AMD x86 Signed Division by Multiplication of Constant, Simpler Code for, Restricted Dividend

22007E/0 — November 1999

AMD Athlon™ Processor x86 Code Optimization

Example 1:

	;In:		EDX = dividend
	;Out:		EDX = quotient
	XOR EDX,		EDX;0
	CMP EAX,		d ;CF = (dividend < divisor) ? 1 : 0
	SBB EDX,		-1 ;quotient = 0+1-CF = (dividend < divisor) ? 0 : 1
	In cases where the dividend does not need to be preserved, the
	division can be accomplished without the use of an additional
	register, thus reducing register pressure. This is shown in
	example 2 below:
	Example 2:
	;In:	EDX = dividend
	;Out: EAX = quotient
	CMP EDX,		d ;CF = (dividend < divisor) ? 1 : 0
	MOV EAX,		0 ;0
	SBB EAX,		-1 ;quotient = 0+1-CF = (dividend < divisor) ? 0 : 1
Simpler Code for	Integer division by a constant can be made faster if the range of
Restricted Dividend	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 40000005h:
	MOV	EAX, dividend
	MOV	EDX, 01999999Ah
	MUL	EDX
	MOV	quotient, EDX

Signed Division by Multiplication of Constant

Algorithm: Divisors These algorithms work if the divisor is positive. If the divisor is

2 <= d < 231negative, use abs(d) instead of d, and append a ‘NEG EDX’ to the code. The code makes use of the fact that n/–d = –(n/d).

;IN: d = divisor, 2 <= d < 2^31

;OUT: a = algorithm

;m = multiplier

;s = shift count

;algorithm		0
MOV	EAX,	m
MOV	EDX,	dividend
MOV	ECX,	EDX
IMUL	EDX
SHR	ECX,	31
SAR	EDX,	s
ADD	EDX,	ECX	;quotient in EDX

Replace Divides with Multiplies 79