Three Fundamental Flaws of SIMD ISAs (2023)

According to Flynn’s taxonomy SIMD refers to a computer architecture that can process multiple data streams with a single instruction (i.e. “Single Instruction stream, Multiple Data streams”). There are different taxonomies, and within those several different sub-categories and architectures that classify as “SIMD”. In this post, however, I refer to packed SIMD ISA:s, i.e. the type of SIMD instruction set architecture that is most common in contemporary consumer grade CPU:s. More specifically, I refer to non-predicated packed SIMD ISA:s where the details of packed SIMD processing is exposed to the software environment. Packed SIMD The common trait of packed SIMD architectures is that several data elements are packed into a single register of a fixed width. Here is an example of possible configurations of a packed 128 bits wide SIMD register: For instance, a 128-bit register can hold sixteen integer bytes or four single precision floating-point values. This type of SIMD architectur ... Read full article.