Intel® 64 and IA-32 Architectures Optimization Reference Manual

APPLICATION PERFORMANCE TOOLSA.1.2.1Multithreading with OpenMP*Both the Intel C++ and Fortran Compilers support shared memory parallelism usingOpenMP compiler directives, library functions and environment variables. OpenMPdirectives are activated by the compiler switch /Qopenmp (-openmp on Linux andMac OS). The available directives are described in the Compiler User's Guides availablewith the Intel C++ and Fortran Compilers. For information about the OpenMPstandard, see http://www.openmp.org.A.1.2.2Automatic MultithreadingBoth the Intel C++ and Fortran Compilers can generate multithreaded code automaticallyfor simple loops with no dependencies. This is activated by the compilerswitch /Qparallel (-parallel in Linux and Mac OS).A.1.3Inline Expansion of Library Functions (/Oi, /Oi-)The compiler inlines a number of standard C, C++, and math library functions bydefault. This usually results in faster execution. Sometimes, however, inline expansionof library functions can cause unexpected results. For explanation, see theIntel C++ Compiler documentation.A.1.4Floating-point Arithmetic Precision (/Op, /Op-, /Qprec,/Qprec_div, /Qpc, /Qlong_double)These options provide a means of controlling optimization that might result in a smallchange in the precision of floating-point arithmetic.A.1.5Rounding Control Option (/Qrcr, /Qrcd)The compiler uses the -Qrcd option to improve the performance of code that requiresfloating-point calculations. The optimization is obtained by controlling the change ofthe rounding mode.The -Qrcd option disables the change to truncation of the rounding mode in floatingpoint-to-integerconversions.For more on code optimization options, see the Intel C++ Compiler documentation.A.1.6Interprocedural and Profile-Guided OptimizationsThe following are two methods to improve the performance of your code based on itsunique profile and procedural dependencies.A-5