Modern CPU cores have special vector units that can process more than one piece of data in a single instruction. For example, a modern X86-64 CPU with AVX vector instructions has a set of different instructions that can work with 4 double values in one instruction: vector load instruction loads 4 double values into a vector register, vector add instruction adds together 2 registers each holding 4 double values, etc.
The process of using special vector instructions to speed up computation is called vectorization. If the code meets certain conditions, the compiler can emit vector instructions instead of scalar instructions in order to speed up computation. This process is called autovectorization.
For the compiler to be able automatically vectorize a loop, certain conditions must be fulfilled:
Sometimes compilers fail to vectorize certain loops automatically for various reasons. It is possible to use compiler pragmas either to force vectorization or provide additional information needed for vectorization. There are portable vectorization pragmas defined by OpenMP standard, but also there are compiler-specific vectorization pragmas as well.
Apart from relying on autovectorization, it is possible for the developer to manually write code that employs vector instructions: either pure assembly, C/C++ assembly intrinsics, or using one of the vectorization frameworks such as EVE.