Optimizer Assistant Advice: Detecting low optimization.

Typical Advice

 #27008-D Detecting compilation with low optimization. Recommend setting --opt_level=3. NOTE: debug ability will decrease.

Why is the Compiler giving this Advice ?

Your compilation uses low-level optimization options (-o2 and below). This prevents the compiler from using its most powerful optimization techniques, since the -o (--opt_level) options are the foundations for most other optimizations. You could get substantially better performance using -o3 (or above) optimization.

NOTE: debug ability will decrease with increased optimization levels. For details see:

• https://processors.wiki.ti.com/index.php/Debug_versus_Optimization_Tradeoff

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, 3, and 4), which controls the type and degree of optimization.

--opt_level=off or -Ooff

• Performs no 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 loop optimizations
• Eliminates global common subexpressions
• Eliminates global unused assignments
• 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

• 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

--opt_level=4 or -O4

• Performs link-time optimization.

Note that -o defaults to -o2 (-o3 for ARM). In the absence of any -o options, MSP430 is set to -o0, ARM is set to -o3, and C2000 is set to -ooff.

For more information, see:

• C28x Code Generation Tips and Tricks
  
• ARM compiler optimizations
  
• MSP430 FAQ
  

Risks, Severity

Use -o3 or higher optimization to see performance improvements.

Suggested Action

Compile with options -o3 or higher.

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