SIMD - Disadvantages

Disadvantages

• Not all algorithms can be vectorized. For example, a flow-control-heavy task like code parsing wouldn't benefit from SIMD.
• It also has large register files which increases power consumption and chip area.
• Currently, implementing an algorithm with SIMD instructions usually requires human labor; most compilers don't generate SIMD instructions from a typical C program, for instance. Vectorization in compilers is an active area of computer science research. (Compare vector processing.)
• Programming with particular SIMD instruction sets can involve numerous low-level challenges.
  • SSE (Streaming SIMD Extension) has restrictions on data alignment; programmers familiar with the x86 architecture may not expect this.
  • Gathering data into SIMD registers and scattering it to the correct destination locations is tricky and can be inefficient.
  • Specific instructions like rotations or three-operand addition are not available in some SIMD instruction sets.
  • Instruction sets are architecture-specific: old processors and non-x86 processors lack SSE entirely, for instance, so programmers must provide non-vectorized implementations (or different vectorized implementations) for them.
  • The early MMX instruction set shared a register file with the floating-point stack, which caused inefficiencies when mixing floating-point and MMX code. However, SSE2 corrects this.