General Optimization Guidelines 2
2-77
(model 9) does incur a penalty. This is because every operation on a
partial register updates the whole register. However, this does mean that
there may be false dependencies between any references to partial
registers.
Example 2-24 demonstrates a series of false and real dependencies
caused by referencing partial registers.
If instructions 4 and 6 (see Example 2-24) are changed to use a
movzx
instruction instead of a
mov, then the dependences of instructions 4 on 2
(and transitively 1 before it), and instructions 6 on 5 are broken. This
creates two independent chains of computation instead of one serial one.
In a tight loop with limited parallelism, the resulting optimization can
yield several percent performance improvement.
Example 2-24 Dependencies Caused by Referencing Partial Registers
1: add ah, bh
2: add al, 3 ; instruction 2 has a false dependency on 1
3: mov bl, al ; depends on 2, but the dependence is real
4: mov ah, ch ; instruction 4 has a false dependency on 2
5: sar eax, 16 ; this wipes out the al/ah/ax part, so the
; result really doesn't depend on them programatically,
; but the processor must deal with the real dependency on
; al/ah/ax
6: mov al, bl ; instruction 6 has a real dependency on 5
7: add ah, 13 ; instruction 7 has a false dependency on 6
8: imul dl ; instruction 8 has a false dependency on 7
; because al is implicitly used
9: mov al, 17 ; instruction 9 has a false dependency on 7
; and a real dependency on 8
10: imul cx : implicitly uses ax and writes to dx, hence
; a real dependency
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