In the quest for faster computing, modern CPUs have turned to innovative techniques to optimize instruction execution. One such technique, register renaming, is a crucial component that helps us achieve the impressive multi-tasking abilities of modern processors. If you’re keen on hacking or tinkering with how CPUs manage tasks, this is one concept you’ll want to understand. Here’s a breakdown of how it works and you can watch the video, below.
In a nutshell, register renaming allows CPUs to bypass the restrictions imposed by a limited number of registers. Consider a scenario where two operations need to access the same register at once: without renaming, the CPU would be stuck, having to wait for one task to complete before starting another. Enter the renaming trick—registers are reassigned on the fly, so different tasks can use the same logical register but physically reside in different slots. This drastically reduces idle time and boosts parallel tasking. Of course, you also have to ensure that the register you are using has the correct contents at the time you are using it, but there are many ways to solve that problem. The basic technique dates back to some IBM System/360 computers and other high-performance mainframes.
Register renaming isn’t the only way to solve this problem. There’s a lot that goes into a superscalar CPU.
“The basic technique dates back to some IBM System/360 computers and other high-performance mainframes.”
Interesting how much can be traced back to them.
Technology confluence and the demand for computing made System 360 possible. At the same time period as System 360 was the rise of the minicomputer as the technology trickled down. The science of computers outpaced the technology by a lot back in those days. Still does, hardware is, well hard. The difference is that all of the low hanging fruit was harvested a while back. So even on the theoretical end of things, progress is hard.
Just look at Itanium.
So it’s kind of like a Virtual Register Space, analogous to a Virtual Address Space which lets each process have an address space to itself and ignore other processes which are using the “same” addresses in memory, with an MMU translating process addresses to the actual memory locations as needed.
Is it just me or does it read as if the processor has a multiple personality disorder? Or multiple personality order? 🤔
As the saying goes,
As the old saying goes, all models are wrong except when they are useful. So your model is useful but it is wrong. Register renaming is analogous to virtual memory, but only the simplest virtual memory model. The analogy is useful, however, in that you can see how the register renaming can extend to support hardware multithreading (SMT, in which hardware threads share the physical registers without blowing away the other thread’s data) and even to multiple processes sharing register pools. You just need to add more tag bits to the renaming (“just”, heh). Where the analogy falls short is that register “translations” are never read from or written to memory the way that address translations are. For RISC folks, note that there is no such thing as “renamed register spilling” when you run out of physical registers, the CPU just stops renaming until some slots are freed. Back to memory management, register renaming also does not have the overhead of permission bits or memory allocation bits (e.g., LRU).
For those of you youngsters willing to learn more, find lessons on Tomasulo algorithm in a real Cas book (or CPU book). For God’s sake, do not try to learn ny reading Wikipedia pages! (or random websites). Regarding new tricks – they keep coming. I listened recently Ventana talk at RISC-V conference, and heard few new tricks that are not in the books. I am not sure if they already have it in Silicon, but clearly people keep coming up with new ideas. Even Itaniun – that was not a bad idea, it was lacking custom software that could have taken advantage. –Zvonimir
yep. I liked the itanium concept back im the day. I think it was ahead of it’s time for a multiple reasons
no. it is fine to learn from wikipedia. depending on your level of involvement, it might not be enough. you shouldn’t limit yourself just to wikipedia. but wikipedia is fine. textbooks and references are fine too. you learn where you learn.
and itanium was a bad idea, too :)
you can move a lot of the work into the compiler, but then the compiler has to truly know the architecture. and then the compiler has to be updated every time the architecture is updated. so in every chip’s life cycle you’ve introduced a period of time where the silicon is available and the compilers haven’t caught up yet, when you’ve paid the costs of development but shouldn’t sell it yet.
on top of that, the compiler can’t know the architecture as well as a running chip does. the chip simply has resources that their usage patterns can’t be 100% predicted. abstraction in an instruction set is good, and brings many benefits, and vliw chips lose those advantages. it’s like the exposed branch delay slot of MIPS and SPARC et al. a compiler can work around it easily but the advantage in silicon is an illusion. the chip will be repeatedly redesigned and the pipeline specifics will change from one version to another. you don’t want the ISA to change with it, so you wind up with the same overhead to ’emulate’ one branch delay slot as you would have had to emulate zero branch delay slots from the start.
I believe this concept is predicated on the idea of pipelining which isn’t a surprise considering all but one (that I know of) modern processor relies on pipelining. XMOS’ XCore processor avoids pipelining entirely by having separate sets of registers/memory so that it provides the appearance of being multiple processors, each with a “slow” clock speed. The upside is that execution is fully deterministic which enables you to bit-bang protocols without relying on IO buffers or interrupts for proper timing.
Given that we’re moving away from the monolithic “one process does everything” software process model and we have absurdly high processing rates, I think it may be time to reconsider the use of pipelining.
Linux at its core still is monolithic, though. And it dominates the IT landscape, sadly.
I hope it will be rewritten as a modular microkernal once.
pipelining isn’t going anywhere. and single-threaded execution speed still matters.