AMD 250 manual Movzx and Movsx, 153

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

7.5MOVZX and MOVSX

Optimization

Use the MOVZX and MOVSX instructions to zero-extend or sign-extend, respectively, an operand to a larger size.

Application

This optimization applies to:

•32-bit software

•64-bit software

Rationale

Typical code for zero extension that replaces MOVZX uses more decode and execution resources than MOVZX. It also has higher latency due to the superset dependency between the XOR and the MOV, which requires a merge operation.

Example

When zero-extending an operand (in this case, a byte), avoid code such as the following:

xor rax, rax mov al, mem

Instead, use the MOVZX instruction:

movzx rax, BYTE PTR mem

Chapter 7 Scheduling Optimizations 153