# Other Considerations

There are some notable differences between the Intel® C++ Compiler Classic and GCC*. Consider the following as you begin compiling your source code with the Intel® C++ Compiler Classic.

## Set the Environment

The Intel® C++ Compiler Classic relies on environment variables for the location of compiler binaries, libraries, man pages, and license files. In some cases these are different from the environment variables that GCC uses. Another difference is that these variables are not set by default after installing the Intel® C++ Compiler Classic. The following environment variables need to be set prior to running the Intel® C++ Compiler Classic:

- PATH: Add the location of the compiler binaries to PATH.

- LD_LIBRARY_PATH: Sets the location where the generated executable picks up the runtime libraries (*.so files).

- MANPATH – add the location of the compiler man pages (`icc or icpc`) to MANPATH.

- INTEL_LICENSE_FILE – sets the location of the Intel® C++ Compiler license file.

To set these environment variables, run the compilervars.sh script.

### Note

Setting these environment variables with compilervars.sh does not impose a conflict with GCC. You should be able to use both compilers in the same shell.

## Use Optimization

The Intel® C++ Compiler Classic is an optimizing compiler that begins with the assumption that you want improved performance from your application when it is executed on Intel® architecture. Consequently, certain optimizations, such as option O2, are part of the default invocation of the Intel® C++ Compiler Classic. Optimization is turned off in GCC by default, the equivalent of compiling with option O0. Other forms of the `O<n>` option compare as follows:

Option | Intel® C++ Compiler Classic | GCC
--- | --- | ---
-O0 | Turns off optimization. | Default. Turns off optimization.
-O1 | Decreases code size with some increase in speed. | Decreases code size with some increase in speed.
-O2 | Default. Favors speed optimization with some increase in code size. Same as option O. Intrinsics, loop unrolling, and inlining are performed. | Optimizes for speed as long as there is not an increase in code size. Loop unrolling and function inlining, for example, are not performed.
-O3 | Enables option O2 optimizations plus more aggressive optimizations, such as prefetching, scalar replacement, and loop and memory access transformations. | Optimizes for speed while generating larger code size. Includes option O2 optimizations plus loop unrolling and inlining. Similar to option O2 -ip on the Intel® C++ Compiler.

## Target Intel® Processors

While many of the same options that target specific processors are supported with both compilers, Intel includes options that utilize processor-specific instruction scheduling to target the latest Intel® processors. If you compile your GCC application with the march or mtune option, consider using the Intel® C++ Compiler options x or ax options for applications that run on IA-32 architecture or Intel® 64 architecture.

## Modify Your Configuration

The Intel® C++ Compiler Classic lets you maintain configuration and response files that are part of compilation. Options stored in the configuration file apply to every compilation, while options stored in response files apply only where they are added on the command line. If you have several options in your makefile that apply to every build, you may find it easier to move these options to the configuration file (`icc.cfg or icpc.cfg`).

In a multi-user, networked environment, options listed in the `icc.cfg or icpc.cfg` files are generally intended for everyone who uses the compiler. If you need a separate configuration, you can use the `ICCCFG or ICPCCFG`environment variable to specify the name and location of your own .cfg file, such as /my_code/my_config.cfg. Anytime you instruct the compiler to use a different configuration file, the system configuration files (`icc.cfg or icpc.cfg`) are ignored.

## Use the Intel Libraries

The Intel® C++ Compiler Classic supplies additional libraries that contain optimized implementations of many commonly used functions. Some of these functions are implemented using CPU dispatch. This means that different code may be executed when run on different processors.

Supplied libraries include the Intel® C++ Compiler Classic Math Library (libimf), the Short Vector Math Library (libsvml), libirc, as well as others. These libraries are linked in by default. Some library functions, such as sin or `memset`, may not require a call to the library, since the compiler may inline the code for the function.

### Note

The Intel Compiler Math Libraries contain performance-optimized implementations for various Intel platforms. By default, the best implementation for the underlying hardware is selected at runtime. The library dispatch of multi-threaded code may lead to apparent data races, which may be detected by certain software analysis tools. However, as long as the threads are running on cores with the same CPUID, these data races are harmless and are not a cause for concern.

Intel® C++ Compiler Classic Math Library (libimf)

With the Intel® Compiler, the math library, libimf, is linked by default. Some functions, such as sin, may not require a call to the library, since the compiler already knows how to compute the `sin` function. The math library also includes some functions not found in the standard math library.

### Note

You cannot make calls to the math library with GCC.

Many routines in the libimf library are more optimized for Intel® microprocessors than for non-Intel microprocessors.

Short Vector Math Library (libsvml)

When vectorization is being done, the compiler may translate some calls to the libimf math library functions into calls to libsvml functions. These functions implement the same basic operations as the math library, but operate on short vectors of operands. This results in greater efficiency. In some cases, the libsvml functions are slightly less precise than the equivalent libimf functions.

Many routines in the libimf library are more optimized for Intel® microprocessors than for non-Intel microprocessors.

libirc

libirc contains optimized implementations of some commonly used string and memory functions. For example, it contains functions that are optimized versions of `memcpy` and `memset`. The compiler will automatically generate calls to these functions when it sees calls to `memcpy` and `memset`. The compiler may also transform loops that are equivalent to `memcpy` or `memset` into calls to these functions.

Many routines in the libirc library are more optimized for Intel® microprocessors than for non-Intel microprocessors.

Product and Performance Information
Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex. Notice revision #20201201

Parent topic: Port from GCC* to the Intel® C++ Compiler Classic

## See Also

ax, Qax
Invoke the Compiler
march
mtune
-O compiler option
Using Configuration Files
Using Response Files
x, Qx