shifting left and then immediately shifting right in asm?

Question

disclaimer : I'm an asm newbie. I probably need to review my 2s complement or something to fully comprehend :(

I'm confused as to the purpose of the following:

....
BL some_func
MOV R3, R0,LSL#16
MOVS R3, R3,LSR#16
....

why the shift back? and the movs?

Answer 1

In the left-shift, the bits overflowed will be lost. So all bits above 32-16 = 16th digit will be zeroed out after right-shift.

     r0 =                   aaaabbbbccccdddd eeeeffffgggghhhh
lsl, 16 -> aaaabbbbccccdddd eeeeffffgggghhhh 0000000000000000
             (overflowed)
        ->                  eeeeffffgggghhhh 0000000000000000
lsr, 16 ->                                   eeeeffffgggghhhh

The instruction is equivalent to

r3 = r0 & 0xffff;