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Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors
August 2006
Developer’s Manual
Order Number: 306262-004US
199
Intel XScale
®
Processor—Intel
®
IXP45X and Intel
®
IXP46X Product Line of Network Processors
Cost of using branches:
As can be seen, we get better performance by using branch instructions in the above
scenario.
3.10.3.1.3
Optimizing Complex Expressions
Conditional instructions should also be used to improve the code generated for complex
expressions such as the C shortcut evaluation feature. Consider the following C code
segment:
The optimized code for the if condition is:
Similarly, the code generated for the following C segment
is:
The use of conditional instructions in the above fashion improves performance by
minimizing the number of branches, thereby minimizing the penalties caused by branch
incorrect predictions. This approach also reduces the utilization of branch prediction
resources.
3.10.3.2
Bit Field Manipulation
The shift and logical operations of the IXP45X/IXP46X network processors provide a
useful way of manipulating bit fields. Bit field operations can be optimized as follows:
1
50
100
---------
10
×
⎝
⎠
⎛
⎞
50
100
---------
10
×
⎝
⎠
⎛
⎞
+
+
11
=
cycles
1
50
100
---------
7
×
⎝
⎠
⎛
⎞
50
100
---------
6
×
⎝
⎠
⎛
⎞
50
100
---------
4
×
⎝
⎠
⎛
⎞
+
+
+
9.5
=
cycles
int foo(int a, int b)
{
if (a != 0 && b != 0)
return 0;
else
return 1;
}
cmp r0,
#0
cmpne r1, #0
int foo(int a, int b)
{
if (a != 0 || b != 0)
return 0;
else
return 1;
}
cmp r0, #0
cmpeq r1, #0