Assembly Language for Beginners

3.20. PACKING 12-BIT VALUES INTO ARRAY

8: sub w1, w1, w2
; W1 = W1-W2 = (idx&(~1))<<1 - idx>>1 = idx*1.5
; now test lowest bit of idx and jump if it is present.
; (ARM64 has single instruction for these operations: TBNZ (Test and Branch Not Zero)).
c: tbnz w0, #0, 30 <get_from_array+0x30>

; idx is even, go on:
10: adrp x2, page of array
14: add w3, w1, #0x1
; W3 = W1+1 = idx1.5 + 1, i.e., offset of middle byte
18: add x2, x2, offset of array within page
; load left byte at X2+W1 = array + idx1.5 with sign-extension ("sxtw")
1c: ldrb w0, [x2,w1,sxtw]
; load middle byte at X2+W3 = array + idx*1.5 + 1 with sign-extension ("sxtw")
20: ldrb w1, [x2,w3,sxtw]
; W0 = left byte
; W1 = middle byte
24: lsl w0, w0, #4
; W0 = W0<<4 = left_byte << 4
; merge parts:
28: orr w0, w0, w1, lsr #4
; W0 = W0 | W1>>4 = left_byte << 4 | middle_byte >> 4
; value in W0 is returned
2c: ret

; idx is odd, go on:
30: adrp x2, page of array
34: add w0, w1, #0x1
; W0 = W1+1 = idx1.5+1, i.e., offset of middle byte
38: add x2, x2, address of array within page
3c: add w1, w1, #0x2
; W1 = W1+2 = idx1.5+2, i.e., offset of right byte
; load middle byte at X2+W0 = array+idx1.5+1 with sign-extension ("sxtw")
40: ldrb w0, [x2,w0,sxtw]
; load right byte at X2+W1 = array+idx1.5+2 with sign-extension ("sxtw")
44: ldrb w1, [x2,w1,sxtw]
; W0 = middle byte
; W1 = right byte
48: ubfiz w0, w0, #8, #4
; W0 = middle_byte<<8
; now merge parts:
4c: orr w0, w0, w1
; W0 = W0 | W1 = middle_byte<<8 | right_byte
; value in W0 is returned
50: ret

ARM64 has new cool instructionUBFIZ(Unsigned bitfield insert in zero, with zeros to left and right), which
can be used to place specified number of bits from one register to another. It’s alias of another instruction,
UBFM(Unsignedbitfieldmove, withzerostoleftandright).UBFMistheinstructionusedinternallyinARM64
instead ofLSL/LSR(bit shifts).

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