New instruction schema

[dhower-qc]

This PR is a WIP to discuss the structure of ELF relocations in the database. Eventually, we will tie these to the variables in instruction encodings.

[AFOliveira]

I just have a couple questions on this:

• Couldn't ABI version be a parameter so that we would allow more than 1 ABI?
• Couldn't "Not 0 " be a parameter or should the immediate have this defined elsewhere?

[dhower-qc]

Good questions. I think both of these can be addressed in how this relocation data gets tied to an encoding. For example, if we had something like:

[common/inst_variable/itype_imm.yaml]

$schema: inst_variable_schema.json
kind: instruction variable
location: 31-20
# ...
elf_psabi_relocation: { $ref: relocation/R_RISCV_PCREL_LO12_I.yaml# }
pe_abi_relocation: # { ... }  future Windows ABI

[inst/addi.yaml]

# ...
encoding:
  fields:
    imm: { $ref: common/inst_variable/itype_imm.yaml# }

Then we can tie as many relocations as needed to an instruction variable. We do probably need to add an abi key to the relocation object.

This also handled the "not 0" case, since that's tied to the instruction variable.

[apazos]

We should document the new relocations only. What is already documented in the RISC-V psABI (https://github.com/riscv-non-isa/riscv-elf-psabi-doc), we should skip.

We should follow the psABI document format for recording the info, so that the resulting PDF has the same look and feel of the RISC-V psABI.

• A table with Vendor relocation types, their vendor enum, and their descriptions (See Table 13. Relocation types in the RISC-V psABI)
• Examples of code sequence
• Relaxation Types with Target Relocation, Description, Condition, Extensions Required, Relaxation result

[dhower-qc]

I've added a new attempt at an instruction schema. Here is an example:

$schema: "inst_schema.json#"
kind: instruction
name: lb
long_name: Load byte
format: { $ref: inst_format/itype.yaml# } # the instruction format 'I-type'
# ...
assembly: xd, offset(rs1)
encoding:
  match: -----------------000-----0000011
  variables:
  - field: { $ref: inst_format/itype.yaml#/fields/name=imm }
    name: offset
  - field: { $ref: inst_format/itype.yaml#/fields/name=rs1 }
    name: xs1
  - field: { $ref: inst_format/itype.yaml#/fields/name=rs1 }
    name: xd
# ...

and itype:

$schema: inst_format.json#
kind: instruction format
name: I-type
size: 32
fields:
- location: 31-20
  name: imm
  kind: immediate
  sign_extend: true
  relocations:
  - $ref: relocation/R_RISCV_PCREL_LO12_I.yaml#
  - $ref: relocation/R_RISCV_TPREL_LO12_I.yaml#
  - $ref: relocation/R_RISCV_TLSDESC_ADD_LO12.yaml#
  - $ref: relocation/R_RISCV_LO12_I.yaml#
- location: 19-15
  name: rs1
  kind: x source register
- location: 14-12
  name: funct3
  kind: opcode
- location: 11-7
  name: rd
  kind: x destination register
- location: 6-0
  name: opcode
  kind: opcode
  opcode: true