AMD 250 manual Efficient 64-Bit Integer Arithmetic in 32-Bit Mode, Bit Addition, Bit Subtraction

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

8.5Efficient 64-Bit Integer Arithmetic in 32-Bit Mode

Optimization

The following section contains a collection of code snippets and subroutines showing the efficient implementation of 64-bit arithmetic in 32-bit mode. Note that these are 32-bit recommendations, in 64-bit mode it is important to use 64-bit integer instructions for best performance.

Addition, subtraction, negation, and shifting are best handled by inline code. Multiplication, division, and the computation of remainders are less common operations and are usually implemented as subroutines. If these subroutines are used often, the programmer should consider inlining them. Except for division and remainder calculations, the following code works for both signed and unsigned integers. The division and remainder code shown works for unsigned integers, but can easily be extended to handle signed integers.

64-Bit Addition

;Add ECX:EBX to EDX:EAX, and place sum in EDX:EAX. add eax, ebx

adc edx, ecx

64-Bit Subtraction

;Subtract ECX:EBX from EDX:EAX and place difference in EDX:EAX. sub eax, ebx

sbb edx, ecx

64-Bit Negation

;Negate EDX:EAX. not edx

neg eax

sbb edx, -1 ; Fix: Increment high word if low word was 0.

64-Bit Left Shift

;Shift EDX:EAX left, shift count in ECX (count

;applied modulo 64).

shld	edx, eax, cl	; First apply shift count.
shl	eax, cl	; mod	32 to EDX:EAX
test	ecx, 32	; Need to shift by another 32?
jz	lshift_done	; No,	done.
mov	edx, eax	; Left shift EDX:EAX
xor	eax, eax	; by	32 bits

lshift_done:

170 Integer Optimizations Chapter 8