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52 Store-to-Load Forwarding Restrictions
AMD Athlon™ Processor x86 Code Optimization
22007E/0—November 1999
Narrow-to-Wide
Store-Buffer Data
Forwarding
Restriction
If the following conditions are present, there is a
narrow-to-wide store-buffer data forwarding restriction:
■ The operand size of the store data is smaller than the
operand size of the load data.
■ The range of addresses spanned by the store data covers
some sub-region of range of addresses spanned by the load
data.
Avoid the type of code shown in the following two examples.
Example 1 (Avoid):
MOV EAX, 10h
MOV WORD PTR [EAX], BX ;word store
...
MOV ECX, DWORD PTR [EAX] ;doubleword load
;cannot forward upper
; byte from store buffer
Example 2 (Avoid):
MOV EAX, 10h
MOV BYTE PTR [EAX + 3], BL ;byte store
...
MOV ECX, DWORD PTR [EAX] ;doubleword load
;cannot forward upper byte
; from store buffer
Wide-to-Narrow
Store-Buffer Data
Forwarding
Restriction
If the following conditions are present, there is a
wide-to-narrow store-buffer data forwarding restriction:
■ The operand size of the store data is greater than the
operand size of the load data.
■ The start address of the store data does not match the start
address of the load.
Example 3 (Avoid):
MOV EAX, 10h
ADD DWORD PTR [EAX], EBX ;doubleword store
MOV CX, WORD PTR [EAX + 2] ;word load-cannot forward high
; word from store buffer
Use example 5 instead of example 4.
Example 4 (Avoid):
MOVQ [foo], MM1 ;store upper and lower half
...
ADD EAX, [foo] ;fine
ADD EDX, [foo+4] ;uh-oh!