AMD 250 manual Summary of Store-to-Load-Forwarding Pitfalls to Avoid, 103

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

4-Kbyte pages away from the load address (address bits 47–12 do not match). Avoid the type of code shown in the following example:

mov eax, 10h
mov [eax], bx	; Word store to address 10
mov cx, [eax+2]	; Word load		to address	12
	; Load detects a false			dependency
	;	on store	because it	is in the
	; same doubleword of memory.
mov cx, [eax+4]	; Word load		to address	14
	; Load does		not detect	a false
	; dependency because it is to a
	;	different doubleword of memory.

Here is another example of the type of code to avoid:

mov eax, 10h mov [eax], bl mov [eax+1], cl mov dl, [eax]

;First store to DWORD at address 10h

;Second store to DWORD at address 10h

;Load detects a false

;dependency on the second store

;because it is the most recent

;store to the same doubleword of

;memory as the load.

Summary of Store-to-Load-Forwarding Pitfalls to Avoid

To avoid store-to-load-forwarding pitfalls, follow these guidelines:

•Maintain consistent use of operand size across all loads and stores. Preferably use doubleword or quadword operand sizes.

•Avoid misaligned data references.

•Avoid narrow-to-wide and wide-to-narrow forwarding cases.

•When using word or byte stores, avoid loading data from anywhere in the same doubleword of memory other than the identical start addresses of the stores.

Application

This optimization applies to:

•32-bit software

•64-bit software

Chapter 5 Cache and Memory Optimizations 103