR reload.]
Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>