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![[INTEL NAVIGATION HEADER]](../../PIX/HEADER.GIF){kind=small}
Intel C/C++ and FORTRAN Compilers
The Intel C/C++ Compiler plug-in and FORTRAN compiler allow software developers to gain superior performance on Intel Architecture processors. The objective of these compilers is to allow software to take advantage of the full potential of Intel Architecture processors. The Intel C/C++ Compiler plug-in and FORTRAN compiler are the first compilers to offer processor-specific optimizations with the introduction of each new Intel processor generation. This allows developers to take immediate advantage of each new processor's improvements. Support of MMX™ technology is a vivid example of the Intel compilers’ evolution in parallel with the latest line of Intel Architecture processors. Feature Summary Intel compilers have been developed to be compatible with Microsoft* Visual C++*. To ensure ease of use, the Intel C/C++ Compiler plug-in is usable as a first generation plug-in to the Microsoft Developer Studio*. This capability combines the power of Intel compilers with the features of Microsoft's Integrated Development Environment (IDE). The following summarizes the features for each of the compilers: C/C++ plug-in:
FORTRAN:
System Requirements The Intel C/C++ and FORTRAN compilers run under the Microsoft Windows* 95 or Windows NT* operating systems. Language Support The Intel C/C++ compiler is a plug-in to Microsoft Visual C++* version 4.x, which provides the development and run-time environments plus the MFC* libraries. Note: There are known compatibility problems with MSVC++ 5.0 (vs. MSVC++ 4.2) which some users may experience. For specifics, please consult the release notes for this release. Intel expects to have all MSVC++ 5.0 issues resolved in a mid-April '97 Beta update. The following FORTRAN languages and extensions are supported:
Microsoft Visual C/C++ 4.x Compatibility The Intel C/C++ compiler is compatible with Microsoft Visual C++ 4.x in the following areas:
Application Support The Intel C/C++ Compiler plug-in is particularly efficient in support of the applications described in the sections that follow. Graphics / Multimedia Applications MMX™ technology adds 57 powerful new assembly instructions to the Intel Architecture instruction set which are designed to efficiently manipulate and process video, audio, and graphical data. The Pentium® and Pentium Pro processors with MMX™ technology include these new instructions to enhance performance of multimedia applications. The Intel C/C++ Compiler plug-in supports these new MMX instructions in C/C++ programs by using special compiler intrinsics that are coded using C function call syntax. The compiler allows you to use C language variables in place of hardware registers, which frees you from managing these registers. The compiler generates the corresponding MMX instructions and reorders them to maximize performance through the Pentium processor’s dual instruction pipeline. In addition, the compiler also handles the loads and stores of the C variables to and from memory. Here is an example of an Intel C Compiler intrinsic and a description of its function: _m64 _m_pmaddwd (__m64 m1, __m64 m2) where (__m64 m1, __m64 m2) means to multiply four 16-bit values in m1 by four 16-bit values in m2 to produce four 32-bit intermediate results, which are then summed by pairs to produce two 32-bit results. The Intel C/C++ Compiler plug-in also provides a rounding control option, which optimizes floating-point-to-integer conversions. The system default rounding mode is round-to-nearest. Because the C language requires that floating-point-to-integer conversions be truncated, the compiler must generate additional instructions to change the rounding mode to truncation before each floating-point instruction and then change it back afterwards. With the -Qrcd switch you can optimize your code by eliminating the additional overhead of instructions required to change the rounding mode back and forth. This option has no effect on floating-point calculations, but conversions to integer will not conform to C semantics. Graphics applications that use floating-point data as input into their rendering operations can benefit from this type of optimization. Consider the following example: int a; void func() The following is the standard code generation that would take place:
Notice that it takes ten instructions to complete this function. Using the rounding control option -Qrcd, the optimized code looks like this:
You can see that it takes only two instructions to complete the function. This has reduced the total number of instructions by 80%. Scientific / Engineering Applications The Intel C/C++ Compiler plug-in provides analysis for interprocedural optimizations to assist you with programs that contain many small or medium-sized frequently used functions, especially for programs that contain calls within loops. Potential optimizations around calling points are normally inhibited due to a lack of information about what happens in the called procedure. Interprocedural analysis examines the relationship between calling and called procedures and enables the following optimizations:
In addition, the Intel C/C++ Compiler plug-in exploits the use of the floating-point (FP) stack by implementing code generation optimizations that allow FP instructions to execute more efficiently. Most floating-point operations require that one operand and the result use the top of stack. This makes each FP instruction dependent on the previous one and inhibits overlapping the instructions. The compiler breaks this dependency by allowing a program to arrange for one of the inputs for the next operation to always be at the top of stack. It provides this capability by effective use of the FXCH instruction, which comes at almost no additional cost on the Pentium® Pro processor. Consider the following expression: a = ((b + c) * b) + ((d + e) * d);. This expression can be presented graphically as follows:
The serial instruction sequence depicts instructions executed one at a time with no overlapping because of the top-of-stack dependency. The parallel instruction sequence uses the FXCH instruction that provides the following gains:
Database Server Applications The Intel C/C++ Compiler plug-in has been proven to assist large database applications through combination of interprocedural analysis and profile-guided optimization. Profile-guided optimization provides detailed information on program execution. Therefore, you can optimize performance-critical areas of large applications where the execution time is mostly spent. Profile-guided optimizations can help eliminate instruction cache thrashing by reorganizing code layout, shrinking code size, and reducing branch mispredictions. Information collected during program execution can be fed back into the compiler to allow a higher degree of optimization. For example, profile-guided optimization might find that a particular section of code is rarely executed. This code would then be moved to the end of the module resulting in the processor fetching instructions more efficiently. The following are the three phases of profile-guided optimization that, when completed, provide the data that can significantly improve the performance of large applications:
Application Optimizations Summary The following table summarizes the optimizations that the compiler applies to your program for each optimization switch. The entry "any" in the Option column means that the compiler automatically performs this optimization, even when optimizations are disabled.
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