Typical Advice

 advice #27001: Detecting compilation with low optimization level.
Use optimization option -o2 or higher.

Why is the Compiler giving this Advice ?

Your compilation uses low-level optimization options (-o1 and below), which prevents the compiler from using its most powerful optimization techniques. You could get substantially better performance using -o2 (or above) optimization. For C6000, optimization option -o2 is required for the software pipelining loop optimization, which is crucial to getting good performance.

What it means:

The C/C++ compiler is able to perform various optimizations, but you can control the level of these optimizations. High-level optimizations are performed in the optimizer and low-level, target-specific optimizations occur in the code generator. You must use high-level optimizations to achieve optimal code. The easiest way to invoke optimization is to specify the --opt_level=n option on the compiler command line. You can use -On to alias the --opt_level option. The n denotes the level of optimization (0, 1, 2, and 3), which controls the type and degree of optimization.

--opt_level=0 or -O0

• Performs control-flow-graph simplification
• Allocates variables to registers
• Performs loop rotation
• Eliminates unused code
• Simplifies expressions and statements
• Expands calls to functions declared inline

--opt_level=1 or -O1 : Performs all --opt_level=0 (-O0) optimizations, plus:

• Performs local copy/constant propagation
• Removes unused assignments
• Eliminates local common expressions

--opt_level=2 or -O2 : Performs all --opt_level=1 (-O1) optimizations, plus:

• Performs software pipelining (see Section 3.2)
• Performs loop optimizations
• Eliminates global common subexpressions
• Eliminates global unused assignments
• Converts array references in loops to incremented pointer form
• Performs loop unrolling

· --opt_level=3 or -O3 : Performs all --opt_level=2 (or -O2) optimizations, plus:Italic text* Removes all functions that are never called

• Simplifies functions with return values that are never used
• Inlines calls to small functions
• Reorders function declarations; the called functions attributes are known when the caller is optimized
• Propagates arguments into function bodies when all calls pass the same value in the same argument position
• Identifies file-level variable characteristics

Note that -o defaults to -o2. In the absence of any -o options, no optimization is performed. By default, the -o switch optimizes for performance. This may increase code size. If code size is an issue, do not reduce the level of optimization. Instead, use the -ms switch to affect the optimization goal (performance versus code size).

For more information, see C6000 Compiler: Recommended Compiler Options.

Risks, Severity

Not using -o2 or higher optimization can significantly hurt performance.

Suggested Action

Increase your optimization level; use -o2 or higher to compile.

More Resources

Want to squeeze a few more Performance Cycles out of your application? Leverage the e2e (Engineer-to-Engineer) online community to get all of your Advice questions answered! Or, give back to the community with your expertise.

Go to the TI Compiler's e2e online forum! 

For technical support please post your questions at http://e2e.ti.com.