# Other Considerations

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

## Setting the Environment

The compiler installation provides a batch file, setvars.bat, that sets the proper environment for the Intel® C++ Compiler Classic. For information on running setvars.bat, see Specifying the Location of Compiler Components.

## Using 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 compiler. By default, Microsoft turns off optimization, which is the equivalent of compiling with options Od or O0. Other forms of the O[n] option compare as follows:

Option | Intel® C++ Compiler Classic | Microsoft Compiler
--- | --- | ---
/Od | Turns off all optimization. Same as O0. | Default. Turns off all optimization.
/O1 | Decreases code size with some increase in speed. | Optimizes code for minimum size.
/O2 | Default. Favors speed optimization with some increase in code size. Intrinsics, loop unrolling, and inlining are performed. | Optimizes code for maximum speed.
/O3 | Enables -O2 optimizations plus more aggressive optimizations, such as prefetching, scalar replacement, and loop and memory access transformations. | Not supported.

## Targeting Specific Intel® Processors

While many of the same options that target specific processors are supported with both compilers, the Intel® C++ Compiler includes options that utilize processor-specific instructions to target the latest Intel® architecture processors. Consider using the Intel® C++ Compiler [Q]x , /arch , or [Q]ax options for applications that run on IA-32 architecture or Intel® 64 architecture. Refer to the descriptions of these compiler options for more specific information.

## Modifying 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 (..\bin\icx.cfg).

In a multi-user, networked environment, options listed in the icx.cfg file are generally intended for everyone who uses the compiler. If you need a separate configuration, you can use the ICXCFG 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 icx.cfg system configuration file is ignored.

## Using 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® Math Library (libm), the Short Vector Math Library (svml_disp), libirc, as well as others. These libraries are linked in by default when the compiler sees that references to them have been generated. 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.

Intel Math Library (libm)

With the Intel® C++ Compiler Classic, the Intel Math Library, libm, is linked by default when calling math functions that require the library. 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 Intel Math Library also includes some functions not found in the standard math library.

### Note

You cannot make calls to the Intel Math Library with the Microsoft Compiler.

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

Short Vector Math Library (svml_disp)

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

Many routines in the Short Vector Math Library (SVML) 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: Porting from Microsoft* Visual C++* to the Intel® C++ Compiler Classic

## See Also

O compiler option
Differences in PCH Support
Using Configuration Files
Using Response Files
Specifying the Location of Compiler Components