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Proposal for instruction set - now on Github
Author: Agner Date: 2016-08-18 09:01
Joe Duarte wrote:
Agner, by the way, I just learned that my idea for small cache for constants (preferably in the form of special registers, or at least an L1 speed cache) has been widely implemented for years in Nvidia GPUs. It's called "constant memory", and reserves 64 KiB.
ForwardCom can have read-only memory areas, but they are not guraranteed to be cached. I assume that you want a small memory area that is guaranteed to always be cached. That might actually give a speed gain since memory access is the most important bottleneck in many applications.

The Itanium did not have success with "explicit parallelism" because it was too difficult to handle for the compiler. Maybe "explicit caching" will be easier to handle. You might want to specify explicit caching for the top of the stack in the innermost loop or for often-used constants, jump tables or global variables.

There is a problem with task switching, though. The entire contents of the on-chip extra cache has to be swapped to main memory on each task switch. If you want fast task switching, you might want to have multiple cache banks with one for each process. That would be cumbersome.

Another possibility would be to reserve the extra cache for a particularly critical process. This process would monopolize the extra cache in one or more CPU cores while less important processes would be prevented from accessing it. That's an extra complication for the operating system to keep track of, but it is a quite common scenario that one process is speed-critical while all other processes are unimportant background processes.

Another possibility, which is perhaps simpler to handle, is an extra set of registers which are used for extra storage. Assume, for example, that we supplement the vector registers v0 - v31 by an extra set of vector registers w0 - w31. The extra registers can perhaps do nothing but read and write, e.g.

    w8 = v5;
    v2 = w8;
These extra registers can be used for often-used constants, for global variables, and for temporarily storing a normal register rather than saving it on the stack.

With an extra set of registers you have the complication of calling conventions. ForwardCom has a mechanism for telling the compiler which registers are modified by e.g. a library function. This mechanism could be extended to the extra registers. A library function could save a callee-save register to an extra register rather than saving it on the stack and reserve this extra register by indicating in the object file which extra registers it is using. The extra registers still have to be saved at every task switch if they are used.

In fact, I don't think ISAs should settle on 32-bit and 64-bit types and registers unless those turn out to be optimal sizes, which I think is unlikely. As long as we can take care of alignment issues, I would determine the right type and register sizes empirically, based on what we know about common workloads and applications, as well as foreseeable future workloads.
The alignment problem is serious. The hardware would have to support misaligned memory accesses as well as vectors of odd-size elements, which would be quite expensive for the hardware design and for performance. There is also a standardization issue.
 
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