Intel® 64 and IA-32 Architectures Optimization Reference Manual

CHAPTER 964-BIT MODE CODING GUIDELINES9.1 INTRODUCTIONThis chapter describes coding guidelines for application software written to run in64-bit mode. Some coding recommendations applicable to 64-bit mode are coveredin Chapter 3. The guidelines in this chapter should be considered as an addendum tothe coding guidelines described in Chapter 3 through Chapter 8.Software that runs in either compatibility mode or legacy non-64-bit modes shouldfollow the guidelines described in Chapter 3 through Chapter 8.9.2 CODING RULES AFFECTING 64-BIT MODE9.2.1 Use Legacy 32-Bit Instructions When Data Size Is 32 Bits64-bit mode makes 16 general purpose 64-bit registers available to applications. Ifapplication data size is 32 bits, there is no need to use 64-bit registers or 64-bit arithmetic.The default operand size for most instructions is 32 bits. The behavior of thoseinstructions is to make the upper 32 bits all zeros. For example, when zeroing out aregister, the following two instruction streams do the same thing, but the 32-bitversion saves one instruction byte:32-bit version:xor eax, eax; Performs xor on lower 32bits and zeroes the upper 32 bits.64-bit version:xor rax, rax; Performs xor on all 64 bits.This optimization holds true for the lower 8 general purpose registers: EAX, ECX,EBX, EDX, ESP, EBP, ESI, EDI. To access the data in registers R9-R15, the REX prefixis required. Using the 32-bit form there does not reduce code size.9-1