AMD 250 Signed Division by Multiplication of Constant, Simpler Code for Restricted Dividend, 162

Software Optimization Guide for AMD64 Processors 25112 Rev. 3.06 September 2005

Simpler Code for Restricted Dividend

Integer division by a constant can be made faster if the range of the dividend is limited, which removes a shift associated with most divisors. For example, for a divide by 10 operation, use the following code if the dividend is less than 4000_0005h:

mov eax, dividend mov edx, 01999999Ah mul edx

mov quotient, edx

Signed Division by Multiplication of Constant

Algorithm: Divisors 2 <= d < 231

These algorithms work if the divisor is positive. If the divisor is negative, use abs(d) instead of d, and append a neg edx instruction to the code. These changes make use of the fact that n/–d= –(n/d).

;a = algorithm

;m = multiplier

;s = shift count

;a == 0

mov	eax, m
imul	dividend
mov	eax, dividend
shr	eax, 31
sar	edx, s
add	edx, eax	; Quotient in EDX

;a == 1 mov eax, m imul dividend

mov eax, dividend add edx, eax

shr eax, 31 sar edx, s

add edx, eax ; Quotient in EDX

Code for determining the algorithm (a), multiplier (m), and shift factor (s) is shown in “Derivation of Algorithm, Multiplier, and Shift Factor for Integer Division by Constants” on page 186.

Signed Division by 2

;In: EAX = dividend

;Out: EAX = quotient

cmp eax, 80000000h			; CF = 1 if		dividend	>= 0.
sbb	eax,	-1	;	Increment	dividend	if it is < 0.
sar	eax,	1	;	Perform right shift.

162 Integer Optimizations Chapter 8