Texas Instruments C28 Series Student Guide

Optimization Levels
Volatile Usage
Optimization Issue: “Volatile” Variables
Optimization Issue: “Volatile” Variables
unsigned
unsigned
int
int
*CTRL
*CTRL
while (*CTRL !=1);
while (*CTRL !=1);
volatile
volatile
unsigned
unsigned
int
int
*CTRL
*CTRL
while (*CTRL !=1);
while (*CTRL !=1);
Wrong: Wait loop for a hardware signal
Wrong: Wait loop for a hardware signal
Solution:
Solution:
Problem:
Problem:
The compiler does not know that this pointer may refer to a
The compiler does not know that this pointer may refer to a
hardware register that may change outside the scope of the C pro
hardware register that may change outside the scope of the C pro
gram.
gram.
Hence it may be eliminated (optimized out of existence!)
Hence it may be eliminated (optimized out of existence!)
CTRL
CTRL
= 1?
= 1?
No
No
Yes
Yes
Optimizer removes
Optimizer removes
empty loop
empty loop
empty
empty
loop
loop


When using optimization, it is important to declare variables as
When using optimization, it is important to declare variables as
volatile
volatile
when:
when:


The memory location may be
The memory location may be
modifed
modifed
by something other than the
by something other than the
compiler (e.g. it’s a memory
compiler (e.g. it’s a memory
-
-
mapped peripheral register).
mapped peripheral register).


The order of operations should not be rearranged by the compiler
The order of operations should not be rearranged by the compiler


Define the pointer as “volatile” to prevent the optimizer from o
Define the pointer as “volatile” to prevent the optimizer from o
ptimizing
ptimizing
C28x – C Programming D - 11

Texas Instruments C28 Series Specifications

General

Brand	Texas Instruments
Model	C28 Series
Category	Computer Hardware
Language	English