Microchip Technology XC8 Standard Compiler (Workstation) SW006021-1 SW006021-1 User Manual
Product codes
SW006021-1
MPLAB
®
XC8 C Compiler User’s Guide
DS52053B-page 62
2012 Microchip Technology Inc.
3.6.3
How Can I Reduce RAM Usage?
Use the smallest data types possible. (This also reduces code size as less code is
needed to access these.) Note that a bit type and non-standard 24-bit integer type
(short long) exists for this compiler. See Section 5.4 “Supported Data Types and
Variables” for all data types and sizes. There are two sizes of floating-point type, as
well, and these are discussed in the same section.
needed to access these.) Note that a bit type and non-standard 24-bit integer type
(short long) exists for this compiler. See Section 5.4 “Supported Data Types and
Variables” for all data types and sizes. There are two sizes of floating-point type, as
well, and these are discussed in the same section.
Consider using auto variables over global or static variables as there is the poten-
tial that these may share memory allocated to other auto variables that are not active
at the same time. Memory allocation of auto variables is made on a compiled stack,
described in Section 5.5.2.2 “Auto Variable Allocation and access”.
tial that these may share memory allocated to other auto variables that are not active
at the same time. Memory allocation of auto variables is made on a compiled stack,
described in Section 5.5.2.2 “Auto Variable Allocation and access”.
Rather than pass large objects to, or from, functions, pass pointers which reference
these objects. This is particularly true when larger structures are being passed, but
there might be RAM savings to be made even when passing long variables.
these objects. This is particularly true when larger structures are being passed, but
there might be RAM savings to be made even when passing long variables.
Objects that do not need to change throughout the program can be located in program
memory using the const qualifier, see Section 5.4.7.1 “Const Type Qualifier” and
Section 5.5.3 “Variables in Program Space”. This frees up precious RAM, but slows
execution.
memory using the const qualifier, see Section 5.4.7.1 “Const Type Qualifier” and
Section 5.5.3 “Variables in Program Space”. This frees up precious RAM, but slows
execution.
Ensure that all optimizations are enabled, see Section 4.8.42 “--OPT: Invoke Com-
piler Optimizations”. Be aware of which optimizations the compiler performs (see
Section 5.13 “Optimizations”) so that you can take advantage of them and don’t
waste your time manually performing optimizations in C code that the compiler already
handles.
piler Optimizations”. Be aware of which optimizations the compiler performs (see
Section 5.13 “Optimizations”) so that you can take advantage of them and don’t
waste your time manually performing optimizations in C code that the compiler already
handles.
3.6.4
How Can I Make My Code Faster?
To a large degree, smaller code is faster code, so efforts to reduce code size often
decrease execution time, see Section 3.6.2 “How Can I Make My Code Smaller?”.
See also, Section 3.6.6 “How Can I Make My Interrupt Routine Faster?”. However,
there are ways some sequences can be sped up at the expense of increased code size.
decrease execution time, see Section 3.6.2 “How Can I Make My Code Smaller?”.
See also, Section 3.6.6 “How Can I Make My Interrupt Routine Faster?”. However,
there are ways some sequences can be sped up at the expense of increased code size.
One of the compiler optimization settings is for speed (the alternate setting is for
space), so ensure this is selected, see Section 4.8.42 “--OPT: Invoke Compiler
Optimizations”. This will use alternate output in some instances that is faster, but
larger.
space), so ensure this is selected, see Section 4.8.42 “--OPT: Invoke Compiler
Optimizations”. This will use alternate output in some instances that is faster, but
larger.
Generally, the biggest gains to be made in terms of speed of execution come from the
algorithm used in a project. Identify which sections of your program need to be fast.
Look for loops that might be linearly searching arrays and choose an alternate search
method such as a hash table and function. Where results are being recalculated, con-
sider if they can be cached.
algorithm used in a project. Identify which sections of your program need to be fast.
Look for loops that might be linearly searching arrays and choose an alternate search
method such as a hash table and function. Where results are being recalculated, con-
sider if they can be cached.
3.6.5
How Does the Compiler Place Everything in Memory?
In most situations, assembly instructions and directives associated with both code and
data are grouped into sections, called psects, and these are then positioned into con-
tainers which represent the device memory. An introductory explanation into this pro-
cess is given in Section 5.15.1 “Program Sections”. The exception is for absolute
variables (see Section 5.5.4 “Absolute Variables”), which are placed at a specific
address when they are defined and which are not placed in a psect.
data are grouped into sections, called psects, and these are then positioned into con-
tainers which represent the device memory. An introductory explanation into this pro-
cess is given in Section 5.15.1 “Program Sections”. The exception is for absolute
variables (see Section 5.5.4 “Absolute Variables”), which are placed at a specific
address when they are defined and which are not placed in a psect.