alt Hacker NewsNo (sorry, I post this every time the Mythical Compiler Myth reappears), the problem with VLIW is that it fundamentally doesn’t work for anything with unpredictable memory access patterns, and modern general purpose computing has moved almost exclusively in this direction. For VLIW to work, you need to either guess correctly what is in cache or not have a cache at all; as soon as you mispredict what has been loaded, you stall while an OoO processor keeps going and a speculative OoO processor even keeps guessing. With multiple workloads on the same hardware (virtualization, multitasking, multitenancy) this becomes an intractable problem even in the presence of the magic compiler which can solve for software based unpredictability (branch likelihood and pointer chasing), because every context switch clobbers an unknown set of cache lines and blows the entire thing up.
VLIW works for single workloads. It works exceptionally well for single workloads with no or explicit cache like DSP. You can trade the footprint and complexity from OoO for a wider execution unit and more SRAM. It works well for HPC, too, for the same reason. But for anything where more than one process exists, it just really doesn’t work, and that’s most modern workload.
Itanium also has a unique set of self inflicted issues due in large part to Intel trying to make a wide variety of cross compatible parts, but IMO even if they’d got it right, it still would have died.
Replies
VLIW is being heavily used in some AI inference tasks (by Apple, AMD, Google, mainly in optimizing latency). It also is a nice way to do inference on edge devices that aren’t using constantly advancing models. It’s pretty much dead in general purpose computing though, and lacks the versatility of GPUs to go beyond inference.
We agree it probably would have still failed, but mostly it was the legacy code-motion compatibility/performance issues that were practically inescapable without refactoring millions of lines of code.
gcc maintained the ia64 target a long time for unclear reasons, but it was also still inefficient on other platforms. The FOSS compiler worked, but that was its only performance metric that counted for many users. =3
Not sure why you think the Intel compilers were a myth, as they are still around working far better than gcc in many use-cases:
"An Overview of the Intel® IA-64 Compiler"
https://webdocs.cs.ualberta.ca/~amaral/courses/605/papers/In...
Honestly, you're conflating two things.
1. VLIW exposes microarchitectural details, locking them in like an ABI. Updates to the microrachitecture will require changes to the ISA, thereby breaking backwards compatibility with every generation.
2. The dominant programming paradigm is sequential code, often just old C code with heavy pointer aliasing and little compile time extractable parallelism. This reduces static parallelism in the code base and shifts it into a runtime problem. The CPU discovers the dependencies at runtime instead. If you built a programming language that exposes more parallelism (think something like ParaSail), this problem wouldn't be as big as it is for C style programming languages.
Nobody will build a language for architectures that suffer from backwards compatibility problems, so why bother? VLIW is primarily suited for ASIPs and not much else.
Itanium had all sorts of specialized machinery so that you could issue a load that's required in the future, and then keep chugging away at the instruction stream, similar to what an OoO processor does with it's reordering logic.
One example was "advanced loads" which allowed you to issue a load as soon as you knew the address, even in the face of potential pointer aliasing in the future, and then later complete the load when you actually hit a data dependency that requires it. https://devblogs.microsoft.com/oldnewthing/20150805-00/?p=91...
Another example is "speculative loads" which lets you issue a load before you even know if it's valid, such as unrolling a loop for an array that you don't know if is a multiple of the unrolled loop chunk length (and therefore might trap with a page fault if fully resolved). https://devblogs.microsoft.com/oldnewthing/20150804-00/?p=91...