Create generalized interface for repetitive load/store instructions

[nadime15]

Problem

Many current instructions and probably many future ones are based on load and store operations. I have noticed that the same code is repeated over and over again. Some examples are in Zcmp (#730), Zcmt (#757), Zilsd/Zcl (#765) and in extensions that are already implemented and lead to:

• Redundant code
• Higher maintenance burden
• Increased risk of inconsistencies when making changes
• High number of lines of code (In case code ends up in the specification)

Proposed solution

Create a generalized interface or template mechanism that can handle the common aspects of load/store operations.

This abstraction would accept common arguments, like AccessType, Aquire, Release etc.

Expected benefits

• Reduced code duplication
• Improved maintainability
• Easier implementation of new load/store variants
• More consistent behavior across related instructions
• Better testability of core load/store logic

As we also plan to integrate Sail Code into the specification we would also reduce the number of lines for all affected instructions (The generalized interface will be provided as part of the appendix)

Implementation approach

I thought of something like I did in Zcmt (#757) but only in better (return error cause + Retired Illegal, in case you want to take further action)

type target_address = bits(xlen)

union FJT_Result = {
  FJT_Success : target_address,
  FJT_Failure : unit
}

function fetch_jump_table(table_address : bits(xlen)) -> FJT_Result = {
  /* Fetching jump table address needs execute permission */
  match ext_data_get_addr_from_bits(table_address, Execute(), xlen_bytes) {
    Ext_DataAddr_Error(e)  => { ext_handle_data_check_error(e); FJT_Failure() },
    Ext_DataAddr_OK(vaddr) => {
      if   check_misaligned(vaddr, size_bytes(xlen_bytes))
      then { handle_mem_exception(vaddr, E_Load_Addr_Align()); FJT_Failure() }
      else match translateAddr(vaddr, Execute()) {
        TR_Failure(e, _) => { handle_mem_exception(vaddr, e); FJT_Failure() },
        TR_Address(paddr, _) =>
          match mem_read(Execute(), paddr, xlen_bytes, false, false, false) {
            Ok(result) => { FJT_Success(result) },
            Err(e)     => { handle_mem_exception(vaddr, e); FJT_Failure() },
          }
      }
    }
  }
}

I would like to discuss the idea of generalizing load and store operations to reduce code redundancy, as many instructions essentially perform the same function. Before I start working on this, I would like to confirm whether this approach makes sense and if there is agreement.

[Timmmm]

Yeah I've thought about this before. Especially wrt. unaligned loads/stores.

In theory it's not too hard and we have implemented a version of this in our fork. The showstopper is the call to announce the effective address for stores. As I understand it it has to happen before the store value is read, but Sail doesn't have first class functions so you can't pass a lambda function or whatever into your generic store to get it to read the register at the right point.

It only matters for the concurrency model which we aren't using so I don't really know too much about it.

But yeah I agree it's annoying and it makes the Sail code look more complicated than it is.

[Alasdair]

I did this in #467, creating generic read/write functions that handled misaligned accesses better. In that PR I just ignored the effective address thing.

You can use scattered functions to work around the lack of lambdas if you want, but it does make the code a little bit more complex. The general pattern would be:

enum clause E = A

function clause foo() = {
  ...
  bar(A)
}

function clause baz(A, x) = ...

then in the file with bar it calls

function bar(A) = {
  let x = ...;
  baz(A, x)
  ...
}

this is effectively the same as passing λx. baz(A, x) as a first class function.

[Timmmm]

Ah clever! So... can we ignore the effective address thing? I don't really care about it personally, but presumably some people do? How much does it matter?

[Alasdair]

Right now nobody is really using it. We could work around it by having the effective address announce take an explicit set of register dependencies if needed, so I don't think it should get in the way of writing the spec in the cleanest way.

[nadime15]

Okay, this looks good! I’m just wondering if it might make sense to generalize LOAD/STORE even further by passing the effective addresses directly. The reason I ask is that Zcmt, for example, reads directly from instruction memory without storing the address in a register. You can’t use LOAD directly, nor can I use the current implementation of LOAD because it relies on ext_data_get_addr.

Another point is that, while I'm not sure how much it matters right now, the imm value for STORE and LOAD is limited to a maximum of 12 bits. I understand this is how the base load and store are defined, but it means that all instructions calling these operations must respect this limit as well. This could be fine, but I wonder if, at some point, future instructions might be defined to allow larger offsets.

We could define some wrapper functions that use our current LOAD/STORE functions and call something like LOAD_RAW (I am pretty sure there is a better name 😄) and define a layered approach.