Intel® 64 and IA-32 Architectures Optimization Reference Manual

APPLICATION PERFORMANCE TOOLSTable A-2. Recommended Processor Optimization Options for 64-bit Code (Contd.)Need Recommendation Comments• /QaxP /QxW (-axP-xW on Linux)Best performance on otherprocessors supporting Intel 64architecture, utilizing SSE3where possible, while stillrunning on older Intel as wellas non-Intel x86-64 processorssupporting SSE2• Multiple code path aregenerated• Be sure to validate yourapplication on all systemswhere it may be deployed.A.1.2Vectorization and Loop OptimizationThe Intel C++ and Fortran Compiler’s vectorization feature can detect sequentialdata access by the same instruction and transforms the code to use SSE, SSE2,SSE3, and SSSE3, depending on the target processor platform. The vectorizersupports the following features:• Multiple data types: Float/double, char/short/int/long (both signed andunsigned), _Complex float/double are supported.• Step by step diagnostics: Through the /Qvec-report[n] (-vec-report[n] on Linuxand Mac OS) switch (see Table A-3), the vectorizer can identify, line-by-line andvariable-by-variable, what code was vectorized, what code was not vectorized,and more importantly, why it was not vectorized. This feedback gives thedeveloper the information necessary to slightly adjust or restructure code, withdependency directives and restrict keywords, to allow vectorization to occur.• Advanced dynamic data-alignment strategies: Alignment strategies include looppeeling and loop unrolling. Loop peeling can generate aligned loads, enablingfaster application performance. Loop unrolling matches the prefetch of a fullcache line and allows better scheduling.• Portable code: By using appropriate Intel compiler switches to take advantagenew processor features, developers can avoid the need to rewrite source code.The processor-specific vectorizer switch options are: -Qx[K,W, N, P, T] and-Qax[K,W, N, P, T]. The compiler provides a number of other vectorizer switchoptions that allow you to control vectorization. The latter switches require the -Qx[,K,W,N,P,T] or -Qax[K,W,N,P,T] switch to be on. The default is off.Table A-3. Vectorization Control Switch Options-Qvec_report[n] Controls the vectorizer’s diagnostic levels, where n is either 0, 1, 2, or 3.-QrestrictEnables pointer disambiguation with the restrict qualifier.A-4