Understanding Compiler Flags for Size Optimization
In GCC, several compiler flags can be utilized to reduce the firmware size when compiling your embedded code. These flags instruct the compiler on how aggressively it should work to optimize the code. Here are some essential flags specifically for size optimization:
<html><b>-Os</b></html>: This flag optimizes the code for size reduction. It enables all the -O2 optimizations that don't increase the code size.
Example usage:
gcc -Os -o output_file source_file.c
<html><b>-ffunction-sections</b></html> and <html><b>-fdata-sections</b></html>: These flags place each function or data item into its own section in the output file. When linked with the --gc-sections option, the linker can remove unused sections.
Example usage:
gcc -Os -ffunction-sections -fdata-sections -o output_file source_file.c
<html><b>-Wl,--gc-sections</b></html>: This linker flag works with -ffunction-sections and -fdata-sections to remove unused code and data sections.
Example usage:
gcc -Os -ffunction-sections -fdata-sections -Wl,--gc-sections -o output_file source_file.c
Examine and Adjust Code Structure
Beyond compiler flags, examining the code structure itself can aid in optimizing the firmware size. Here are some strategies:
- Minimize Memory Usage: Use
const where applicable to store data in flash instead of RAM.
Example:
const char message[] = "Stored in flash memory";
- Inline Functions Judiciously: Use
inline functions sparingly when they are small, frequently called, and time-critical, but avoid inlining large functions that can inflate the code size.
Example:
static inline int add(int a, int b) {
return a + b;
}
Utilize Link-Time Optimization (LTO)
Link-Time Optimization can significantly reduce the binary size by optimizing across module boundaries.
<html><b>-flto</b></html>: Enables Link-Time Optimization, allowing more comprehensive inter-procedural optimizations.
Example usage:
gcc -Os -flto -o output_file source_file.c
Analyzing and Tuning Libraries
The libraries used in your project can also impact the overall firmware size.
Use Standard Libraries Carefully: Standard libraries can sometimes add more code than necessary. If applicable, consider using alternative smaller libraries suited for embedded systems.
Strip Unnecessary Symbols: Use the <html><b>-s</b></html> flag to strip out symbol information for a smaller executable if debugging information is not required.
Utilize Profile-Guided Optimization (PGO)
PGO is a more advanced feature that can help in generating the smallest possible code by optimizing based on actual usage patterns.
Generate Profiling Data by compiling and running your application with the -fprofile-generate flag.
gcc -fprofile-generate -o output_file source_file.c
./output_file # Run the program to generate profiling data
Use Profiling Data for Optimization by recompiling the application with -fprofile-use.
gcc -fprofile-use -o output_file source_file.c
By using these strategies and options effectively, you can substantially reduce the size of your embedded firmware, making it more efficient and suitable for resource-constrained environments.
