Optimizer Assistant Advice: Detecting optimization level 3.

Typical Advice

 #27009-D Detecting compilation at optimization level 3. Recommend setting --opt_level=4. NOTE: Compilation time will increase and debug ability will decrease.

Why is the Compiler giving this Advice ?

Your compilation uses optimization level 3. Optimization level 4 enables whole program optimization for improved performance and code size.

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:

Risks, Severity

Use -o4 optimization to see performance improvements.

Suggested Action

Compile with option -o4.

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