Developer Guide and Reference


Automatic Vectorization Overview

The automatic vectorizer (also called the auto-vectorizer) is a component of the Intel® compiler that automatically uses SIMD instructions in the Intel® Streaming SIMD Extensions (Intel® SSE, Intel® SSE2, Intel® SSE3 and Intel® SSE4), Supplemental Streaming SIMD Extensions (SSSE3) instruction sets, and the Intel® Advanced Vector Extensions (Intel® AVX, Intel® AVX2) instruction sets.
The vectorizer detects operations in the program that can be done in parallel and converts the sequential operations to parallel; for example, the vectorizer converts the sequential SIMD instruction that processes up to 16 elements into a parallel operation, depending on the data type.
Automatic vectorization occurs when the Intel® Compiler generates packed SIMD instructions to unroll a loop. Because the packed instructions operate on more than one data element at a time, the loop executes more efficiently. This process is referred to as auto-vectorization only to emphasize that the compiler identifies and optimizes suitable loops on its own, without requiring any special action by you. However, it is useful to note that in some cases, certain keywords or directives may be applied in the code for auto-vectorization to occur.
The compiler supports a variety of auto-vectorizing hints that can help the compiler to generate effective vector instructions. Automatic vectorization is supported on IA-32 and Intel® 64 architectures. Intel® Advisor XE, a separate tool included in some editions of Intel® Parallel Studio XE, provides a Vectorization Advisor feature that can analyze the compiler's optimization reports and make recommendations for enhancing vectorization.
Using this option enables vectorization at default optimization levels for both Intel® microprocessors and non-Intel microprocessors. Vectorization may call library routines that can result in additional performance gain on Intel® microprocessors than on non-Intel microprocessors.
The vectorization can also be affected by certain options, such as
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