User's Manual

Intel
®
IXP42X product line and IXC1100 control plane processors—Intel XScale
®
Processor
Intel
®
IXP42X Product Line of Network Processors and IXC1100 Control Plane Processor
DM September 2006
176 Order Number: 252480-006US
The above code segment would not incur any branch misprediction penalties and would
take three cycles to execute assuming best case conditions. As can be seen, using
conditional instructions speeds up execution significantly. However, the use of
conditional instructions should be carefully considered to ensure that it does improve
performance. To decide when to use conditional instructions over branches consider the
following hypothetical code segment:
Assume that we have the following data:
•N1
B
.............. Number of cycles to execute the if_stmt assuming the use of branch
instructions
•N2
B
...........Number of cycles to execute the else_stmt assuming the use of branch
instructions
• P1...................................Percentage of times the if_stmt is likely to be executed
• P2.......... Percentage of times we are likely to incur a branch misprediction penalty
•N1
C
...Number of cycles to execute the if-else portion using conditional instructions
assuming the if-condition to be true
•N2
C
...Number of cycles to execute the if-else portion using conditional instructions
assuming the if-condition to be false
Once we have the above data, use conditional instructions when:
The following example illustrates a situation in which we are better off using branches
over conditional instructions. Consider the code sample shown below:
In the above code sample, the cmp instruction takes 1 cycle to execute, the if-part
takes 7 cycles to execute and the else-part takes 6 cycles to execute. If we were to
change the code above so as to eliminate the branch instructions by making use of
conditional instructions, the if-else part would always take 10 cycles to complete.
if (cond)
if_stmt
else
else_stmt
N1
C
P1
100
---------
×
⎝⎠
⎛⎞
N2
C
100 P1–
100
----------------------
×
⎝⎠
⎛⎞
N1
B
P1
100
---------
×
⎝⎠
⎛⎞
N2
B
100 P1–
100
----------------------
×
⎝⎠
⎛⎞
P2
100
---------
4×
⎝⎠
⎛⎞
++≤+
cmp r0, #0
bne L1
add r0, r0, #1
add r1, r1, #1
add r2, r2, #1
add r3, r3, #1
add r4, r4, #1
b L2
L1:
sub r0, r0, #1
sub r1, r1, #1
sub r2, r2, #1
sub r3, r3, #1
sub r4, r4, #1
L2: