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60 General 64-Bit Optimizations Chapter 3
25112 Rev. 3.06 September 2005
Software Optimization Guide for AMD64 Processors
3.1 64-Bit Registers and Integer Arithmetic
Optimization
Use 64-bit registers for 64-bit integer arithmetic.
Rationale
Using 64-bit registers instead of their 32-bit equivalents can dramatically reduce the amount of code
necessary to perform 64-bit integer arithmetic.
Example 1
This code performs 64-bit addition using 32-bit registers:
; Add ECX:EBX to EDX:EAX, and place sum in EDX:EAX.
00000000 03 C3 add eax, ebx
00000002 13 D1 adc edx, ecx
Using 64-bit registers, the previous code can be replaced by one simple instruction (assuming that
RAX and RBX contain the 64-bit integer values to add):
00000000 48 03 C3 add rax, rbx
Although the preceding instruction requires one additional byte for the REX prefix, it is still one byte
shorter than the original code. More importantly, this instruction still has a latency of only one cycle,
uses two fewer registers, and occupies only one decode slot.