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IA-32 Intel® Architecture Optimization
2-76
CMPXCHG8B, various rotate instructions, STC, and STD. An example
of assembly with a partial flag register stall and alternative code without
the stall is shown in Table 2-2.
Integer Divide
Typically, an integer divide is preceded by a cwd or cdq instruction.
Depending on the operand size, divide instructions use
DX:AX or
EDX:EAX
for the dividend. The cwd or cdq instructions sign-extend AX or
EAX into DX or EDX, respectively. These instructions are denser encoding
than a
shift and move would be, but they generate the same number of
μops. If AX or EAX are known to be positive, replace these instructions
with
xor dx, dx
or
xor edx, edx
Operand Sizes and Partial Register Accesses
The Pentium 4 processor, Pentium M processor (with CPUID signature
family 6, model 13), Intel Core Solo and Intel Core Duo processors do
not incur a penalty for partial register accesses; Pentium M processor
Table 2-2 Avoiding Partial Flag Register Stall
A Sequence with
Partial Flag Register Stall
Alternate Sequence without
Partial Flag Register Stall
xor eax,eax
mov ecx,a
sar ecx,2
setz al ; no partial register
stall,
; flag stall as sar may change
; the flags
xor eax,eax
mov ecx,a
sar ecx,2
test ecx,ecx
setz al ; no partial reg or flag
; stall, test
; always updates all the flags