AMD 250 manual Short Instruction Encodings

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

4.6Short Instruction Encodings

Optimization

Use instruction forms with shorter encodings rather than those with longer encodings. For example, use 8-bit displacements instead of 32-bit displacements, and use the single-byte form of simple integer instructions instead of the 2-byte opcode-ModRM form.

Application

This optimization applies to:

•32-bit software

•64-bit software

Rationale

Using shorter instructions increases the number of instructions that can fit into the L1 instruction cache and increases the average decode rate.

Example

Avoid the use of instructions with longer encodings, such as those shown here:

81	C0	78	56	34	12	add eax,	12345678h	; 2-byte opcode form (with ModRM)
81	C3	FB FF FF FF				add ebx,	-5	;	32-bit	immediate value
0F	84	05	00	00	00	jz label1		;	2-byte	opcode, 32-bit immediate value

Instead, choose instructions with shorter encodings, like these:

05	78	56	34 12	add	eax, 12345678h	; 1-byte opcode form
83	C3	FB		add	ebx, -5	;	8-bit sign-extended immediate value
74	05			jz	label1	;	1-byte opcode, 8-bit immediate value

80 Instruction-Decoding Optimizations Chapter 4