optimization_level

Controls optimization for one function or all functions after its first occurrence.

Syntax

#pragma [intel|GCC] optimization_level n

Arguments

Argument	Description
intel\|GCC	indicates the interpretation to use
n	an integer value specifying an optimization level; valid values are: 0 same optimizations as -O0 1 same optimizations as -O1 2 same optimizations as -O2 3 same optimizations as -O3

Argument

Description

intel|GCC

indicates the interpretation to use

an integer value specifying an optimization level; valid values are:

0 same optimizations as -O0
1 same optimizations as -O1
2 same optimizations as -O2
3 same optimizations as -O3

Note

For more information on the optimizations levels, see Enabling Automatic Optimizations.

Description

The optimization_level pragma is used to restrict optimization for a specific function while optimizing the remaining application using a different, higher optimization level. For example, if you specify -O3 (Linux and Mac OS) for the application and specify #pragma optimization_level 1, the marked function will be optimized at the -O1 option level, while the remaining application will be optimized at the higher level.

In general, the pragma optimizes the function at the level specified as n; however, certain compiler optimizations, like Inter-procedural Optimization (IPO), are not enabled or disabled during translation unit compilation. For example, if you enable IPO and a specific optimization level, IPO is enabled even for the function targeted by this pragma; however, IPO might not be fully implemented regardless of the optimization level specified at the command line. The reverse is also true.

Scope of optimization restriction

On Linux* and Mac OS*, the scope of the optimization restriction can be affected by arguments passed to the -pragma-optimization-level compiler option as explained in the following table.

Syntax	Behavior
#pragma intel optimization_level n	Applies pragma only to the next function, using the specified optimization level, regardless of the argument passed to the -pragma-optimization-level option.
#pragma GCC optimization_level n or #pragma GCC optimization_level reset	Applies pragma to all subsequent functions, using the specified optimization level, regardless of the argument passed to the -pragma-optimization-level option. Specifying reset reverses the effect of the most recent #pragma GCC optimization_level statement, by returning to the optimization level previously specified.
#pragma optimization_level n	Applies either the intel or GCC interpretation. Interpretation depends on argument passed to the -pragma-optimization-level option.

Syntax

Behavior

#pragma intel optimization_level n

Applies pragma only to the next function, using the specified optimization level, regardless of the argument passed to the -pragma-optimization-level option.

#pragma GCC optimization_level n

#pragma GCC optimization_level reset

Applies pragma to all subsequent functions, using the specified optimization level, regardless of the argument passed to the -pragma-optimization-level option.

Specifying reset reverses the effect of the most recent #pragma GCC optimization_level statement, by returning to the optimization level previously specified.

#pragma optimization_level n

Applies either the intel or GCC interpretation. Interpretation depends on argument passed to the -pragma-optimization-level option.

On Windows*, the pragma always uses the intel interpretation; the pragma is applied only to the next function.

Using the intel interpretation of the pragma

Place the pragma immediately before the function being affected. See Example below.

Using the GCC interpretation of the pragma

Place the pragma in any location prior to the functions being affected.

Example

Example: intel interpretation of pragma
#pragma intel optimization_level 1 gamma() { ... }

Example: intel interpretation of pragma

#pragma intel optimization_level 1

gamma() {

...

}