Developer’s Manual
January, 2004
183
Intel XScale® Core
Developer’s Manual
Optimization Guide
A.3
Basic Optimizations
This chapter outlines optimizations specific to ARM architecture. These optimizations have been
modified to suit the core where needed.
A.3.1
Conditional Instructions
The Intel XScale
®
core architecture provides the ability to execute instructions conditionally. This
feature combined with the ability of the core instructions to modify the condition codes makes
possible a wide array of optimizations.
A.3.1.1.
Optimizing Condition Checks
The Intel XScale
®
core instructions can selectively modify the state of the condition codes. When
generating code for if-else and loop conditions it is often beneficial to make use of this feature to
set condition codes, thereby eliminating the need for a subsequent compare instruction. Consider
the C code segment:
if (a + b)
Code generated for the if condition without using an add instruction to set condition codes is:
;Assume r0 contains the value a, and r1 contains the value b
add r0,r0,r1
cmp r0,
#0
However, code can be optimized as follows making use of add instruction to set condition codes:
;Assume r0 contains the value a, and r1 contains the value b
adds r0,r0,r1
The instructions that increment or decrement the loop counter can also be used to modify the
condition codes. This eliminates the need for a subsequent compare instruction. A conditional
branch instruction can then be used to exit or continue with the next loop iteration.
Consider the following C code segment:
for (i = 10; i != 0; i--)
{
do something;
}
The optimized code generated for the above code segment would look like:
L6:
.
.
subs r3, r3, #1
bne .L6
It is also beneficial to rewrite loops whenever possible so as to make the loop exit conditions check
against the value 0. For example, the code generated for the code segment below will need a
compare instruction to check for the loop exit condition.
for (i = 0; i < 10; i++)
{
do something;
}
If the loop were rewritten as follows, the code generated avoids using the compare instruction to
check for the loop exit condition.
for (i = 9; i >= 0; i--)
{
do something;
}
