Typical Advice

 advice #27000: Detecting compilation without optimization.
Use optimization option -o2 or higher.

Why is the Compiler giving this Advice ?

Your compilation is being done without any optimization options (-o0 and above). This prevents the compiler from using its most powerful optimization techniques, since the -o (--opt_level) options are the foundtions for most other optimizations. 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 need to specify optimization options on the command line so that these optimizations are performed. 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 any optimization can significantly hurt performance. You must use -o2 or higher optimization to see performance improvements.

Suggested Action

Compile with options -o2 or higher.

More Resources

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