Typical Advice

 advice #27003: Detecting compilation with debug option -g, which hinders
optimization. To optimize, remove -g or compile with
--optimize_with_debug_option.

Why is the Compiler giving this Advice ?

The compiler detects that your compilation is being done using -g, which limits software-pipelining. Software-pipelining is a key optimization for achieving good performance. This Advice is issued to alert you to NOT use compiler option -g. -g is a good option for debugging, but it is recommended that this option not be used for production code because of the negative performance implications.

What it means

For improved performance, at optimization levels --opt_level=2 (or -O2) and -O3, the compiler attempts to software pipeline your loops. Using -g limits this software-pipelining; you may see a performance loss with this option. For best performance, use compiler options -o2 or higher; this is critical for generating efficient code sequences. In general, to achieve maximal performance, avoid using the following in production code :

• -g: Support full symbolic debug. Good option for debugging. This inhibits code reordering across source line boundaries and limits optimizations around function boundaries. This results in less parallelism, more nops and generally less efficient schedules. Using this option can cause a 30-50% performance degradation for control code, generally somewhat less but still significant degradation for performance critical code.
• -ss: Interlist source code into assembly file. As with -g, this option can negatively impact performance.
• -mu: Turn off software-pipelining, which is a key optimization for achieving good performance. Good option for debugging. Not recommended for use in production code due to negative performance implications.

The easiest way to improve performance is to invoke optimization by specifying 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

Using -g results in loss of performance.

Suggested Action

Remove -g from compile options; compile with options -o2 or higher.

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.