A VLIW has a lot of functional units that are used simultaneously.* Modern processors (out of order, superscalar, using speculative execution, pick your techniques and buzzwords) allow these units to be used simultaneously and dynamically depending on the instruction mix. With VLIW some instruction units won’t be used and so won’t contribute to performance.
The compilers can give hints to the execution CPU but it also decides what to do when. With VLIW (EPIC is probably a better name in this regard) you have to guess right up front, without knowing what the data will be.
* So does SIMD, but in VLIW you don’t have to have a single instruction.
A VLIW has a lot of functional units that are used simultaneously.* Modern processors (out of order, superscalar, using speculative execution, pick your techniques and buzzwords) allow these units to be used simultaneously and dynamically depending on the instruction mix. With VLIW some instruction units won’t be used and so won’t contribute to performance.
The compilers can give hints to the execution CPU but it also decides what to do when. With VLIW (EPIC is probably a better name in this regard) you have to guess right up front, without knowing what the data will be.
* So does SIMD, but in VLIW you don’t have to have a single instruction.