Difference between revisions of "ARM: Count leading zeros"
From ScienceZero
| Line 15: | Line 15: | ||
;R1 = count of leading zeros | ;R1 = count of leading zeros | ||
;Uses R0-R1 | ;Uses R0-R1 | ||
| − | ; | + | ;16 cycles constant |
movs r1,r0,lsr #16 | movs r1,r0,lsr #16 | ||
mov r1,#0 | mov r1,#0 | ||
addeq r1,r1,#16 | addeq r1,r1,#16 | ||
moveqs r0,r0,lsl #16 | moveqs r0,r0,lsl #16 | ||
| − | addeq r1,r1,# | + | addeq r1,r1,#1 |
| − | + | ||
tst r0,#0xff000000 | tst r0,#0xff000000 | ||
Revision as of 17:08, 12 October 2007
Minimum size
;R0 - value
;R1 = count of leading zeros
;Uses R0-R1 mov r1,#0
;Worst case 159 cycles, best case 4 cycles
mov r1,#0
loop movs r0,r0,lsl #1
addcc r1,r1,#1
bcc loop
mov pc,r14
Maximum performance
;R0 - value
;R1 = count of leading zeros
;Uses R0-R1
;16 cycles constant
movs r1,r0,lsr #16
mov r1,#0
addeq r1,r1,#16
moveqs r0,r0,lsl #16
addeq r1,r1,#1
tst r0,#0xff000000
addeq r1,r1,#8
moveq r0,r0,lsl #8
tst r0,#0xf0000000
addeq r1,r1,#4
moveq r0,r0,lsl #4
tst r0,#0xc0000000
addeq r1,r1,#2
moveq r0,r0,lsl #2
tst r0,#0x80000000
addeq r1,r1,#1
mov pc,r14
