Optimization Options Summary

/O1

-O1

Optimizes to favor code size and code locality. Disables loop unrolling. Disables intrinsics inlining to reduce code size. Enables optimizations for speed. Also disables intrinsic recognition and the -fp (Linux) or /Oy- (Windows) option.

May improve performance for applications with very large code size, many branches, and execution time not dominated by code within loops.

In most cases, -O2 (Linux) or /O2 (Windows) is recommended over this option.

This option behaves differently depending on architecture and operating system. To see which options this option sets or to get detailed information about the architecture- and operating system-specific behaviors, see the following topic:

• -O1 compiler option

/O2, /Ox

-O2, -O

Optimizes for code speed. This is the default optimization and is the generally recommended optimization level.

IA-32 systems:

• Linux: This option is the same as the -O1 (Linux) or /O1 (Windows) option.

Itanium®-based systems:

• Enables optimizations for speed, including global code scheduling, software pipelining, predication, speculation, and data prefetch.

• On these systems, the -O2 (Linux) or /O2 (Windows) option enables inlining of intrinsics. It also enables the following capabilities for performance gain: constant propagation, copy propagation, dead-code elimination, global register allocation, global instruction scheduling and control speculation, loop unrolling, optimized code selection, partial redundancy elimination, strength reduction/induction variable simplification, variable renaming, exception handling optimizations, tail recursions, peephole optimizations, structure assignment lowering and optimizations, and dead store elimination.

For more information, see the following topic:

• -O2 compiler option

/O3

-O3

Enables -O2 (Linux) or /O2 (Windows) optimizations and in addition, enables more aggressive optimizations such as prefetching, scalar replacement, and loop and memory access transformations. Enables optimizations for maximum speed, but does not guarantee higher performance unless loop and memory access transformation take place. The optimizations enabled by this option can slow down code in some cases compared to -O2 (Linux) or /O2 (Windows) optimizations.

Recommended for applications that have loops that heavily use floating-point calculations and process large data sets.

IA-32 systems:

• Causes the compiler to perform more aggressive data dependency analysis than for -O2 (Linux) or /O2 (Windows), which might result in longer compilation times, when used with the -ax or -x (Linux) or /Qax or /Qx (Windows)

Itanium®-based systems:

• Enables optimizations for technical computing applications (loop-intensive code): loop optimizations and data prefetch.

For more information, see the following topic:

• -O3 compiler option

/fast

-fast

This option is a single, simple method to enable a collection of optimizations for run-time performance. This option sets specific options, depending on architecture and operating system:

Itanium®-based systems:

• Linux: -ipo, -O3, and -static

• Windows: /O3 and /Qipo

IA-32 and Intel® EM64T systems:

• Linux: -ipo, -O3, -no-prec-div, -static, and -xP

• Windows: /O3, /Qipo, /Qprec-div-, and /QxP

Additionally for IA-32 and Intel® EM64T systems, the -xP (Linux) or /QxP (Windows) option that is set by the fast option cannot be overridden by other command line options. If you specify this option and a different processor-specific option, such as -xN (Linux) or /QxN (Windows), the compiler will issue a warning stating the -xP or /QxP option cannot be overridden. In such cases, use this option along with processor-specific options. For example, if you want to use the processor-specific option -xW (Linux) or /QxW (Windows), do not specify the fast option; instead, specify the following combination of options:

• Linux systems: -O3 -ipo -no-prec-div -static -xW

• Windows systems: /O3 /Qipo /Qprec-div- /QxW

For more information on restrictions and usage, see the following topic:

• -fast compiler option

/Od

-O0

Disables optimizations.

Linux:

• Sets option -fp and option -fmath-errno.

Window:

• Use /Od to specify particular optimizations, such as: /Od /Ob1 (disables all optimizations, but only enables inlining)

For more information, see the following topic:

• -O0 compiler option

/Zi, /Z7

-g

Specifying the option turns off the default -O2 (Linux) or /O2 (Windows) option and makes -O0 (Linux) or /Od (Windows) the default unless -O2 (Linux) or /O2 (Windows), or -O1 or -O3 (Linux) or /O1 or /O3 (Windows), is explicitly specified in the command line together with this option.

For more information, see the following topic:

• -g compiler option

No equivalent

-fmath-errno,
-fno-math-errno

Instructs the compiler to assume that the program tests errno after calls to math library functions.

• -fmath-errno compiler option

/Qsox

-sox

Instructs the compiler to save the compiler options and version number in the executable. During the linking process, the linker places information strings into the resulting executable.

For more information, see the following topic:

• -sox compiler option