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Developer’s Manual March, 2003 B-15
Intel
®
80200 Processor based on Intel
®
XScale
™
Microarchitecture
Optimization Guide
B.3.4 Optimizing Integer Multiply and Divide
Multiplication by an integer constant should be optimized to make use of the shift operation
whenever possible.
;Multiplication of R0 by 2
n
mov r0, r0, LSL #n
;Multiplication of R0 by 2
n
+1
add r0, r0, r0, LSL #n
Multiplication by an integer constant that can be expressed as can similarly be
optimized as:
;Multiplication of r0 by an integer constant that can be
;expressed as (2
n
+1)*(2
m
)
add r0, r0, r0, LSL #n
mov r0, r0, LSL #m
Please note that the above optimization should only be used in cases where the multiply operation
cannot be advanced far enough to prevent pipeline stalls.
Dividing an unsigned integer by an integer constant should be optimized to make use of the shift
operation whenever possible.
;Dividing r0 containing an unsigned value by an integer constant
;that can be represented as 2
n
mov r0, r0, LSR #n
Dividing a signed integer by an integer constant should be optimized to make use of the shift
operation whenever possible.
;Dividing r0 containing a signed value by an integer constant
;that can be represented as 2
n
mov r1, r0, ASR #31
add r0, r0, r1, LSR #(32 - n)
mov r0, r0, ASR #n
The add instruction would stall for 1 cycle. The stall can be prevented by filling in another
instruction before add.
2
n
1+()
·
2
m
()⋅